Barium (Ba)
Barium (Ba) is an alkaline earth metal with atomic number 56 and a standard atomic weight of 137.33. At room temperature it is a solid that melts at 727 °C and boils at 1,845 °C.
Barium key facts
Barium at a glance
Identity
| Symbol | Ba |
|---|---|
| Atomic number (Z) | 56 |
| Atomic weight | 137.33 ± 0.01 (CIAAW 2024) |
| Category | Alkaline earth metal |
| Group | 2 — alkaline earth metals |
| Period | 6 |
| Block | s-block |
| CAS number | 7440-39-3 |
Atomic properties
| Electron configuration | [Xe] 6s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 18, 8, 2 |
| Oxidation states | +2 Less common: +1 |
| Electronegativity (Pauling) | 0.89 |
| First ionisation energy | 5.212 eV (502.9 kJ/mol) 2nd: 10.004 eV · 3rd: 35.844 eV |
| Electron affinity | 0.145 eV |
| Atomic radius (empirical) | 215 pm |
| Covalent radius | 196 pm |
| Van der Waals radius | 268 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 727 °C · 1,340.6 °F · 1,000.15 K |
| Boiling point | 1,845 °C · 3,353 °F · 2,118.15 K |
| Density | 3.51 g/cm³ |
| Specific heat capacity | 0.204 J/(g·K) |
| Crystal structure | Body-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 138Ba 71.7%, 137Ba 11.23%, 136Ba 7.85%, 135Ba 6.59%, 134Ba 2.42%, 130Ba 0.11% |
| Stable isotopes | 7 |
| Known isotopes | 42 |
| Abundance in Earth's crust | 425 mg/kg |
| Abundance in seawater | 0.013 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Barium electron configuration
Barium has 56 electrons. Written in short form, its ground-state configuration is [Xe] 6s2 — the electrons of the noble gas xenon, plus 6s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2.
As a member of group 2 it has 2 valence electrons in its outer shell (shell 6), which largely decide its chemistry.
Discovery and name
1772 / 1808 — Carl Wilhelm Scheele (identified); isolated by Humphry Davy · Sweden / England.
Greek barys, heavy (via the mineral baryte).
Known for
Barium sulfate is drunk as a 'barium meal' to show the gut on X-rays and is used in drilling muds. Barium salts colour fireworks green.
Isotopes of barium
Naturally occurring barium is a mix of 7 isotopes. Each has 56 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 138Ba | 56 | 82 | 71.7% | stable |
| 137Ba | 56 | 81 | 11.23% | stable |
| 136Ba | 56 | 80 | 7.85% | stable |
| 135Ba | 56 | 79 | 6.59% | stable |
| 134Ba | 56 | 78 | 2.42% | stable |
| 130Ba | 56 | 74 | 0.11% | stable |
| 132Ba | 56 | 76 | 0.1% | stable |
42 isotopes of barium are known in our data set (half-lives as published in nuclear data tables).
Barium compared with group 2
The other elements in group 2 (alkaline earth metals), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Beryllium | 4 | 9.0122 | 1,287 °C | 2,468 °C | 1.85 g/cm³ | 1.57 |
| Magnesium | 12 | 24.305 | 650 °C | 1,090 °C | 1.74 g/cm³ | 1.31 |
| Calcium | 20 | 40.078 | 842 °C | 1,484 °C | 1.54 g/cm³ | 1 |
| Strontium | 38 | 87.62 | 777 °C | 1,377 °C | 2.64 g/cm³ | 0.95 |
| Barium | 56 | 137.33 | 727 °C | 1,845 °C | 3.51 g/cm³ | 0.89 |
| Radium | 88 | [226] | 696 °C | 1,737 °C | 5 g/cm³ | 0.9 |