Neptunium

Neptunium

Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element, and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production, and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.

Occurrence

The most stable isotope of neptunium is 237Np, with a half-life of two million years. Thus, all primordial neptunium should have decayed by now. However, trace amounts of the neptunium isotopes neptunium-237 through neptunium-240, are found naturally as decay products from transmutation reactions in uranium ores.

Artificial 237Np is produced through a reaction of 237NpF3 with liquid barium or lithium at around 1200 °C and is most often extracted from spent nuclear fuel rods in kilogram amounts 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. However, even this fraction still amounts to more than fifty tons per year.

Symbol		Np
Atomic Number		93
Atomic Weight		237.048
Oxidation States		+3, +4, +5, +6
Electronegativity, Pauling		1.28
State at RT		Solid, Metal
Melting Point, K		913
Boiling Point, K		4175

Appearance and Characteristics

Harmful effects:

Neptunium is harmful due to its radioactivity.

Characteristics:

• Neptunium is a silvery radioactive synthetic metal. (Miniscule quantities of neptunium-237 and neptunium-239 are found in nature as a result of beta decay of uranium.)
• Neptunium exists in three allotropes: it has an orthorhombic structure at normal temperatures, a tetragonal structure above 280oC and a cubic structure above 577oC. 
• Neptunium has five oxidation states (+3 to +7) producing different colors in solution:

III:   Np3+ (violet)

IV:   Np4+ (yellow green)

V:   NpO2+ in acidic solution (green) and in alkaline solution (yellow)

VI:   NpO22+ (pink red)

VII:  Np(VII) in alkaline solution (green) or possibly in acidic conditions (brownish-red). (3)

Uses of Neptunium

• Neptunium is used mainly for research purposes.
• When bombarded with neutrons neptunium-237 is used to produce plutonium-238 which is used for spacecraft generators and terrestrial navigation beacons.
• Neptunium is also used in neutron detection equipment.