Input: What undertaking included the person who discovered that the number of protons in each element's atoms is unique?

Context:
Passage 1:
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes in air and moisture. When oxidized, neodymium reacts quickly producing pink, purple/blue and yellow compounds in the +2, +3 and +4 oxidation states. It is generally regarded as having one of the most complex spectra of the elements. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach, who also discovered praseodymium. It is present in significant quantities in the minerals monazite and bastnäsite. Neodymium is not found naturally in metallic form or unmixed with other lanthanides, and it is usually refined for general use. Neodymium is fairly common—about as common as cobalt, nickel, or copper—and is widely distributed in the Earth's crust. Most of the world's commercial neodymium is mined in China, as is the case with many other rare-earth metals.
Neodymium compounds were first commercially used as glass dyes in 1927 and remain a popular additive. The color of neodymium compounds comes from the Nd3+ ion and is often a reddish-purple. However, this changes with the type of lighting because of the interaction of the sharp light absorption bands of neodymium with ambient light enriched with the sharp visible emission bands of mercury, trivalent europium or terbium. Neodymium-doped glasses are used in lasers that emit infrared with wavelengths between 1047 and 1062 nanometers. These lasers have been used in extremely high-power applications, such as experiments in inertial confinement fusion. Neodymium is also used with various other substrate crystals, such as yttrium aluminium garnet in the Nd:YAG laser.
Neodymium alloys are used to make high-strength neodymium magnets—a type of powerful permanent magnet. These magnets are widely used in products like microphones, professional loudspeakers, in-ear headphones, high-performance hobby DC electric motors, and computer hard disks, where low magnet mass (or volume) or strong magnetic fields are required. Larger neodymium magnets are used in electric motors with a high power-to-weight ratio (e.g., in hybrid cars) and generators (e.g., aircraft and wind turbine electric generators).

Characteristics
Physical properties
Metallic neodymium has a bright, silvery metallic luster. Neodymium commonly exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C. Neodymium, like most of the lanthanides, is paramagnetic at room temperature and becomes an antiferromagnet upon cooling to 20 K (−253.2 °C). Neodymium is a rare-earth metal that was present in the classical mischmetal at a concentration of about 18%. To make neodymium magnets it is alloyed with iron, which is a ferromagnet.

Electron configuration
In the periodic table, it appears between the lanthanides praseodymium to its left and the radioactive element promethium to its right, and above the actinide uranium. Its 60 electrons are arranged in the configuration [Xe]4f46s2, of which the six 4f and 6s electrons are valence. Like most other metals in the lanthanide series, neodymium usually only uses three electrons as valence electrons, as afterwards the remaining 4f electrons are strongly bound: this is because the 4f orbitals penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this increases with higher ionic charge. Neodymium can still lose a fourth electron because it comes early in the lanthanides, where the nuclear charge is still low enough and the 4f subshell energy high enough to allow the removal of further valence electrons.

Chemical properties
Neodymium is the fourth member of the lanthanide series. It has a melting point of 1,024 °C (1,875 °F) and a boiling point of 3,074 °C (5,565 °F). Neodymium, like other lanthanides, usually has the oxidation state +3, but it can also form in the +2 and +4 oxidation states, and even, in very rare conditions, +0. Neodymium metal quickly oxidizes at ambient conditions, forming an oxide layer like iron rust that spalls off and exposes the metal to further oxidation; a centimeter-sized sample of neodymium corrodes completely in about a year. Nd3+ is generally soluble in water. Like its neighbor praseodymium, it readily burns at about 150 °C to form neodymium(III) oxide; the oxide peels off, exposing the bulk metal to the further oxidation:
4Nd + 3O2 → 2Nd2O3Neodymium is a quite electropositive element, and it reacts slowly with cold water, or quickly with hot water, to form neodymium(III) hydroxide:

2Nd (s) + 6H2O (l) → 2Nd(OH)3 (aq) + 3H2 (g)Neodymium metal reacts vigorously with all the stable halogens:
2Nd (s) + 3F2 (g) → 2NdF3 (s) [a violet substance]
2Nd (s) + 3Cl2 (g) → 2NdCl3 (s) [a mauve substance]
2Nd (s) + 3Br2 (g) → 2NdBr3 (s) [a violet substance]
2Nd (s) + 3I2 (g) → 2NdI3 (s) [a green substance]Neodymium dissolves readily in dilute sulfuric acid to form solutions that contain the lilac Nd(III) ion. These exist as a [Nd(OH2)9]3+ complexes:
2Nd (s) + 3H2SO4 (aq) → 2Nd3+ (aq) + 3SO2−4 (aq) + 3H2 (g)

Compounds
Some of the most important neodymium compounds include:

halides: NdF3; NdCl2; NdCl3; NdBr3; NdI2; NdI3
oxides: Nd2O3
hydroxide: Nd(OH)3
carbonate: Nd2(CO3)3
sulfate: Nd2(SO4)3
acetate: Nd(CH3COO)3
neodymium magnets (Nd2Fe14B)Some neodymium compounds have colors that vary based on the type of lighting.

Organoneodymium compounds
Organoneodymium compounds are compounds that have a neodymium–carbon bond. These compounds are similar to those of the other lanthanides, characterized by an inability to undergo π backbonding. They are thus mostly restricted to the mostly ionic cyclopentadienides (isostructural with those of lanthanum) and the σ-bonded simple alkyls and aryls, some of which may be polymeric.

Isotopes
Naturally occurring neodymium (60Nd) is composed of five stable isotopes—142Nd, 143Nd, 145Nd, 146Nd and 148Nd, with 142Nd being the most abundant (27.2% of the natural abundance)—and two radioisotopes with extremely long half-lives, 144Nd (alpha decay with a half-life (t1/2) of 2.29×1015 years) and 150Nd (double beta decay, t1/2 ≈ 7×1018 years). In all, 33 radioisotopes of neodymium have been detected as of 2022, with the most stable radioisotopes being the naturally occurring ones: 144Nd and 150Nd. All of the remaining radioactive isotopes have half-lives that are shorter than twelve days, and the majority of these have half-lives that are shorter than 70 seconds; the most stable artificial isotope is 147Nd with a half-life of 10.98 days.
Neodymium also has 13 known metastable isotopes, with the most stable one being 139mNd (t1/2 = 5.5 hours), 135mNd (t1/2 = 5.5 minutes) and 133m1Nd (t1/2 ~70 seconds). The primary decay modes before the most abundant stable isotope, 142Nd, are electron capture and positron decay, and the primary mode after is beta minus decay. The primary decay products before 142Nd are element Pr (praseodymium) isotopes and the primary products after are element Pm (promethium) isotopes. Four of the five stable isotopes have been predicted to decay to isotopes of cerium or samarium and are only observationally stable. Additionally, some observationally stable isotopes of samarium are predicted to decay to isotopes of neodymium.Neodymium isotopes are used in various scientific applications. 142Nd has been used for the production of short-lived Tm and Yb isotopes. 146Nd has been suggested for the production of 147Pm, which is a source of radioactive power. Several neodymium isotopes have been used for the production of other promethium isotopes. The decay from 147Sm (t1/2 = 1.06×1011 y) to the stable 143Nd allows samarium–neodymium dating. 150Nd has also been used to study double beta decay.

History
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite. Thirty years later, fifteen-year-old Wilhelm Hisinger, a member of the family owning the mine, sent a sample to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide, which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier. Ceria was simultaneously and independently isolated in Germany by Martin Heinrich Klaproth. Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius; he separated out two other oxides, which he named lanthana and didymia. He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid. The metals that formed these oxides were thus named lanthanum and didymium, officially discovered in Vienna in 1885 by Carl Gustaf Mosander. Von Welsbach confirmed the separation by spectroscopic analysis, but the products were of relatively low purity. Didymium was discovered by Carl Gustaf Mosander in 1841, and pure neodymium was isolated from it in 1925. The name neodymium is derived from the Greek words neos (νέος), new, and didymos (διδύμος), twin.Double nitrate crystallization was the means of commercial neodymium purification until the 1950s. Lindsay Chemical Division was the first to commercialize large-scale ion-exchange purification of neodymium. Starting in the 1950s, high purity (>99%) neodymium was primarily obtained through an ion exchange process from monazite, a mineral rich in rare-earth elements. The metal is obtained through electrolysis of its halide salts. Currently, most neodymium is extracted from bastnäsite and purified by solvent extraction. Ion-exchange purification is used for the highest purities (typically >99.99%). The evolving technology, and improved purity of commercially available neodymium oxide, was reflected in the appearance of neodymium glasses in collections today. Early neodymium glasses made in the 1930s have a more reddish or orange tinge than modern versions, which are more cleanly purple, because of the difficulties in removing traces of praseodymium using early technology, namely fractional crystallization.Because of its role in permanent magnets used for direct-drive wind turbines, it has been argued that neodymium will be one of the main objects of geopolitical competition in a world running on renewable energy. This perspective has been criticised for failing to recognise that most wind turbines do not use permanent magnets, and for underestimating the power of economic incentives for expanded production.

Occurrence and production
Occurrence
Neodymium is rarely found in nature as a free element, instead occurring as ores, such as monazite and bastnäsite (these are mineral group names rather than single mineral names) that contain small amounts of all rare-earth metals. In these minerals neodymium is rarely dominant; some exceptions include monazite-(Nd) and kozoite-(Nd). The main mining areas are in China, United States, Brazil, India, Sri Lanka, and Australia. World reserves of neodymium are estimated at eight million tonnes.The Nd3+ ion is similar in size to the early lanthanides of the cerium group (those from lanthanum up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (MIIIPO4) and bastnäsite (MIIICO3F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y, with somewhat less Pr and Nd). Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.

In space
Neodymium's per-particle abundance in the Solar System is 0.083 ppb (parts per billion). This figure is about two thirds of that of platinum, but two and a half times more than mercury, and nearly five times more than gold. The lanthanides are not usually found in space, and are much more abundant in the Earth's crust.

In the Earth's crust
Neodymium is classified as a lithophile under the Goldschmidt classification, meaning that it is generally found combined with oxygen. Although it belongs to the rare-earth metals, neodymium is not rare at all. Its abundance in the Earth's crust is about 38 mg/kg, making it the 27th most common element. It is similar in abundance to lanthanum. Cerium is the most common rare-earth metal, followed by neodymium, and then lanthanum.

Production
The world's production of neodymium was about 7,000 tons in 2004. The bulk of current production is from China. Historically, the Chinese government imposed strategic material controls on the element, causing large fluctuations in prices. The uncertainty of pricing and availability have caused companies (particularly Japanese ones) to create permanent magnets and associated electric motors with fewer rare-earth metals; however, so far they have been unable to eliminate the need for neodymium. According to the US Geological Survey, Greenland holds the largest reserves of undeveloped rare-earth deposits, particularly neodymium. Mining interests clash with native populations at those sites, due to the release of radioactive substances during the mining process.

Neodymium is typically 10–18% of the rare-earth content of commercial deposits of the light rare-earth-element minerals bastnäsite and monazite. With neodymium compounds being the most strongly colored for the trivalent lanthanides, it can occasionally dominate the coloration of rare-earth minerals when competing chromophores are absent. It usually gives a pink coloration. Outstanding examples of this include monazite crystals from the tin deposits in Llallagua, Bolivia; ancylite from Mont Saint-Hilaire, Quebec, Canada; or lanthanite from the Saucon Valley, Pennsylvania, United States. As with neodymium glasses, such minerals change their colors under the differing lighting conditions. The absorption bands of neodymium interact with the visible emission spectrum of mercury vapor, with the unfiltered shortwave UV light causing neodymium-containing minerals to reflect a distinctive green color. This can be observed with monazite-containing sands or bastnäsite-containing ore.The demand for mineral resources, such as rare-earth elements (including neodymium) and other critical materials, has been rapidly increasing owing to the growing human population and industrial development. Recently, the requirement for a low-carbon society has led to a significant demand for energy-saving technologies such as batteries, high-efficiency motors, renewable energy sources, and fuel cells. Among these technologies, permanent magnets are often used to fabricate high-efficiency motors, with neodymium-iron-boron magnets (Nd2Fe14B sintered and bonded magnets; hereinafter referred to as NdFeB magnets) being the main type of permanent magnet in the market since their invention. NdFeB magnets are used in hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs), and fuel cell vehicles (FCVs) (hereinafter referred to as xEVs), wind turbines, home appliances, computers, and many small consumer electronic devices. Furthermore, they are indispensable for energy savings. Toward achieving the objectives of the Paris Agreement, the demand for NdFeB magnets is expected to increase significantly in the future.

Applications
Neodymium has an unusually large specific heat capacity at liquid-helium temperatures, so is useful in cryocoolers.
Neodymium acetate can be a substitute for the radioactive and toxic uranyl acetate (used as a standard contrasting agent in electron microscopy).
Probably because of similarities to Ca2+, Nd3+ has been reported to promote plant growth. Rare-earth element compounds are frequently used in China as fertilizer.
Samarium–neodymium dating is useful for determining the age relationships of rocks. and meteorites
Neodymium isotopes recorded in marine sediments are used to reconstruct changes in past ocean circulation.

Magnets
Neodymium magnets (an alloy, Nd2Fe14B) are the strongest permanent magnets known. A neodymium magnet of a few tens of grams can lift a thousand times its own weight, and can snap together with enough force to break bones. These magnets are cheaper, lighter, and stronger than samarium–cobalt magnets. However, they are not superior in every aspect, as neodymium-based magnets lose their magnetism at lower temperatures and tend to corrode, while samarium–cobalt magnets do not.Neodymium magnets appear in products such as microphones, professional loudspeakers, headphones, guitar and bass guitar pick-ups, and computer hard disks where low mass, small volume, or strong magnetic fields are required. Neodymium is used in the electric motors of hybrid and electric automobiles and in the electricity generators of some designs of commercial wind turbines (only wind turbines with "permanent magnet" generators use neodymium). For example, drive electric motors of each Toyota Prius require one kilogram (2.2 pounds) of neodymium per vehicle.In 2020, physics researchers at Radboud University and Uppsala University announced they had observed a behavior known as "self-induced spin glass" in the atomic structure of neodymium. One of the researchers explained, "…we are specialists in scanning tunneling microscopy. It allows us to see the structure of individual atoms, and we can resolve the north and south poles of the atoms. With this advancement in high-precision imaging, we were able to discover the behavior in neodymium, because we could resolve the incredibly small changes in the magnetic structure."  Neodymium behaves in a complex magnetic way that had not been seen before in a periodic table element.

Glass
Neodymium glass (Nd:glass) is produced by the inclusion of neodymium oxide (Nd2O3) in the glass melt. Usually in daylight or incandescent light neodymium glass appears lavender, but it appears pale blue under fluorescent lighting. Neodymium may be used to color glass in delicate shades ranging from pure violet through wine-red and warm gray.The first commercial use of purified neodymium was in glass coloration, starting with experiments by Leo Moser in November 1927. The resulting "Alexandrite" glass remains a signature color of the Moser glassworks to this day. Neodymium glass was widely emulated in the early 1930s by American glasshouses, most notably Heisey, Fostoria ("wisteria"), Cambridge ("heatherbloom"), and Steuben ("wisteria"), and elsewhere (e.g. Lalique, in France, or Murano). Tiffin's "twilight" remained in production from about 1950 to 1980. Current sources include glassmakers in the Czech Republic, the United States, and China.The sharp absorption bands of neodymium cause the glass color to change under different lighting conditions, being reddish-purple under daylight or yellow incandescent light, but blue under white fluorescent lighting, or greenish under trichromatic lighting. This color-change phenomenon is highly prized by collectors. In combination with gold or selenium, red colors are produced. Since neodymium coloration depends upon "forbidden" f-f transitions deep within the atom, there is relatively little influence on the color from the chemical environment, so the color is impervious to the thermal history of the glass. However, for the best color, iron-containing impurities need to be minimized in the silica used to make the glass. The same forbidden nature of the f-f transitions makes rare-earth colorants less intense than those provided by most d-transition elements, so more has to be used in a glass to achieve the desired color intensity. The original Moser recipe used about 5% of neodymium oxide in the glass melt, a sufficient quantity such that Moser referred to these as being "rare-earth doped" glasses. Being a strong base, that level of neodymium would have affected the melting properties of the glass, and the lime content of the glass might have had to be adjusted accordingly.Light transmitted through neodymium glasses shows unusually sharp absorption bands; the glass is used in astronomical work to produce sharp bands by which spectral lines may be calibrated. Another application is the creation of selective astronomical filters to reduce the effect of light pollution from sodium and fluorescent lighting while passing other colours, especially dark red hydrogen-alpha emission from nebulae. Neodymium is also used to remove the green color caused by iron contaminants from glass.

Neodymium is a component of "didymium" (referring to mixture of salts of neodymium and praseodymium) used for coloring glass to make welder's and glass-blower's goggles; the sharp absorption bands obliterate the strong sodium emission at 589 nm. The similar absorption of the yellow mercury emission line at 578 nm is the principal cause of the blue color observed for neodymium glass under traditional white-fluorescent lighting. Neodymium and didymium glass are used in color-enhancing filters in indoor photography, particularly in filtering out the yellow hues from incandescent lighting. Similarly, neodymium glass is becoming widely used more directly in incandescent light bulbs. These lamps contain neodymium in the glass to filter out yellow light, resulting in a whiter light which is more like sunlight. During World War I, didymium mirrors were reportedly used to transmit Morse Code across battlefields. Similar to its use in glasses, neodymium salts are used as a colorant for enamels.

Lasers
Certain transparent materials with a small concentration of neodymium ions can be used in lasers as gain media for infrared wavelengths (1054–1064 nm), e.g. Nd:YAG (yttrium aluminium garnet), Nd:YAP (yttrium aluminium perovskite), Nd:YLF (yttrium lithium fluoride), Nd:YVO4 (yttrium orthovanadate), and Nd:glass. Neodymium-doped crystals (typically Nd:YVO4) generate high-powered infrared laser beams which are converted to green laser light in commercial DPSS hand-held lasers and laser pointers.

Trivalent neodymium ion Nd3+ was the first lanthanide from rare-earth elements used for the generation of laser radiation. The Nd:CaWO4 laser was developed in 1961. Historically, it was the third laser which was put into operation (the first was ruby, the second the U3+:CaF laser). Over the years the neodymium laser became one of the most used lasers for application purposes. The success of the Nd3+ ion lies in the structure of its energy levels and in the spectroscopic properties suitable for the generation of laser radiation. In 1964 Geusic et al. demonstrated the operation of neodymium ion in YAG matrix Y3Al5O12. It is a four-level laser with lower threshold and with excellent mechanical and temperature properties. For optical pumping of this material it is possible to use non-coherent flashlamp radiation or a coherent diode beam.

The current laser at the UK Atomic Weapons Establishment (AWE), the HELEN (High Energy Laser Embodying Neodymium) 1-terawatt neodymium-glass laser, can access the midpoints of pressure and temperature regions and is used to acquire data for modeling on how density, temperature, and pressure interact inside warheads. HELEN can create plasmas of around 106 K, from which opacity and transmission of radiation are measured.Neodymium glass solid-state lasers are used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Nd:glass lasers are usually frequency tripled to the third harmonic at 351 nm in laser fusion devices.

Substitute for uranyl acetate
Uranyl acetate has been the standard contrasting agent in transmission electron microscopy (TEM) for decades. However, its use is increasingly hampered by regulations by governments due to its radioactive properties as well as its high toxicity. Therefore, alternatives are being searched for, including lanthanide acetates or platinum blue  as well as the use of less defined substances such as oolong tea extract. Despite these published alternatives, uranyl acetate (UAc) is still the standard for EM contrasting.In the periodic table the vertical ordering of elements in groups is based on the presence of the same number of electrons in their outermost shell, which determines their chemical and physical properties. Because neodymium (Nd) is right above uranium (U) the chemical properties of UAc and NdAc would be very similar in binding to tissue in ultrathin sections thus leading to a similar amount of contrast.

Biological role and precautions
The early lanthanides have been found to be essential to some methanotrophic bacteria living in volcanic mudpots, such as Methylacidiphilum fumariolicum: lanthanum, cerium, praseodymium, and neodymium are about equally effective. Neodymium is otherwise not known to have a biological role in any other organisms.Neodymium metal dust is combustible and therefore an explosion hazard. Neodymium compounds, as with all rare-earth metals, are of low to moderate toxicity; however, its toxicity has not been thoroughly investigated. Neodymium salts are regarded as more toxic if they are soluble than if they are insoluble if they are ingested. Neodymium dust and salts are very irritating to the eyes and mucous membranes, and moderately irritating to skin. Breathing the dust can cause lung embolisms, and accumulated exposure damages the liver. Neodymium also acts as an anticoagulant, especially when given intravenously.Neodymium magnets have been tested for medical uses such as magnetic braces and bone repair, but biocompatibility issues have prevented widespread application. Commercially available magnets made from neodymium are exceptionally strong and can attract each other from large distances. If not handled carefully, they come together very quickly and forcefully, causing injuries. For example, there is at least one documented case of a person losing a fingertip when two magnets he was using snapped together from 50 cm away.Another risk of these powerful magnets is that if more than one magnet is ingested, they can pinch soft tissues in the gastrointestinal tract. This has led to an estimated 1,700 emergency room visits and necessitated the recall of the Buckyballs line of toys, which were construction sets of small neodymium magnets.

See also
Neodymium compounds
Category:Neodymium compounds
Lanthanides
Period 6 elements
Rare earth metals

Notes
Passage 2:
James Chadwick
Sir James Chadwick,  (20 October 1891 – 24 July 1974) was a British physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atom bomb research efforts. He was the head of the British team that contributed to the Apollo Program during the Space Race. He was knighted in Britain in 1945 for his achievements in physics.
Chadwick graduated from the Victoria University of Manchester in 1911, where he studied under Ernest Rutherford (known as the "father of nuclear physics"). At Manchester, he continued to study under Rutherford until he was awarded his MSc in 1913. The same year, Chadwick was awarded an 1851 Research Fellowship from the Royal Commission for the Exhibition of 1851. He elected to study beta radiation under Hans Geiger in Berlin. Using Geiger's recently developed Geiger counter, Chadwick was able to demonstrate that beta radiation produced a continuous spectrum, and not discrete lines as had been thought. Still in Germany when World War I broke out in Europe, he spent the next four years in the Ruhleben internment camp.
After the war, Chadwick followed Rutherford to the Cavendish Laboratory at the University of Cambridge, where Chadwick earned his Doctor of Philosophy degree under Rutherford's supervision from Gonville and Caius College, Cambridge, in June 1921. He was Rutherford's assistant director of research at the Cavendish Laboratory for over a decade at a time when it was one of the world's foremost centres for the study of physics, attracting students like John Cockcroft, Norman Feather, and Mark Oliphant. Chadwick followed his discovery of the neutron by measuring its mass. He anticipated that neutrons would become a major weapon in the fight against cancer. Chadwick left the Cavendish Laboratory in 1935 to become a professor of physics at the University of Liverpool, where he overhauled an antiquated laboratory and, by installing a cyclotron, made it an important centre for the study of nuclear physics.
During the Second World War, Chadwick carried out research as part of the Tube Alloys project to build an atom bomb, while his Manchester lab and environs were harassed by Luftwaffe bombing. When the Quebec Agreement merged his project with the American Manhattan Project, he became part of the British Mission, and worked at the Los Alamos Laboratory and in Washington, D.C. He surprised everyone by earning the almost-complete trust of project director Leslie R. Groves, Jr. For his efforts, Chadwick received a knighthood in the New Year Honours on 1 January 1945. In July 1945, he viewed the Trinity nuclear test. After this, he served as the British scientific advisor to the United Nations Atomic Energy Commission. Uncomfortable with the trend toward Big Science, he became the Master of Gonville and Caius College in 1948. He retired in 1959.

Education and early life
James Chadwick was born in Bollington, Cheshire, on 20 October 1891, the first child of John Joseph Chadwick, a cotton spinner, and Anne Mary Knowles, a domestic servant. He was named James after his paternal grandfather. In 1895, his parents moved to Manchester, leaving him in the care of his maternal grandparents. He went to Bollington Cross Primary School, and was offered a scholarship to Manchester Grammar School, which his family had to turn down as they could not afford the small fees that still had to be paid. Instead he attended the Central Grammar School for Boys in Manchester, rejoining his parents there. He now had two younger brothers, Harry and Hubert; a sister had died in infancy. At the age of 16, he sat two examinations for university scholarships, and won both of them.Chadwick chose to attend Victoria University of Manchester, which he entered in 1908. He meant to study mathematics, but enrolled in physics by mistake. Like most students, he lived at home, walking the 4 miles (6.4 km) to the university and back each day. At the end of his first year, he was awarded a Heginbottom Scholarship to study physics. The physics department was headed by Ernest Rutherford, who assigned research projects to final-year students, and he instructed Chadwick to devise a means of comparing the amount of radioactive energy of two different sources. The idea was that they could be measured in terms of the activity of 1 gram (0.035 oz) of radium, a unit of measurement which would become known as the curie. Rutherford's suggested approach was unworkable—something Chadwick knew but was afraid to tell Rutherford—so Chadwick pressed on, and eventually devised the required method. The results became Chadwick's first paper, which, co-authored with Rutherford, was published in 1912. He graduated with first class honours in 1911.Having devised a means of measuring gamma radiation, Chadwick proceeded to measure the absorption of gamma rays by various gases and liquids. This time the resulting paper was published under his name alone. He was awarded his Master of Science (MSc) degree in 1912, and was appointed a Beyer Fellow. The following year he was awarded an 1851 Exhibition Scholarship, which allowed him to study and research at a university in continental Europe. He elected to go to the Physikalisch-Technische Reichsanstalt in Berlin in 1913, to study beta radiation under Hans Geiger. Using Geiger's recently developed Geiger counter, which provided more accuracy than the earlier photographic techniques, he was able to demonstrate that beta radiation did not produce discrete lines, as has been previously thought, but rather a continuous spectrum with peaks in certain regions. On a visit to Geiger's laboratory, Albert Einstein told Chadwick that: "I can explain either of these things, but I can't explain them both at the same time." The continuous spectrum would remain an unexplained phenomenon for many years.Chadwick was still in Germany at the start of the First World War, and was interned in the Ruhleben internment camp near Berlin, where he was allowed to set up a laboratory in the stables and conduct scientific experiments using improvised materials such as radioactive toothpaste. With the help of Charles Drummond Ellis, he worked on the ionisation of phosphorus, and the photochemical reaction of carbon monoxide and chlorine. He was released after the Armistice with Germany came into effect in November 1918, and returned to his parents' home in Manchester, where he wrote up his findings over the previous four years for the 1851 Exhibition commissioners.Rutherford gave Chadwick a part-time teaching position at Manchester, allowing him to continue research. He looked at the nuclear charge of platinum, silver, and copper, and experimentally found that this was the same as the atomic number within an error of less than 1.5 per cent. In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick joined him there a few months later. Chadwick was awarded a Clerk-Maxwell studentship in 1920, and enrolled as a Doctor of Philosophy (PhD) student at Gonville and Caius College, Cambridge. The first half of his thesis was his work with atomic numbers. In the second, he looked at the forces inside the nucleus. His degree was awarded in June 1921. In November, he became a Fellow of Gonville and Caius College.

Researcher
Cambridge
Chadwick's Clerk-Maxwell studentship expired in 1923, and he was succeeded by the Russian physicist Pyotr Kapitza. The Chairman of the Advisory Council of the Department of Scientific and Industrial Research, Sir William McCormick arranged for Chadwick to become Rutherford's assistant director of research. In this role, Chadwick helped Rutherford select PhD students. Over the next few years these would include John Cockcroft, Norman Feather and Mark Oliphant, who would become firm friends with Chadwick. As many students had no idea what they wanted to research, Rutherford and Chadwick would suggest topics. Chadwick edited all the papers produced by the laboratory.

In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. The two were married in August 1925, with Kapitza as Best Man. The couple had twin daughters, Joanna and Judith, who were born in February 1927.In his research, Chadwick continued to probe the nucleus. In 1925, the concept of spin had allowed physicists to explain the Zeeman effect, but it also created unexplained anomalies. At the time it was believed that the nucleus consisted of protons and electrons, so nitrogen's nucleus, for example, with a mass number of 14, was assumed to contain 14 protons and 7 electrons. This gave it the right mass and charge, but the wrong spin.At a conference at Cambridge on beta particles and gamma rays in 1928, Chadwick met Geiger again. Geiger had brought with him a new model of his Geiger counter, which had been improved by his post-doctoral student Walther Müller. Chadwick had not used one since the war, and the new Geiger–Müller counter was potentially a major improvement over the scintillation techniques then in use at Cambridge, which relied on the human eye for observation. The major drawback with it was that it detected alpha, beta and gamma radiation, and radium, which the Cavendish laboratory normally used in its experiments, emitted all three, and was therefore unsuitable for what Chadwick had in mind. However, polonium is an alpha emitter, and Lise Meitner sent Chadwick about 2 millicuries (about 0.5 μg) from Germany.In Germany, Walther Bothe and his student Herbert Becker had used polonium to bombard beryllium with alpha particles, producing an unusual form of radiation. Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate their results. To Chadwick, this was evidence of something that he and Rutherford had been hypothesising for years: the neutron, a theoretical nuclear particle with no electric charge. Then in January 1932, Feather drew Chadwick's attention to another surprising result. Frédéric and Irène Joliot-Curie had succeeded in knocking protons from paraffin wax using polonium and beryllium as a source for what they thought was gamma radiation. Rutherford and Chadwick disagreed; protons were too heavy for that. But neutrons would need only a small amount of energy to achieve the same effect. In Rome, Ettore Majorana came to the same conclusion: the Joliot-Curies had discovered the neutron but did not know it.

Chadwick dropped all his other responsibilities to concentrate on proving the existence of the neutron, assisted by Feather and frequently working late at night. He devised a simple apparatus that consisted of a cylinder containing a polonium source and beryllium target. The resulting radiation could then be directed at a material such as paraffin wax; the displaced particles, which were protons, would go into a small ionisation chamber where they could be detected with an oscilloscope.
In February 1932, after only about two weeks of experimentation with neutrons, Chadwick sent a letter to Nature titled "Possible Existence of a Neutron". He communicated his findings in detail in an article sent to Proceedings of the Royal Society A titled "The Existence of a Neutron" in May. His discovery of the neutron was a milestone in understanding the nucleus. Reading Chadwick's paper, Robert Bacher and Edward Condon realised that anomalies in the then-current theory, like the spin of nitrogen, would be resolved if the neutron has a spin of 1/2 and that a nitrogen nucleus consisted of seven protons and seven neutrons.The theoretical physicists Niels Bohr and Werner Heisenberg considered whether the neutron could be a fundamental nuclear particle like the proton and electron, rather than a proton–electron pair.  Heisenberg showed that the neutron was best described as a new nuclear particle, but its exact nature remained unclear.  In his 1933 Bakerian Lecture, Chadwick estimated that a neutron had a mass of about 1.0067 u. Since a proton and an electron had a combined mass of 1.0078 u, this implied the neutron as a proton–electron composite had a binding energy of about 2 MeV, which sounded reasonable, although it was hard to understand how a particle with so little binding energy could be stable.  Estimating such a small mass difference required challenging precise measurements, however, and several conflicting results were obtained in 1933–4.  By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained a large value for the mass of a neutron, but Ernest Lawrence's team at the University of California produced a small one.  Then Maurice Goldhaber, a refugee from Nazi Germany and a graduate student at the Cavendish Laboratory, suggested to Chadwick that deuterons could be photodisintegrated by the 2.6 MeV gamma rays of 208Tl (then known as thorium C"):

An accurate value for the mass of the neutron could be determined from this process. Chadwick and Goldhaber tried this and found that it worked.  They measured the kinetic energy of the proton produced as 1.05 MeV, leaving the mass of the neutron as the unknown in the equation.  Chadwick and Goldhaber calculated that it was either 1.0084 or 1.0090 atomic units, depending on the values used for the masses of the proton and deuteron.  (The modern accepted value for the mass of the neutron is 1.00866 u.) The mass of the neutron was too large to be a proton–electron pair.For his discovery of the neutron, Chadwick was awarded the Hughes Medal by the Royal Society in 1932, the Nobel Prize in Physics in 1935, the Copley Medal in 1950 and the Franklin Medal in 1951. His discovery of the neutron made it possible to produce elements heavier than uranium in the laboratory by the capture of slow neutrons followed by beta decay. Unlike the positively charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier, and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium. This inspired Enrico Fermi to investigate the nuclear reactions brought about by collisions of nuclei with slow neutrons, work for which Fermi would receive the Nobel Prize in 1938.Wolfgang Pauli proposed another kind of particle on 4 December 1930 in order to explain the continuous spectrum of beta radiation that Chadwick had reported in 1914. Since not all of the energy of beta radiation could be accounted for, the law of conservation of energy appeared to be violated, but Pauli argued that this could be redressed if another, undiscovered, particle was involved. Pauli also called this particle a neutron, but it was clearly not the same particle as Chadwick's neutron. Fermi renamed it the neutrino, Italian for "little neutron". In 1934, Fermi proposed his theory of beta decay which explained that the electrons emitted from the nucleus were created by the decay of a neutron into a proton, an electron, and a neutrino.  The neutrino could account for the missing energy, but a particle with little mass and no electric charge was difficult to observe. Rudolf Peierls and Hans Bethe calculated that neutrinos could easily pass through the Earth, so the chances of detecting them were slim. Frederick Reines and Clyde Cowan would confirm the neutrino on 14 June 1956 by placing a detector within a large antineutrino flux from a nearby nuclear reactor.

Liverpool
With the onset of the Great Depression in the United Kingdom, the government became more parsimonious with funding for science. At the same time, Lawrence's recent invention, the cyclotron, promised to revolutionise experimental nuclear physics, and Chadwick felt that the Cavendish laboratory would fall behind unless it also acquired one. He therefore chafed under Rutherford, who clung to the belief that good nuclear physics could still be done without large, expensive equipment, and turned down the request for a cyclotron.

Chadwick was himself a critic of Big Science in general, and Lawrence in particular, whose approach he considered careless and focused on technology at the expense of science. When Lawrence postulated the existence of a new and hitherto unknown particle that he claimed was a possible source of limitless energy at the Solvay Conference in 1933, Chadwick responded that the results were more likely attributable to contamination of the equipment. While Lawrence rechecked his results at Berkeley only to find that Chadwick was correct, Rutherford and Oliphant conducted an investigation at the Cavendish that found that deuterium fuses to form helium-3, thereby causing the effect that Lawrence had observed. This was another major discovery, but the Oliphant-Rutherford particle accelerator was an expensive state-of-the-art piece of equipment.In March 1935, Chadwick received an offer of the Lyon Jones Chair of physics at the University of Liverpool, in his wife's home town, to succeed Lionel Wilberforce. The laboratory was so antiquated that it still ran on direct current electricity, but Chadwick seized the opportunity, assuming the chair on 1 October 1935. The university's prestige was soon bolstered by Chadwick's Nobel Prize, which was announced in November 1935. His medal was sold at auction in 2014 for $329,000.Chadwick set about acquiring a cyclotron for Liverpool. He started by spending £700 to refurbish the antiquated laboratories at Liverpool, so some components could be made in-house. He was able to persuade the university to provide £2,000 and obtained a grant for another £2,000 from the Royal Society. To build his cyclotron, Chadwick brought in two young experts, Bernard Kinsey and Harold Walke, who had worked with Lawrence at the University of California. A local cable manufacturer donated the copper conductor for the coils. The cyclotron's 50-ton magnet was manufactured in Trafford Park by Metropolitan-Vickers, which also made the vacuum chamber. The cyclotron was completely installed and running in July 1939. The total cost of £5,184 was more than Chadwick had received from the University and the Royal Society, so Chadwick paid the rest from his 159,917 kr (£8,243) Nobel Prize money.At Liverpool the Medicine and Science faculties worked together closely. Chadwick was automatically a committee member of both faculties, and in 1938 he was appointed to a commission headed by Lord Derby to investigate the arrangements for cancer treatment in Liverpool. Chadwick anticipated that neutrons and radioactive isotopes produced with the 37-inch cyclotron could be used to study biochemical processes, and might become a weapon in the fight against cancer.

Second World War
Tube Alloys and the MAUD Report
In Germany, Otto Hahn and Fritz Strassmann bombarded uranium with neutrons, and noted that barium, a lighter element, was among the products produced. Hitherto, only the same or heavier elements had been produced by the process. In January 1939, Meitner and her nephew Otto Frisch astounded the physics community with a paper that explained this result. They theorised that uranium atoms bombarded with neutrons can break into two roughly equal fragments, a process they called fission. They calculated that this would result in the release of about 200 MeV, implying an energy release orders of magnitude greater than chemical reactions, and Frisch confirmed their theory experimentally. It was soon noted by Hahn that if neutrons were released during fission, then a chain reaction was possible. French scientists, Pierre Joliot, Hans von Halban and Lew Kowarski, soon verified that more than one neutron was indeed emitted per fission. In a paper co-authored with the American physicist John Wheeler, Bohr theorised that fission was more likely to occur in the uranium-235 isotope, which made up only 0.7 percent of natural uranium.

Chadwick did not believe that there was any likelihood of another war with Germany in 1939, and took his family for a holiday on a remote lake in northern Sweden. The news of the outbreak of the Second World War therefore came as a shock. Determined not to spend another war in an internment camp, Chadwick made his way to Stockholm as fast as he could, but when he arrived there with his family, he found that all air traffic between Stockholm and London had been suspended. They made their way back to England on a tramp steamer. When he reached Liverpool, Chadwick found Joseph Rotblat, a Polish post-doctoral fellow who had come to work with the cyclotron, was now destitute, as he was cut off from funds from Poland. Chadwick promptly hired Rotblat as a lecturer, despite his poor grasp of English.In October 1939, Chadwick received a letter from Sir Edward Appleton, the Secretary of the Department of Scientific and Industrial Research, asking for his opinion on the feasibility of an atom bomb. Chadwick responded cautiously. He did not dismiss the possibility, but carefully went over the many theoretical and practical difficulties involved. Chadwick decided to investigate the properties of uranium oxide further with Rotblat. In March 1940, Otto Frisch and Rudolf Peierls at the University of Birmingham re-examined the theoretical issues involved in a paper that became known as the Frisch–Peierls memorandum. Instead of looking at unenriched uranium oxide, they considered what would happen to a sphere of pure uranium-235, and found that not only could a chain reaction occur, but that it might require as little as 1 kilogram (2.2 lb) of uranium-235, and unleash the energy of tons of dynamite.

A special subcommittee of the Committee for the Scientific Survey of Air Warfare (CSSAW), known as the MAUD Committee, was created to investigate the matter further. It was chaired by Sir George Thomson and its original membership included Chadwick, along with Mark Oliphant, John Cockcroft and Philip Moon. While other teams investigated uranium enrichment techniques, Chadwick's team at Liverpool concentrated on determining the nuclear cross section of uranium-235. By April 1941, it had been experimentally confirmed that the critical mass of uranium-235 might be 8 kilograms (18 lb) or less. His research into such matters was complicated by all-but-incessant Luftwaffe bombings of the environs of his Liverpool lab; the windows were blown out so often that they were replaced by cardboard.In July 1941, Chadwick was chosen to write the final draft of the MAUD Report, which, when presented by Vannevar Bush to President Franklin D. Roosevelt in October 1941, inspired the U.S. government to pour millions of dollars into the pursuit of an atom bomb. When George B. Pegram and Harold Urey visited Britain to see how the project, now known as Tube Alloys, was going, Chadwick was able to tell them: "I wish I could tell you that the bomb is not going to work, but I am 90 per cent sure that it will."In a recent book about the Bomb project, Graham Farmelo wrote that "Chadwick did more than any other scientist to give Churchill the Bomb. ... Chadwick was tested almost to the breaking point." So worried that he could not sleep, Chadwick resorted to sleeping pills, which he continued to take for most of his remaining years. Chadwick later said that he realised that "a nuclear bomb was not only possible—it was inevitable. Sooner or later these ideas could not be peculiar to us. Everybody would think about them before long, and some country would put them into action". Sir Hermann Bondi suggested that it was fortunate that Chadwick, not Rutherford, was the doyen of UK physics at the time, as the latter's prestige might otherwise have overpowered Chadwick's interest in "looking forward" to the Bomb's prospects.

Manhattan Project
Owing to the danger from aerial bombardment, the Chadwicks sent their twins to Canada as part of a government evacuation scheme. Chadwick was reluctant to move Tube Alloys there, believing that the United Kingdom was a better location for the isotope separation plant. The enormous scope of the effort became more apparent in 1942: even a pilot separation plant would cost over £1 million and strain Britain's resources, to say nothing of a full-scale plant, which was estimated to cost somewhere in the vicinity of £25 million. It would have to be built in America. At the same time that the British became convinced that a joint project was necessary, the progress of the American Manhattan Project was such that British cooperation seemed less essential, although the Americans were still eager to utilise Chadwick's talents.The matter of cooperation had to be taken up at the highest level. In September 1943, the Prime Minister, Winston Churchill, and President Roosevelt negotiated the Quebec Agreement, which reinstated cooperation between Britain, the United States and Canada. Chadwick, Oliphant, Peierls and Simon were summoned to the United States by the director of Tube Alloys, Sir Wallace Akers, to work with the Manhattan Project. The Quebec Agreement established a new Combined Policy Committee to direct the joint project. The Americans disliked Akers, so Chadwick was appointed technical advisor to the Combined Policy Committee, and the head of the British Mission.Leaving Rotblat in charge in Liverpool, Chadwick began a tour of the Manhattan Project facilities in November 1943, except for the Hanford Site where plutonium was produced, which he was not allowed to see. He became the only man apart from Groves and his second in command to have access to all the American research and production facilities for the uranium bomb. Observing the work on the K-25 gaseous diffusion facility at Oak Ridge, Tennessee, Chadwick realised how wrong he had been about building the plant in wartime Britain. The enormous structure could never have been concealed from the Luftwaffe. In early 1944, he moved to Los Alamos, New Mexico, with his wife and their twins, who now spoke with Canadian accents. For security reasons, he was given the cover name of James Chaffee.

Chadwick accepted that the Americans did not need British help, but that it could still be useful in bringing the project to an early and successful conclusion. Working closely with the director of the Manhattan Project, Major General Leslie R. Groves, Jr., he attempted to do everything he could to support the effort. He also aimed to involve British scientists in various aspects of the Apollo Program to facilitate a post-space era British aerospace initiative to which Chadwick was committed. Requests from Groves via Chadwick for particular scientists tended to be met with an immediate rejection by the company, ministry or university currently employing them, only to be overcome by the overriding priority accorded to Tube Alloys. As a result, the British team was critical to the Project's success.Although he had more knowledge of the project than anyone else from Britain, Chadwick had no access to the Hanford site. Lord Portal was offered a tour of Hanford in 1946. "This was the only plant to which Chadwick had been denied access in wartime, and now he asked Groves if he could accompany Portal. Groves replied that he could, but if he did then 'Portal will not see very much'." For his efforts, Chadwick received a knighthood in the New Year Honours on 1 January 1945. He considered this to be a recognition of the work of the whole Tube Alloys project.By early 1945, Chadwick was spending most of his time in Washington, D.C., and his family relocated from Los Alamos to a house on Washington's Dupont Circle in April 1945. He was present at the meeting of the Combined Policy Committee on 4 July when Field Marshal Sir Henry Maitland Wilson gave Britain's agreement to use the atom bomb against Japan, and at the Trinity nuclear test on 16 July, when the first atom bomb was detonated. Inside its pit was a polonium-beryllium modulated neutron initiator, a development of the technique that Chadwick had used to discover the neutron over a decade before. William L. Laurence, The New York Times reporter attached to the Manhattan Project, wrote that "never before in history had any man lived to see his own discovery materialize itself with such telling effect on the destiny of man."

Later life
Shortly after the war ended, Chadwick was appointed to the Advisory Committee on Atomic Energy (ACAE). He was also appointed as the British scientific advisor to the United Nations Atomic Energy Commission. He clashed with fellow ACAE member Patrick Blackett, who disagreed with Chadwick's conviction that Britain needed to acquire its own nuclear weapons; but it was Chadwick's position that was ultimately adopted. He returned to Britain in 1946, to find a country still beset by wartime rationing and shortages.At this time, Sir James Mountford, the Vice Chancellor of the University of Liverpool, wrote in his diary "he had never seen a man 'so physically, mentally and spiritually tired" as Chadwick, for he "had plumbed such depths of moral decision as more fortunate men are never called upon even to peer into ... [and suffered] ... almost insupportable agonies of responsibility arising from his scientific work'."In 1948, Chadwick accepted an offer to become the Master of Gonville and Caius College. The job was prestigious but ill-defined; the Master was the titular head of the College, but authority actually resided in a council of 13 fellows, of whom one was the Master. As Master, Chadwick strove to improve the academic reputation of the college. He increased the number of research fellowships from 31 to 49, and sought to bring talent into the college. This involved controversial decisions, such as hiring in 1951 the Chinese biochemist Tien-chin Tsao and the Hungarian-born economist Peter Bauer. In what became known as the Peasants' Revolt, fellows led by Patrick Hadley voted an old friend of Chadwick's off the council and replaced him with Bauer. More friends of Chadwick's were removed over the following years, and he retired in November 1958. It was during his mastership that Francis Crick, a PhD student at Gonville and Caius College, and James Watson discovered the structure of DNA.Over the years, Chadwick received many honours, including the Medal for Merit from the United States, and the Pour le Mérite from Germany. He was elected a Fellow of the Royal Society in 1927. He became a foreign member of the Royal Netherlands Academy of Arts and Sciences in 1946 and an International member of the American Philosophical Society in 1948. He was made a Companion of Honour in the New Year Honours on 1 January 1970 for "services to science", and went to Buckingham Palace for the investiture ceremony. He became more frail, and seldom left his flat, although he travelled to Liverpool for celebrations of his eightieth birthday.  A lifelong atheist, he saw no reason to adopt religious faith in later life. He died in his sleep on 24 July 1974.His papers are held at the Churchill Archives Centre in Cambridge, and are accessible to the public. The Chadwick Laboratory at the University of Liverpool is named after him, as is its Sir James Chadwick Chair of Experimental Physics, which was named after him in 1991 as part of celebrations of the centenary of his birth. A crater on the moon is also named after him. The James Chadwick Building, which houses part of the School of Chemical Engineering and Analytical Sciences, University of Manchester is named in his honour. He was described by the United Kingdom Atomic Energy Authority official historian Lorna Arnold as "a physicist, a scientist-diplomat, and a good, wise, and humane man."

Notes
Passage 3:
Synthetic element
A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; thus, they are called "synthetic", "artificial", or "man-made". The synthetic elements are those with atomic numbers 95–118, as shown in purple on the accompanying periodic table: these 24 elements were first created between 1944 and 2010. The mechanism for the creation of a synthetic element is to force additional protons into the nucleus of an element with an atomic number lower than 95. All known (see: Island of stability) synthetic elements are unstable, but they decay at widely varying rates: the half-lives of their longest-lived isotopes range from microseconds to millions of years.
Five more elements that were first created artificially are strictly speaking not synthetic because they were later found in nature in trace quantities: 43Tc, 61Pm, 85At, 93Np, and 94Pu, though are sometimes classified as synthetic alongside exclusively artificial elements. The first, technetium, was created in 1937. Plutonium (Pu, atomic number 94), first synthesized in 1940, is another such element. It is the element with the largest number of protons (atomic number) to occur in nature, but it does so in such tiny quantities that it is far more practical to synthesize it. Plutonium is known mainly for its use in atomic bombs and nuclear reactors.No elements with atomic numbers greater than 99 have any uses outside of scientific research, since they have extremely short half-lives, and thus have never been produced in large quantities.

Properties
All elements with atomic number greater than 94 decay quickly enough into lighter elements such that any atoms of these that may have existed when the Earth formed (about 4.6 billion years ago) have long since decayed. Synthetic elements now present on Earth are the product of atomic bombs or experiments that involve nuclear reactors or particle accelerators, via nuclear fusion or neutron absorption.Atomic mass for natural elements is based on weighted average abundance of natural isotopes in Earth's crust and atmosphere. For synthetic elements, there is no "natural isotope abundance". Therefore, for synthetic elements the total nucleon count (protons plus neutrons) of the most stable isotope, i.e., the isotope with the longest half-life—is listed in brackets as the atomic mass.

History
Technetium
The first element to be synthesized, rather than discovered in nature, was technetium in 1937. This discovery filled a gap in the periodic table, and the fact that technetium has no stable isotopes explains its natural absence on Earth (and the gap). With the longest-lived isotope of technetium, 97Tc, having a 4.21-million-year half-life, no technetium remains from the formation of the Earth. Only minute traces of technetium occur naturally in Earth's crust—as a product of spontaneous fission of 238U, or from neutron capture in molybdenum—but technetium is present naturally in red giant stars.

Curium
The first entirely synthetic element to be made was curium, synthesized in 1944 by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso by bombarding plutonium with alpha particles.

Eight others
Synthesis of americium, berkelium, and californium followed soon. Einsteinium and fermium were discovered by a team of scientists led by Albert Ghiorso in 1952 while studying the composition of radioactive debris from the detonation of the first hydrogen bomb. The isotopes synthesized were einsteinium-253, with a half-life of 20.5 days, and fermium-255, with a half-life of about 20 hours. The creation of mendelevium, nobelium, and lawrencium followed.

Rutherfordium and dubnium
During the height of the Cold War, teams from the Soviet Union and the United States independently created rutherfordium and dubnium. The naming and credit for synthesis of these elements remained unresolved for many years, but eventually shared credit was recognized by IUPAC/IUPAP in 1992. In 1997, IUPAC decided to give dubnium its current name honoring the city of Dubna where the Russian team worked since American-chosen names had already been used for many existing synthetic elements, while the name rutherfordium (chosen by the American team) was accepted for element 104.

The last thirteen
Meanwhile, the American team had created seaborgium, and the next six elements had been created by a German team: bohrium, hassium, meitnerium, darmstadtium, roentgenium, and copernicium. Element 113, nihonium, was created by a Japanese team; the last five known elements, flerovium, moscovium, livermorium, tennessine, and oganesson, were created by Russian–American collaborations and complete the seventh row of the periodic table.

List of synthetic elements
The following elements do not occur naturally on Earth. All are transuranium elements and have atomic numbers of 95 and higher.

Other elements usually produced through synthesis
All elements with atomic numbers 1 through 94 occur naturally at least in trace quantities, but the following elements are often produced through synthesis: technetium, promethium, astatine, neptunium, and plutonium, which were discovered through synthesis before being found in nature.
Passage 4:
Atomic number
The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons.
For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.
The conventional symbol Z comes from the German word Zahl 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.
The rules above do not always apply to exotic atoms which contain short-lived elementary particles other than protons, neutrons and electrons.

History
The periodic table and a natural number for each element
Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and so they can be numbered in order.
Dmitri Mendeleev claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of atomic weight ("Atomgewicht"). However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time.

A simple numbering based on periodic table position was never entirely satisfactory, however. Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time).

The Rutherford-Bohr model and van den Broek
In 1911, Ernest Rutherford gave a model of the atom in which a central nucleus held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms. This central charge would thus be approximately half the atomic weight (though it was almost 25% different from the atomic number of gold (Z = 79, A = 197), the single element from which Rutherford made his guess). Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also known as element number, atomic number, and symbolized Z). This proved eventually to be the case.

Moseley's 1913 experiment
The experimental position improved dramatically after research by Henry Moseley in 1913. Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of Z.
To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminium (Z = 13) to gold (Z = 79) used as a series of movable anodic targets inside an x-ray tube. The square root of the frequency of these photons (x-rays) increased from one target to the next in an arithmetic progression. This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.

Missing elements
After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen to uranium (Z = 92) were examined by his method. There were seven elements (with Z < 92) which were not found and therefore identified as still undiscovered, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, all seven of these missing elements were discovered. By this time, the first four transuranium elements had also been discovered, so that the periodic table was complete with no gaps as far as curium (Z = 96).

The proton and the idea of nuclear electrons
In 1915, the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood. An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one. However, as early as 1907, Rutherford and Thomas Royds had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times. If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms.
In 1917, Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas, and believed he had proven Prout's law. He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles). It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei. A helium nucleus was presumed to be composed of four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two of the charges. At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of +79, consistent with its atomic number.

The neutron's discovery was crucial for understanding atomic structure, but it was during the Apollo Program that the unique proton count was explored. An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive nuclear charge now was realized to come entirely from a content of 79 protons. While Moseley had earlier indicated that atomic number Z was tied to positive charge, the Apollo Program revealed Z as the specific proton count of an atom's nucleus.

Chemical properties
Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z (the atomic number). The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. Hence, it is the atomic number alone that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of any mixture of atoms with a given atomic number.

New elements
The quest for new elements is usually described using atomic numbers. As of 2023, all elements with atomic numbers 1 to 118 have been observed. Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created. In general, the half-life of a nuclide becomes shorter as atomic number increases, though undiscovered nuclides with certain "magic" numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability.
A hypothetical element composed only of neutrons has also been proposed and would have atomic number 0.

See also

Answer: Apollo Program