---
title: "Thallium (Tl): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/thallium.html
description: "Thallium (Tl), element 81: atomic mass 204.38, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, group 13, period 6, p-block post-transition metal. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
81Tl204.38

# Thallium (Tl) – element 81

Thallium is a chemical element with the symbol **Tl** and atomic number **81**. It is a post-transition metal in group 13, period 6 of the periodic table (p-block), and is a solid at room temperature.

Thallium (Tl) has atomic number 81, atomic mass 204.38 u and electron configuration [Xe] 4f14 5d10 6s2 6p1.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Thallium facts

| Symbol | Tl |
| --- | --- |
| Atomic number | 81 |
| Atomic mass | 204.38 u (g/mol) |
| Category | post-transition metal |
| Group | group 13 |
| Period | 6 |
| Block | p-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1 |
| Noble-gas notation | [Xe] 4f14 5d10 6s2 6p1 |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Valence electrons | 3 |

## Bohr model and shells

Thallium has 81 electrons in 6 shells: 2, 8, 18, 32, 18, 3. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 577 K (303.9 °C) |
| --- | --- |
| Boiling point | 1746 K (1472.8 °C) |
| Density | 11.85 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| Electron affinity | 36.4 kJ/mol |
| Ionization energies | 589.4, 1971, 2878 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 26.32 J/(mol·K) |
| Discovered by | William Crookes |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing Tl.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of thallium?

The atomic mass of thallium is 204.38 u, so one mole of thallium atoms weighs 204.38 g.

### What is the electron configuration of thallium?

Thallium has 81 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹, or [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ in noble-gas notation. Electrons per shell: 2, 8, 18, 32, 18, 3.

### How many valence electrons does thallium have?

Thallium has 3 valence electrons (group 13).

### Is thallium a metal?

Yes. Thallium is a post-transition metal.

### Where is thallium on the periodic table?

Thallium is in group 13, period 6, in the p-block.

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.