• Element 93 Neptunium Np, 1940
• Chemist Edwin Mattison McMillan, 1907
• Physicist Philip Hauge Abelson, 1913

The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also a publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.

At least three times, discoveries of the element 93 were falsely reported, as bohemium and ausonium in 1934 and then sequanium in 1939. The name neptunium has previously been considered for other elements.

The search for element 93 in minerals was encumbered by the fact that the predictions on the chemical properties of element 93 were based on a periodic table which lacked the actinides series and therefore placed thorium below hafnium, protactinium below tantalum and uranium below tungsten. This periodic table suggested that element 93, at that point often named eka - rhenium, should be similar to manganese or rhenium. With this misconception it was impossible to isolate element 93 from minerals although later neptunium was found in uranium ore in 1952.

Enrico Fermi believed that bombarding uranium with neutrons and subsequent beta decay would lead to the formation of element 93. Chemical separation of the new formed elements from the uranium yielded material with low half-life and therefore Fermi announced the discovery of a new element in 1934, though this was soon found to be mistaken. Soon it was speculated and later proven that most of the material is created by nuclear fission of uranium by neutrons. Small quantities of neptunium had to be produced in Otto Hahn's experiments in late 1930s as a result of decay of ²³⁹U. Hahn and his colleagues experimentally confirmed production and chemical properties of ²³⁹U, but were unsuccessful at isolating and detecting neptunium.

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was discovered by Edwin McMillan and Philip H. Abelson in 1940 at the Berkeley Radiation Laboratory of the University of California, Berkeley. The team produced the neptunium isotope ²³⁹Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. Artificial ²³⁷Np is produced through the reduction of ²³⁷NpF₃ with barium or lithium vapor at around 1200 °C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.

2 NpF₃ + 3 Ba → 2 Np + 3 BaF₂

By weight, neptunium - 237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges.

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three allotropes:

• α-neptunium, orthorhombic, density 20.45 g/cm³
• β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm³
• γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm³

Neptunium has the largest liquid range of any element, 3363 K, between the melting point and boiling point. It is the densest element of all actinoids.

19 neptunium radioisotopes have been characterized, with the most stable being ²³⁷Np with a half-life of 2.14 million years, ²³⁶Np with a half-life of 154,000 years, and ²³⁵Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being ^236mNp (t_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (²²⁵Np) to 244.068 u (²⁴⁴Np). The primary decay mode before the most stable isotope, ²³⁷Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

²³⁷Np is fissionable. ²³⁷Np eventually decays to form bismuth - 209 and thallium - 205, unlike most other common heavy nuclei which decay to make isotopes of lead. This decay chain is known as the neptunium series.

Chemically, neptunium is prepared by the reduction of NpF₃ with barium or lithium vapor at about 1200 °C. Most Np is produced in nuclear reactions:

• When an ²³⁵U atom captures a neutron, it is converted to an excited state of ²³⁶U. About 81% of the excited ²³⁶U nuclei undergo fission, but the remainder decay to the ground state of ²³⁶U by emitting gamma radiation. Further neutron capture creates ²³⁷U which has a half-life of 7 days and thus quickly decays to ²³⁷Np through beta decay. During beta decay, the excited ²³⁷U emits an electron, while the atomic weak interaction converts a neutron to a proton, thus creating ²³⁷Np.

• ²³⁷U is also produced via an (n,2n) reaction with ²³⁸U. This only happens with very energetic neutrons.
• ²³⁷Np is the product of alpha decay of ²⁴¹Am.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure ²³⁷Np.

This element has four ionic oxidation states while in solution:

• Np³⁺ (pale purple), analogous to the rare earth ion Pm³⁺
• Np⁴⁺ (yellow green)
• NpO₂⁺ (green blue)
• NpO₂²⁺ (pale pink)

Neptunium (III) hydroxide is not soluble in water and does not dissolve in excess alkali. Neptunium (III) is susceptible to oxidation in contact to air forming neptunium (IV).

Neptunium forms tri- and tetrahalides such as NpF₃, NpF₄, NpCl₄, NpBr₃, NpI₃, and oxides of the various compositions such as are found in the uranium - oxygen system, including Np₃O₈ and NpO₂.

Neptunium hexafluoride, NpF₆, is volatile like uranium hexafluoride.

Neptunium, like protactinium, uranium, plutonium, and americium readily forms a linear dioxo neptunyl core (NpO₂ⁿ⁺), in its 5+ and 6+ oxidation states, which readily complexes with hard O-donor ligands such as OH^–, NO₂^–, NO₃^–, and SO₄^2– to form soluble anionic complexes which tend to be readily mobile with low affinities to soil.

• NpO₂(OH)₂^–
• NpO₂(CO₃)^–
• NpO₂(CO₃)₂^3–
• NpO₂(CO₃)₃^5–

²³⁷Np is irradiated with neutrons to create ²³⁸Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. ²³⁷Np will capture a neutron to form ²³⁸Np and beta decay with a half-life of two days to ²³⁸Pu.

²³⁸Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that neptunium - 237 "can be used for a nuclear explosive device". It is not believed that an actual weapon has ever been constructed using neptunium. As of 2009, the world production of neptunium - 237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

In September 2002, researchers at the University of California's Los Alamos National Laboratory briefly created the first known nuclear critical mass using neptunium in combination with shells of enriched uranium (U-235), discovering that the critical mass of a bare sphere of neptunium - 237 "ranges from kilogram weights in the high fifties to low sixties," showing that it "is about as good a bomb material as U-235." The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear waste disposal site in Nevada.

²³⁷Np is used in devices for detecting high energy (MeV) neutrons.

Neptunium-237 is the most mobile actinide in the deep geological repository environment. This makes it and its predecessors such as americium - 241 candidates of interest for destruction by nuclear transmutation. Neptunium accumulates in commercial household ionization - chamber smoke detectors from decay of the (typically) 0.2 microgram of americium - 241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium - 241 in a smoke detector includes about 3% neptunium after 20 years, and about 15% after 100 years.

Due to its long half-life neptunium becomes the major contributor of the total radiation in 10,000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.

McMillan was born in Redondo Beach, California, but his family moved to Pasadena the following year. He attended some of the public lectures at the California Institute of Technology as a high school student and began his studies there in 1924. He did a research project with Linus Pauling as an undergraduate and received his Bachelor of Science degree in 1928 and his Master of Science degree in 1929, both from the California Institute of Technology.

He then took his Doctor of Philosophy from Princeton University in 1932 for the thesis: "Deflection of a Beam of HCI Molecules in a Non - Homogeneous Electric Field" under the supervision of Edward Condon.

He joined the group of Ernest Lawrence at the University of California, Berkeley, upon receiving his doctorate, moving to the Berkeley Radiation Laboratory when it was founded at Berkeley in 1934.

His experimental skills lead to the discovery of oxygen - 15 with M. Stanley Livingston and beryllium - 10 with Samuel Ruben

In 1940 he and Philip Abelson created neptunium, while conducting a fission experiment of uranium - 239 with neutrons, using the cyclotron at Berkeley. The newly found isotope of neptunium was created by absorption of neutron into the uranium - 239 and a subsequent beta decay. McMillan understood the underlying principle of the reaction and started to bombard the uranium - 239 with deuterium to create the element 94. He moved to the radar research at the Massachusetts Institute of Technology and Glenn T. Seaborg finished the work.

In World War II, he was involved in research on radar at MIT in Cambridge, Massachusetts, sonar near San Diego, and about November 1942 was recruited to the Manhattan Project at the Los Alamos Laboratory, being involved in the initial selection of Los Alamos and in implosion research.

After World War II, he rejoined the Berkeley Radiation Laboratory, and became head of the institute after the death of Ernest Lawrence in 1958.

In 1945 he developed ideas for the improvement of the cyclotron, leading to the development of the synchrotron. The synchrotron was used to create new elements at Berkeley Radiation Laboratory extending the periodic system of elements far beyond the 92 elements known before 1940.

With Glenn T. Seaborg, he shared the Nobel Prize in Chemistry in 1951 for "discoveries in the chemistry of the transuranium elements." This medal is currently held at the National Museum of American History, a division of The Smithsonian.

In 1946, he became a full professor at Berkeley, and in 1954 he was appointed associate director of the Lawrence Radiation Laboratory, being promoted to director in 1958, where he stayed until his retirement in 1973.

He was elected to the National Academy of Sciences in 1947, serving as its chairman from 1968 to 1971. He received the Atoms for Peace Award in 1963.

Abelson was born in 1913 in Tacoma, Washington. He attended Washington State University where he received degrees in chemistry and physics, and the University of California, Berkeley (UC Berkeley), where he earned his PhD in nuclear physics. As a young physicist, he worked for Ernest Lawrence at the UC Berkeley. He was among the first American scientists to verify nuclear fission in an article submitted to the Physical Review in February 1939. In addition, he collaborated with the Nobel Prize laureate Luis Alvarez in early nuclear research, and was the co-discoverer of neptunium on 8 June 1940 with Edwin McMillan. McMillan was awarded the Nobel Prize for this discovery amongst other elements.

Abelson was a key contributor to the Manhattan Project during World War II. Although he was not formally associated with the atom bomb project, the liquid thermal diffusion isotope separation technique that he invented at the Philadelphia Navy Yard was used in the S-50 plant in Oak Ridge, Tennessee; and proved a critical step in creating the large amount of nuclear fuel required for building atomic bombs.

After the war, he turned his attention under the guidance of Ross Gunn to applying nuclear power to naval propulsion. While not written at an engineering - design level, he wrote the first physics report detailing how a nuclear reactor could be installed in a submarine, providing both propulsion and electrical power. His report anticipated the nuclear submarine's role as a missile platform. This concept was later supported by Admiral Hyman G. Rickover and others. Under Rickover, the concept became reality in the form of USS Nautilus, the world's first nuclear submarine.

From 1951 until 1971 he served as the director of the Carnegie Institution of Washington's Geophysical Laboratory, and as president from 1971 to 1978. From 1962 to 1984 he was editor of Science, one of the most prestigious academic journals, and served as its acting executive officer in 1974, 1975 and 1984. From 1972 until 1974 he served as the president of the American Geophysical Union.

Abelson was outspoken and well known for his opinions on science. In a 1964 editorial published in Science magazine, Abelson identified over - specialization in science as a form of bigotry. He outlined his view that the pressure towards specialization beginning in undergraduate study and intensifying in PhD programs has the effect on students of leading them to believe that their area of specialization is the most important, even to the extreme of considering other intellectual pursuits to be worthless. He reasoned that such over - specialization led to obsolescence of one's work, often through a focus on trivial aspects of a field, and that avoidance of such bigotry was essential to guiding the direction of one's work.

In a 1965 article he described his work in paleobiology and reported evidence of amino acids recovered from fossils hundreds of millions of years in age. He estimated that based on his experiments alanine would be stable for billions of years.

Perhaps his most famous work from this time period is an editorial entitled "Enough of Pessimism" ("enough of pessimism, it only leads to paralysis and decay"). This became the title of a 100 essay collection.

During the 1970s he became interested in the problem of world energy supplies. Books on the topic include Energy for Tomorrow (1975), from a series of lectures at the University of Washington, and Energy II: Use Conservation and Supply. He pointed out the possibilities of mining the Athabascan tar sands, as well as oil shale in the Colorado Rockies. In addition he urged conservation and a change of attitude towards public transit.

After 1984, he remained associated with the magazine. Some have claimed him to be an early skeptic of the case for global warming on the basis of a lead editorial in the magazine dated March 31, 1990 in which he wrote, "[I]f the global warming situation is analyzed applying the customary standards of scientific inquiry one must conclude that there has been more hype than solid fact." However, in 1977 in a US National Research Council, Energy and Environment report he wrote,

What is important is not that there are differences [in the models] but that the span of agreement embraces a fourfold to eightfold increase in atmospheric carbon dioxide in the latter part of the twenty - second century. Our best understanding of the relation between an increase in carbon dioxide in the atmosphere and change in global temperature suggests a corresponding increase in average world temperature of more than 6°C, with polar temperature increases of as much as three times this figure. This would exceed by far the temperature fluctuations of the past several thousand years and would very likely, along the way, have a highly significant impact on global precipitation.

—Philip H. Abelson, Thomas F. Malone, Cochairmen, Geophysics Study Committee

Abelson died on August 1, 2004 from respiratory complications following a brief illness. He was married to Neva Abelson, a distinguished research physician who co-discovered the Rh blood factor test (with L.K. Diamond). Their daughter, Ellen Abelson Cherniavsky, worked as an aviation researcher for the MITRE corporation in Virginia.

Abelson received many distinguished awards, including the National Medal of Science in 1987, the National Science Foundation's Distinguished Achievement Award, the American Medical Association's Scientific Achievement Award, the Distinguished Civilian Service Medal and the Waldo E. Smith Medal in 1988. In 1992 he was awarded the Public Welfare Medal, the National Academy of Sciences's highest honor. He was elected a Fellow of the American Academy of Arts and Sciences in 1958.