---
title: "Barium (Ba) — element 56"
description: "Barium is element 56, symbol Ba, atomic weight 137.33. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/barium/
site: "At One Place"
---

# Barium (Ba)

**Barium — element 56: Ba**

137.33 u · Alkaline earth metal · period 6, group 2

Barium compounds are toxic, but barium sulfate is so insoluble that a patient can drink half a litre of it for an X-ray and absorb essentially none — the difference between a poison and a medicine is entirely a matter of which counter-ion it is paired with. Around three-quarters of world barite production goes into drilling mud, where its density holds back the pressure of the well. Barium was the tell-tale in the discovery of nuclear fission: Otto Hahn found it in uranium bombarded with neutrons in 1938, could not explain how a heavy element had produced a much lighter one, and Lise Meitner worked out that the nucleus had split.

## Properties

- **Atomic number:** 56
- **Symbol:** Ba
- **Standard atomic weight:** 137.33 u
- **Category:** Alkaline earth metal
- **Group:** 2
- **Period:** 6
- **Block:** s-block
- **Electron configuration:** [Xe] 6s²
- **Electrons per shell:** 2, 8, 18, 18, 8, 2
- **Melting point:** 1,000 K — 726.9 °C — 1340.3 °F
- **Boiling point:** 2,170 K — 1896.8 °C — 3446.3 °F
- **Density:** 3.62 g/cm³
- **Electronegativity:** 0.89 (Pauling)
- **Oxidation states:** +2
- **Discovered:** 1808 by Humphry Davy

## Electron configuration

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

## What it is used for

- Barium sulfate as the swallowed contrast agent for gut X-rays
- Weighting agent in oil and gas drilling mud
- Green fireworks
- YBCO high-temperature superconductors
- Getters that absorb stray gas inside vacuum tubes

## Hazards

- Soluble barium salts are acutely toxic, causing muscle paralysis and cardiac arrhythmia
- Barium metal ignites in air and reacts with water

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 54 | Xenon | Xe | 131.29 |
| 55 | Cesium | Cs | 132.91 |
| 56 | Barium ← | Ba | 137.33 |
| 57 | Lanthanum | La | 138.91 |
| 58 | Cerium | Ce | 140.12 |

## Group 2

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Barium sits in group 2 with beryllium, magnesium, calcium, strontium, radium.

## Common questions

### What is the atomic number of barium?

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

### What is the symbol for barium?

Ba.

### What is the electron configuration of barium?

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

### What is the melting point of barium?

1,000 K — 726.9 °C — 1340.3 °F

## Related pages

- [Xenon (Xe)](https://atoneplace.net/element/xenon/)
- [Cesium (Cs)](https://atoneplace.net/element/cesium/)
- [Lanthanum (La)](https://atoneplace.net/element/lanthanum/)
- [Cerium (Ce)](https://atoneplace.net/element/cerium/)
- [Beryllium (Be)](https://atoneplace.net/element/beryllium/)
- [Magnesium (Mg)](https://atoneplace.net/element/magnesium/)
- [Calcium (Ca)](https://atoneplace.net/element/calcium/)
- [Strontium (Sr)](https://atoneplace.net/element/strontium/)
- [Radium (Ra)](https://atoneplace.net/element/radium/)

## Sources

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