---
title: "Protactinium (Pa) — element 91"
description: "Protactinium is element 91, symbol Pa, atomic weight 231.04. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/protactinium/
site: "At One Place"
---

# Protactinium (Pa)

**Protactinium — element 91: Pa**

231.04 u · Actinide · period 7

Protactinium is one of the rarest and most expensive naturally occurring elements: the UK Atomic Energy Authority extracted about 125 grams from 60 tonnes of uranium processing waste in 1961, at a cost that worked out around $500,000, and that batch supplied most of the world's research for decades. The name means "before actinium", because protactinium-231 decays into it. It is awkward to handle for reasons beyond radioactivity — it hydrolyses and sticks to the walls of any container that is not carefully chosen.

## Properties

- **Atomic number:** 91
- **Symbol:** Pa
- **Standard atomic weight:** 231.04 u
- **Category:** Actinide
- **Period:** 7
- **Block:** f-block
- **Electron configuration:** [Rn] 7s² 5f² 6d¹
- **Electrons per shell:** 2, 8, 18, 32, 20, 9, 2
- **Melting point:** 1,845 K — 1571.8 °C — 2861.3 °F
- **Density:** 15.37 g/cm³
- **Electronegativity:** 1.5 (Pauling)
- **Oxidation states:** +5, +4
- **Discovered:** 1913 by Kasimir Fajans and Oswald Göhring

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹
- **Noble gas shorthand:** [Rn] 7s² 5f² 6d¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 20 · P: 9 · Q: 2

## What it is used for

- Protactinium-231 and thorium-230 ratios for dating marine sediments and corals

## Hazards

- Highly radioactive and highly toxic; protactinium-231 has a 32,700-year half-life

> Anomalous electron configuration: [Rn] 7s² 5f² 6d¹ rather than the predicted 7s² 5f³. The boiling point is omitted: published values around 4,300 K are extrapolations, not measurements.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 89 | Actinium | Ac | [227] |
| 90 | Thorium | Th | 232.04 |
| 91 | Protactinium ← | Pa | 231.04 |
| 92 | Uranium | U | 238.03 |
| 93 | Neptunium | Np | [237] |

## Common questions

### What is the atomic number of protactinium?

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

### What is the symbol for protactinium?

Pa.

### What is the electron configuration of protactinium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹, or [Rn] 7s² 5f² 6d¹ in noble-gas shorthand.

### What is the melting point of protactinium?

1,845 K — 1571.8 °C — 2861.3 °F

## Related pages

- [Actinium (Ac)](https://atoneplace.net/element/actinium/)
- [Thorium (Th)](https://atoneplace.net/element/thorium/)
- [Uranium (U)](https://atoneplace.net/element/uranium/)
- [Neptunium (Np)](https://atoneplace.net/element/neptunium/)

## Sources

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