Neptunium (Np)
Element 93 · Actinide · atomic mass [237] u · f-block, period 7
Electron shells: 2, 8, 18, 32, 22, 9, 2
Neptunium was the first transuranic element made, and it exists in nature only in trace amounts. It is a by-product of every operating nuclear reactor.
Named for Neptune because it sits just past uranium, exactly as the planet sits just past Uranus.
- Atomic number
- 93
- Atomic mass
- [237] u
- Electron configuration
- [Rn] 7s² 5f⁴ 6d¹
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +3, +4, +5, +6
- Electronegativity
- 1.36 (Pauling)
- Melting point
- 644 °C
- Boiling point
- 3,902 °C
- Density
- 20.25 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery metal
- Discovered
- Edwin McMillan and Philip Abelson, 1940
93 protons, and 93 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Neptunium is used for
- Neutron detection instruments
- Precursor for plutonium-238 production
Atomic structure of Neptunium
A neutral neptunium atom has 93 protons and 93 electrons. Its most common isotope carries about 144 neutrons, which is where the mass number 237 comes from. Change the neutron count and you have a different isotope of neptunium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 22, 9, 2. In orbital notation that is [Rn] 7s² 5f⁴ 6d¹. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Neptunium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Neptunium sits in the periodic table
Discovery and naming
Neptunium was discovered by Edwin McMillan and Philip Abelson in 1940. Its name comes from The planet Neptune, next out from Uranus, which is also why its symbol (Np) does not match its English name.
Every isotope of neptunium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Neptunium?
Neptunium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 237 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Neptunium have?
Neptunium has 93 protons, because the atomic number is the proton count and that is what makes an atom Neptunium at all. A neutral Neptunium atom has the same number of electrons, 93. The neutron count varies by isotope; the most common one has about 144, since 237 − 93 = 144.
What is the electron configuration of Neptunium?
The electron configuration of Neptunium is [Rn] 7s² 5f⁴ 6d¹. Its electrons are arranged 2, 8, 18, 32, 22, 9, 2 across 7 shells.
Is Neptunium a metal?
Yes. Neptunium is classified as a actinide, on the metallic side of the table.
What group and period is Neptunium in?
Neptunium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Neptunium?
Neptunium most commonly forms a 5+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound. Across all its compounds it shows oxidation states of +3, +4, +5, +6.
What is the melting point of Neptunium?
Neptunium melts at 644 °C and boils at 3,902 °C. That puts it in the solids at everyday temperatures.
What is Neptunium used for?
Neutron detection instruments, Precursor for plutonium-238 production. Neptunium was the first transuranic element made, and it exists in nature only in trace amounts.
Who discovered Neptunium, and where does its name come from?
Neptunium (Np) was discovered by Edwin McMillan and Philip Abelson in 1940. The name comes from The planet Neptune, next out from Uranus.
Is Neptunium radioactive?
Yes. Every isotope of Neptunium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.