Neptunium (Np)

• Element Symbol: Np
• Atomic Number: 93
• Atomic Mass: 237
• Group # in Periodic Table: n/a
• Group Name: Actinides
• Period in Periodic Table: 7
• Block of Periodic Table: f-block
• Discovered by: Edwin M. McMillan, Philip H. Abelson (1940)

Neptunium is a synthetic actinide metal. The actinide metals are the fifteen elements with atomic numbers from 89 to 103. Neptunium’s symbol is Np, and its atomic number is 93. It is the first transuranic element. These unstable, or radioactive, elements are all those with atomic numbers greater than 92, the atomic number of uranium. Neptunium metal is silvery and rusts easily when exposed to air. It is also pyrophoric, which means that it ignites spontaneously (without a catalyst’s help) when exposed to moisture. Due to these properties, neptunium actually has a few useful applications.

Neptunium was the first synthetic transuranic element of the actinide series to be discovered. It was first produced in 1940 by Edwin McMillan and Philip H. Abelson. This milestone took place at the Berkeley Radiation Laboratory of the University of California. These two research scientists discovered neptunium by bombarding uranium-238 with neutrons. As a result, they were able to show chemically that they had produced neptunium-239, which has a half-life of just 2.3 days.

A longer-lived isotope, neptunium-237, was discovered in 1942. Scientists A. C. Wahl and Glenn T. Seaborg bombarded uranium-238 with fast neutrons using the Berkeley sixty-inch cyclotron. As a result, they were able to isolate several hundred milligrams of neptunium. These two scientists then went on to make a thorough study of its properties.

Element 93 is named after the planet Neptune, the solar system’s eighth planet. The name "neptunium" continued the naming convention established by Martin Klaproth when he named uranium after the planet Uranus, the seventh planet. This theme then continued with plutonium, which follows neptunium in the actinide series. When plutonium was named, Pluto was considered to be the ninth planet in the solar system, the one after Neptune. (In 2006 Pluto was downgraded to a dwarf planet by the International Astronomical Union, or IAU.)

Physical Properties

Neptunium is a hard, silvery, ductile, radioactive actinide metal. The word "ductile" means that a metal can be stretched into a wire. "Radioactive" means that an element is unstable chemically and gives off energy in the form of radiation. Neptunium also ignites spontaneously and tarnishes readily when exposed to humid air or water. The first property, spontaneous ignition, is referred to as pyrophoricity. This comes from the Greek word pyrophoros, which means "fire-bearing." In essence any pyrophoric substance gives off sparks without help from another substance when it is exposed to moisture. As for the second property, tarnishing is scientifically known as oxidation, which means that a substance rusts when it is exposed to air.

Unlike the other actinide elements, neptunium’s physical properties are in fact known. Its standard state at 298 kelvins (K) is solid. Neptunium metal is dense, at 20.2 grams per cubic centimeter (g/cm³), and heavy, with an atomic mass of 237). Its melting point, 640 degrees Celsius (°C), is comparatively low. Conversely, neptunium’s boiling point, 3900 °C, is exceptionally high. The distance between these two temperatures means that neptunium has the largest liquid range of any element.

As for its specific heat, electrical conductivity, and thermal conductivity, these physical properties are unknown and are being investigated by theoretical chemists and physicists.

Chemical Properties

Neptunium has five oxidation states, +3 through +7, that produce different colors when in solution. Specifically, neptunium(III), Np³⁺, is violet; neptunium(IV), Np⁴⁺, is yellow-green; neptunium(V), NpO₂⁺, is green when mixed with acid but yellow when mixed with an alkaline base; neptunium(VI), NpO₂²⁺, is pink-red; and finally, neptunium(VII), NpO₃⁺, is brownish-red in acidic solutions but green in basic solutions. The most stable of these five oxidation states is neptunium(V), but neptunium(IV) is preferred in solid neptunium compounds. Since it has the most oxidation states, neptunium is the heaviest actinide that can lose all of its valence electrons in a stable compound. These facts mean that neptunium metal is extremely reactive chemically.

Due to the fact that neptunium is so chemically reactive and radioactive, it has absolutely no stable isotopes. It does, however, have twenty known radioactive isotopes with known half-lives. These twenty radioisotopes have atomic mass numbers 225 through 244. Neptunium’s longest-lived isotopes are neptunium-237 (2.14 million years), neptunium-236 (154,000 years), and neptunium-235 (396 days). All remaining isotopes have half-lives that are less than 4.5 days, with the majority lasting less than fifty minutes. This means that neptunium radioisotopes have one of the largest half-life ranges of any element.

Applications

Neptunium does not really occur in nature, except in trace amounts in uranium ores. Instead, this transuranic element is usually synthesized in a nuclear physics lab. Neptunium is obtained as a by-product from nuclear reactors. It is extracted from the spent uranium fuel rods. Specifically, neptunium-237 is produced in kilogram quantities from these rods. Also, neptunium-238 is a by-product of plutonium-238 synthesis.

Due to its various chemical and physical properties, neptunium actually has a few useful applications. It is primarily used for research purposes in theoretical chemistry and nuclear physics labs. However, when bombarded with neutrons (charged particles), neptunium-237 is used to produce plutonium-238. Plutonium-238 is used in spacecraft generators and terrestrial navigation beacons. Neptunium has also been used in neutron detection equipment. Since it is radioactive and therefore highly toxic, this transuranic element does not have any biological uses.

Bibliography

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