---
title: "Thulium (Tm) — element 69"
description: "Thulium is element 69, symbol Tm, atomic weight 168.93. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/thulium/
site: "At One Place"
---

# Thulium (Tm)

**Thulium — element 69: Tm**

168.93 u · Lanthanide · period 6

Thulium is the least abundant of the stable lanthanides, though still more common in the crust than gold or platinum. Thulium-170, made by irradiating the stable isotope in a reactor, emits soft X-rays and needs no power supply, which makes it useful for radiography where there is no electricity — inspecting pipeline welds in the field, for instance. It is named after Thule, the classical name for the far north.

## Properties

- **Atomic number:** 69
- **Symbol:** Tm
- **Standard atomic weight:** 168.93 u
- **Category:** Lanthanide
- **Period:** 6
- **Block:** f-block
- **Electron configuration:** [Xe] 6s² 4f¹³
- **Electrons per shell:** 2, 8, 18, 31, 8, 2
- **Melting point:** 1,818 K — 1544.8 °C — 2812.7 °F
- **Boiling point:** 2,223 K — 1949.8 °C — 3541.7 °F
- **Density:** 9.32 g/cm³
- **Electronegativity:** 1.25 (Pauling)
- **Oxidation states:** +3
- **Discovered:** 1879 by Per Teodor Cleve

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹³
- **Noble gas shorthand:** [Xe] 6s² 4f¹³
- **Shells:** K: 2 · L: 8 · M: 18 · N: 31 · O: 8 · P: 2

## What it is used for

- Thulium-170 as a portable X-ray source for field radiography
- Thulium fibre lasers for surgery and materials processing
- Blue phosphors in euro banknote security features
- Dopant in high-temperature superconductors

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 67 | Holmium | Ho | 164.93 |
| 68 | Erbium | Er | 167.26 |
| 69 | Thulium ← | Tm | 168.93 |
| 70 | Ytterbium | Yb | 173.05 |
| 71 | Lutetium | Lu | 174.97 |

## Common questions

### What is the atomic number of thulium?

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

### What is the symbol for thulium?

Tm.

### What is the electron configuration of thulium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹³, or [Xe] 6s² 4f¹³ in noble-gas shorthand.

### What is the melting point of thulium?

1,818 K — 1544.8 °C — 2812.7 °F

## Related pages

- [Holmium (Ho)](https://atoneplace.net/element/holmium/)
- [Erbium (Er)](https://atoneplace.net/element/erbium/)
- [Ytterbium (Yb)](https://atoneplace.net/element/ytterbium/)
- [Lutetium (Lu)](https://atoneplace.net/element/lutetium/)

## Sources

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