---
title: "Actinium (Ac) — element 89"
description: "Actinium is element 89, symbol Ac, most stable isotope 227. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/actinium/
site: "At One Place"
---

# Actinium (Ac)

**Actinium — element 89: Ac**

[227] · Actinide · period 7

Actinium glows pale blue in the dark, its own radiation exciting the surrounding air — about 150 times more radioactive than radium, weight for weight. Actinium-225 has become one of the most sought-after isotopes in medicine, because a single alpha particle deposits its energy within a few cell diameters and will kill a targeted tumour cell without touching its neighbours; global supply is measured in a few grams a year. Its configuration is anomalous, [Rn] 7s² 6d¹ with no 5f electron at all, exactly as lanthanum has no 4f electron.

## Properties

- **Atomic number:** 89
- **Symbol:** Ac
- **Mass number of most stable isotope:** [227]
- **Category:** Actinide
- **Period:** 7
- **Block:** d-block
- **Electron configuration:** [Rn] 7s² 6d¹
- **Electrons per shell:** 2, 8, 18, 32, 18, 9, 2
- **Melting point:** 1,324 K — 1050.8 °C — 1923.5 °F
- **Boiling point:** 3,471 K — 3197.8 °C — 5788.1 °F
- **Density:** 10.07 g/cm³
- **Electronegativity:** 1.1 (Pauling)
- **Oxidation states:** +3
- **Discovered:** 1899 by André-Louis Debierne

## Electron configuration

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

## What it is used for

- Actinium-225 for targeted alpha therapy against prostate and other cancers
- Actinium-beryllium neutron sources

## Hazards

- Intensely radioactive alpha emitter; actinium-227 has a 21.8-year half-life

> Anomalous electron configuration: [Rn] 7s² 6d¹ rather than the predicted 7s² 5f¹. No stable isotope; the mass given is the mass number of actinium-227, the longest-lived at 21.8 years.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 87 | Francium | Fr | [223] |
| 88 | Radium | Ra | [226] |
| 89 | Actinium ← | Ac | [227] |
| 90 | Thorium | Th | 232.04 |
| 91 | Protactinium | Pa | 231.04 |

## Common questions

### What is the atomic number of actinium?

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

### What is the symbol for actinium?

Ac.

### What is the electron configuration of actinium?

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

### What is the melting point of actinium?

1,324 K — 1050.8 °C — 1923.5 °F

## Related pages

- [Francium (Fr)](https://atoneplace.net/element/francium/)
- [Radium (Ra)](https://atoneplace.net/element/radium/)
- [Thorium (Th)](https://atoneplace.net/element/thorium/)
- [Protactinium (Pa)](https://atoneplace.net/element/protactinium/)

## Sources

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