---
title: "Thorium (Th) — element 90"
description: "Thorium is element 90, symbol Th, atomic weight 232.04. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/thorium/
site: "At One Place"
---

# Thorium (Th)

**Thorium — element 90: Th**

232.04 u · Actinide · period 7

Thorium is about three times more abundant than uranium and every atom of it can in principle be bred into fissile uranium-233, which is why thorium reactors keep being proposed; the practical obstacle is that the fuel cycle produces uranium-232, whose decay chain includes a hard gamma emitter that makes the fuel awkward to handle. Thoriated gas mantles were the standard for outdoor lighting for a century and old ones are still measurably radioactive. Its configuration is anomalous — [Rn] 7s² 6d², with no 5f electrons, so thorium behaves chemically much more like hafnium than like a rare earth.

## Properties

- **Atomic number:** 90
- **Symbol:** Th
- **Standard atomic weight:** 232.04 u
- **Category:** Actinide
- **Period:** 7
- **Block:** d-block
- **Electron configuration:** [Rn] 7s² 6d²
- **Electrons per shell:** 2, 8, 18, 32, 18, 10, 2
- **Melting point:** 2,023 K — 1749.8 °C — 3181.7 °F
- **Boiling point:** 5,061 K — 4787.9 °C — 8650.1 °F
- **Density:** 11.72 g/cm³
- **Electronegativity:** 1.3 (Pauling)
- **Oxidation states:** +4
- **Discovered:** 1829 by Jöns Jacob Berzelius

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d²
- **Noble gas shorthand:** [Rn] 7s² 6d²
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 10 · Q: 2

## What it is used for

- Thoriated tungsten electrodes for TIG welding
- High-refractive-index optical glass, in older lenses
- Gas mantles for camping and street lanterns, historically
- Proposed fuel for molten salt and thorium-cycle reactors
- Thorium-lead dating of rocks and meteorites

## Hazards

- Radioactive alpha emitter, though thorium-232's 14-billion-year half-life makes it weakly so
- Thorium dust is the real hazard: inhaled, it lodges in the lungs
- Old thoriated lenses yellow with age from their own radiation

> Anomalous electron configuration: [Rn] 7s² 6d² rather than the predicted 7s² 5f². Thorium is one of only three actinides — with protactinium and uranium — that has a characteristic terrestrial isotopic composition and therefore a real standard atomic weight rather than a bracketed mass number.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 88 | Radium | Ra | [226] |
| 89 | Actinium | Ac | [227] |
| 90 | Thorium ← | Th | 232.04 |
| 91 | Protactinium | Pa | 231.04 |
| 92 | Uranium | U | 238.03 |

## Common questions

### What is the atomic number of thorium?

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

### What is the symbol for thorium?

Th.

### What is the electron configuration of thorium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d², or [Rn] 7s² 6d² in noble-gas shorthand.

### What is the melting point of thorium?

2,023 K — 1749.8 °C — 3181.7 °F

## Related pages

- [Radium (Ra)](https://atoneplace.net/element/radium/)
- [Actinium (Ac)](https://atoneplace.net/element/actinium/)
- [Protactinium (Pa)](https://atoneplace.net/element/protactinium/)
- [Uranium (U)](https://atoneplace.net/element/uranium/)

## Sources

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