Neptunium

The first element heavier than uranium, named for the next planet out.

At a glance

Symbol: Np
Atomic number: 93
Mass number of its longest-lived isotope: [237]
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 5f⁴ 6d¹ 7s²
Electronegativity: 1.36
Common ions: +5 in water, as NpO₂⁺; anywhere from +3 to +7
Isotopes: none are stable; ²³⁷Np, with a half-life of 2.1 million years, is the longest-lived

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: silvery, radioactive metal
Melting point: 644 °C
Boiling point: 3902 °C
Density: 20.3 g/cm³

Where it is found

Tiny traces form in uranium ores. Nearly all of it comes from spent nuclear fuel.

Discovery and name

Edwin McMillan and Philip Abelson made it in 1940 at Berkeley by bombarding uranium with neutrons, the first element beyond uranium. Uranium had been named for Uranus, so they named this one for Neptune.

What it is used for

Neptunium-237 is irradiated to make plutonium-238 for spacecraft power sources, and it goes into some neutron detectors.

In compounds

NpO₂neptunium dioxideNpF₆neptunium hexafluoride

Handle with care

Radioactive, and it builds up in bone and the liver.