Atomic number 93

Neptunium

Np
+ phase
− phase

Shells Orbitals 5f 6d 7s

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93 electrons in 7 shells: 2, 8, 18, 32, 22, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 237.048172 Da (longest-lived isotope)
Electron configuration: [Rn] 5f⁴ 6d¹ 7s²
Phase at 25 °C: solid
Melting point: 644 °C
Boiling point: 4174 °C
Density: 20.25 g/cm³
Electronegativity (Pauling): 1.36
Discovered: 1940, Edwin McMillan and Philip Abelson

Neighbours in the table

U Pu

A neptunium solution changes colour with the metal’s oxidation state, a bookkeeping charge that assumes ionic bonds. Los Alamos lists five states in water, from +3 to +7. Np³⁺ is dark blue-purple and Np⁴⁺ grass green. At +5 neptunium exists as NpO₂⁺, emerald green and the most stable of the five, while NpO₂²⁺ at +6 is light burgundy. The +7 state survives only in strongly basic solution, where it turns dark green.

Only the +3 and +4 ions are bare metal ions. At higher charges, Los Alamos explains, the charge is spread over a larger cation carrying oxygen. Those neptunyl ions bind readily to hydroxide, nitrate, sulfate and carbonate, forming soluble complexes. Rare-earth ions in water, by comparison, stay simple.

The 93 electrons are arranged as [Rn] 5f⁴ 6d¹ 7s². The metal is silvery and chemically reactive, with at least three allotropes, crystal forms of one element. Alpha-neptunium, stable at room temperature, is orthorhombic, a cell with three unequal axes at right angles, whereas most metals crystallise in body- or face-centred cubic forms, Chem1 notes. Above 280 °C it becomes tetragonal beta-neptunium, and above 577 °C cubic gamma-neptunium. PubChem gives its density as 20.25 g/cm³ and Los Alamos 20.45 g/cm³ for the alpha form; Los Alamos calls neptunium the densest actinide.

Where it is found

Any neptunium present when the Earth formed has decayed away. PubChem gives the mass of its longest-lived isotope, 237.048 Da. That isotope, neptunium-237, halves in 2.14 million years according to Los Alamos, far too short a span to have lasted since then. The International Atomic Energy Agency (IAEA) evaluation gives 2.144 million years. In that evaluation alpha emission accounts for 100 per cent of decays and spontaneous fission, the nucleus splitting unprompted, for 2 × 10⁻¹⁰ per cent.

Tiny amounts of neptunium-237 to neptunium-240 still arise in concentrated uranium ores. In Jefferson Lab’s description, uranium atoms there capture neutrons released by the decay of other uranium atoms.

Reactors supply what is used. Neptunium-237 builds up as a by-product of making plutonium: Jefferson Lab speaks of gram quantities, while Los Alamos says spent fuel rods yield it by the kilogram. Reactor plutonium production discharges only 5 per cent as much neptunium as plutonium, Los Alamos estimates, yet that still exceeds fifty tons a year. Americium-241 offers a second route, alpha-decaying into neptunium-237 with a half-life of 432.2 years.

Why it matters

In 1934 Enrico Fermi fired neutrons at uranium and announced elements 93 and 94. He had in fact split uranium into many radioactive fragments, Los Alamos records. In 1940 at Berkeley, Edwin McMillan and Philip Abelson struck uranium with slowed neutrons from a cyclotron. Uranium-238 took up a neutron, and the uranium-239 formed beta-decayed into neptunium-239, the first transuranium element made in a laboratory, in Los Alamos’s account.

Neptunium-237 is irradiated with neutrons to make plutonium-238. After capturing a neutron it becomes neptunium-238, which beta-decays to plutonium-238. Los Alamos puts that half-life at two days, the International Union of Pure and Applied Chemistry at 2.117 and the Atomic Mass Data Center at 2.099. That plutonium powers radioisotope thermoelectric generators on spacecraft, and radioisotope heater units. Neptunium-237 also goes into detectors for high-energy neutrons.

Its alpha decay feeds protactinium-233, which halves in 26.9 days and settles into equilibrium with its parent. Measuring that balance shows how long ago an intercepted neptunium sample was last separated and purified. The chain continues to bismuth-209 and thallium-205, unlike most common heavy nuclei, which end as lead, and is called the neptunium series. The alloy NpPd₅Al₂ superconducts, carrying an electric current with no resistance, a surprise in Los Alamos’s view because such materials are often strongly magnetic, and magnetism usually destroys superconductivity.

Properties

Properties of neptunium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	93
Atomic mass	237.048172 Da	Da	PubChem, longest-lived isotope
Melting point	644 °C	°C	Los Alamos: 644 °C
Boiling point	4174 °C	°C	Los Alamos: 4174 °C
Density	20.25 g/cm³	g/cm³
Electronegativity (Pauling)	1.36
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Neptunium’s hazard is ionising radiation, above all from atoms taken into the body. Haz-Map, in PubChem, calls it toxic after ingestion or inhalation, names bone as the critical organ and rates internal toxicity very high. Neptunium-237 decays slowly, but its daughter protactinium-233 gives off beta particles, Los Alamos notes. The PubChem Laboratory Chemical Safety Summary is the reference sheet.

Questions

What was the first transuranium element ever made?

Neptunium-239, produced at Berkeley in 1940, holds that place in Los Alamos's account, and it was also the first actinide beyond uranium to be found. The isotope is short-lived: it halves in 2.3565 days according to Los Alamos, 2.356 days for the Atomic Mass Data Center and about 2.4 days in Jefferson Lab's figure. Every decay is a beta decay, and the daughter is plutonium-239.

Why is neptunium named after Neptune?

Los Alamos says the element was named for the planet Neptune, the next planet outward from the Sun beyond Uranus, just as uranium had been named for Uranus. Los Alamos records the same precedent behind plutonium: element 94 was made from neptunium-238, which beta-decayed into it, and its name continued the planetary pattern set by uranium and neptunium.

How long does neptunium-237 last?

Its half-life is 2.14 million years for Los Alamos, 2.144 million for the IAEA and Jefferson Lab, and 2.1 million in Haz-Map's summary. Lighter neptunium isotopes decay mainly by electron capture, with much alpha emission too, mostly to uranium, while heavier ones beta-decay to plutonium, Los Alamos explains. Neptunium-236 halves in 154,000 years on the Los Alamos figure and 153,000 on the Atomic Mass Data Center's; neptunium-235 halves in 396 days.

Is neptunium dangerous?

Yes, as a radioactive element. Haz-Map rates its internal toxicity very high and adds that neptunium-236 and neptunium-237 carry a small risk of significant external radiation. Los Alamos says the digestive tract does not absorb it, and Haz-Map lists only a very small uptake there; injected neptunium collects in bone and leaves only slowly. Its long-lived isotopes and the mobile NpO₂⁺ ion make it a concern for nuclear-waste storage. The PubChem safety summary is the reference sheet.