---
title: "Thallium (Tl) — element 81"
description: "Thallium is element 81, symbol Tl, atomic weight 204.38. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/thallium/
site: "At One Place"
---

# Thallium (Tl)

**Thallium — element 81: Tl**

204.38 u · Post-transition metal · period 6, group 13

Thallium sulfate was called the poisoner's poison: colourless, tasteless, water-soluble, and producing symptoms — nerve pain, confusion, then hair loss about two weeks in — that look like a dozen ordinary illnesses. Agatha Christie described it accurately enough in The Pale Horse that a nurse who had read the book recognised a real case of thallium poisoning in a baby in 1977. It kills by impersonating potassium: the body's potassium channels wave it through and it then disrupts every process potassium is involved in.

## Properties

- **Atomic number:** 81
- **Symbol:** Tl
- **Standard atomic weight:** 204.38 u
- **Category:** Post-transition metal
- **Group:** 13
- **Period:** 6
- **Block:** p-block
- **Electron configuration:** [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹
- **Electrons per shell:** 2, 8, 18, 32, 18, 3
- **Melting point:** 577 K — 303.9 °C — 578.9 °F
- **Boiling point:** 1,746 K — 1472.8 °C — 2683.1 °F
- **Density:** 11.8 g/cm³
- **Electronegativity:** 1.62 (Pauling)
- **Oxidation states:** +1, +3
- **Discovered:** 1861 by William Crookes

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
- **Noble gas shorthand:** [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 3

## What it is used for

- Thallium-201 for cardiac stress imaging
- Infrared optics and low-melting specialty glass
- High-temperature superconductor formulations
- Historically, rat poison and ant bait — now banned almost everywhere

## Hazards

- Highly toxic by ingestion, inhalation and skin contact; the lethal dose is around 1 gram
- Cumulative, with a body half-life of several weeks
- Symptoms are delayed and non-specific, which is what made it a favoured poison

> IUPAC publishes an interval, [204.382, 204.385]; 204.38 is the conventional value.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 79 | Gold | Au | 196.97 |
| 80 | Mercury | Hg | 200.59 |
| 81 | Thallium ← | Tl | 204.38 |
| 82 | Lead | Pb | 207.2 |
| 83 | Bismuth | Bi | 208.98 |

## Group 13

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Thallium sits in group 13 with boron, aluminum, gallium, indium, nihonium.

## Common questions

### What is the atomic number of thallium?

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

### What is the symbol for thallium?

Tl.

### What is the electron configuration of thallium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹, or [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹ in noble-gas shorthand.

### What is the melting point of thallium?

577 K — 303.9 °C — 578.9 °F

## Related pages

- [Gold (Au)](https://atoneplace.net/element/gold/)
- [Mercury (Hg)](https://atoneplace.net/element/mercury/)
- [Lead (Pb)](https://atoneplace.net/element/lead/)
- [Bismuth (Bi)](https://atoneplace.net/element/bismuth/)
- [Boron (B)](https://atoneplace.net/element/boron/)
- [Aluminum (Al)](https://atoneplace.net/element/aluminum/)
- [Gallium (Ga)](https://atoneplace.net/element/gallium/)
- [Indium (In)](https://atoneplace.net/element/indium/)
- [Nihonium (Nh)](https://atoneplace.net/element/nihonium/)

## Sources

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