---
title: "Neptunium (Np) — element 93"
description: "Neptunium is element 93, symbol Np, most stable isotope 237. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/neptunium/
site: "At One Place"
---

# Neptunium (Np)

**Neptunium — element 93: Np**

[237] · Actinide · period 7

Neptunium was the first transuranium element made, in 1940, and it was named for Neptune because it comes after uranium exactly as Neptune comes after Uranus — plutonium and Pluto followed the same logic the next year. It has the lowest critical mass of any actinide after plutonium, around 60 kg for the bare metal, a fact that was classified until 2002. Small quantities occur naturally in uranium ore, produced by neutron capture, but the world's supply comes from spent reactor fuel, where it is one of the longer-lived headaches: neptunium-237 has a 2.14-million-year half-life and is unusually mobile in groundwater.

## Properties

- **Atomic number:** 93
- **Symbol:** Np
- **Mass number of most stable isotope:** [237]
- **Category:** Actinide
- **Period:** 7
- **Block:** f-block
- **Electron configuration:** [Rn] 7s² 5f⁴ 6d¹
- **Electrons per shell:** 2, 8, 18, 32, 22, 9, 2
- **Melting point:** 917 K — 643.9 °C — 1190.9 °F
- **Boiling point:** 4,175 K — 3901.8 °C — 7055.3 °F
- **Density:** 20.25 g/cm³
- **Electronegativity:** 1.36 (Pauling)
- **Oxidation states:** +5, +6, +4, +3
- **Discovered:** 1940 by Edwin McMillan and Philip Abelson

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹
- **Noble gas shorthand:** [Rn] 7s² 5f⁴ 6d¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 22 · P: 9 · Q: 2

## What it is used for

- A precursor for making plutonium-238 for space power sources
- Neptunium-237 in neutron detection instruments

## Hazards

- Radioactive and fissile; neptunium-237 has a half-life of 2.14 million years

> Anomalous electron configuration: [Rn] 7s² 5f⁴ 6d¹ rather than the predicted 7s² 5f⁵. No stable isotope; the mass given is the mass number of neptunium-237, the longest-lived at 2.14 million years.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 91 | Protactinium | Pa | 231.04 |
| 92 | Uranium | U | 238.03 |
| 93 | Neptunium ← | Np | [237] |
| 94 | Plutonium | Pu | [244] |
| 95 | Americium | Am | [243] |

## Common questions

### What is the atomic number of neptunium?

93. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for neptunium?

Np.

### What is the electron configuration of neptunium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹, or [Rn] 7s² 5f⁴ 6d¹ in noble-gas shorthand.

### What is the melting point of neptunium?

917 K — 643.9 °C — 1190.9 °F

## Related pages

- [Protactinium (Pa)](https://atoneplace.net/element/protactinium/)
- [Uranium (U)](https://atoneplace.net/element/uranium/)
- [Plutonium (Pu)](https://atoneplace.net/element/plutonium/)
- [Americium (Am)](https://atoneplace.net/element/americium/)

## Sources

- Official vendor product documentation — At One Place (/about/)