Barium
Atomic Data
| Atomic Number | 56 |
| Symbol | Ba |
| Atomic Weight | 137.33 u |
| Density (STP) | 3.51 g/cm³ |
| Melting Point | 726.85 °C (1000 K) |
| Boiling Point | 1844.85 °C (2118 K) |
| Electronegativity | 0.89 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2 |
| Oxidation States | +2 |
| Phase at STP | Solid |
| Category | Alkaline-Earth Metal |
| Period / Group | 6 / 2 |
| CAS Number | 7440-39-3 |
Electron Configuration
[Xe] 6s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| O | 5 | 5s | 2 | 48 |
| O | 5 | 5p | 6 | 54 |
| P | 6 | 6s | 2 | 56 |
| Total | 56 | 56 | ||
Isotopes of Barium
Barium has seven naturally occurring stable isotopes. The most abundant is ¹³⁸Ba, comprising 71.698% of all naturally occurring Barium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Barium-130 | ¹³⁰Ba | 56 | 74 | 0.106 | Stable |
| Barium-132 | ¹³²Ba | 56 | 76 | 0.101 | Stable |
| Barium-134 | ¹³⁴Ba | 56 | 78 | 2.417 | Stable |
| Barium-135 | ¹³⁵Ba | 56 | 79 | 6.592 | Stable |
| Barium-136 | ¹³⁶Ba | 56 | 80 | 7.854 | Stable |
| Barium-137 | ¹³⁷Ba | 56 | 81 | 11.232 | Stable |
| Barium-138 | ¹³⁸Ba | 56 | 82 | 71.698 | Stable |
Abundance & Occurrence
Barium is present in Earth's crust at approximately 425 ppm by mass and at approximately 1 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Barium soluble salts: acute toxicity: Soluble barium salts (barium chloride, barium carbonate) are acutely toxic; oral ingestion causes hypokalaemia, muscle paralysis, and cardiac arrhythmia: the lethal dose for barium chloride is approximately 3.5 g in adults.
- Barium sulfate: safe for medical use: Barium sulfate is insoluble and non-toxic; it is widely used as a radiopaque contrast agent for gastrointestinal X-rays and is safe for oral ingestion; confusion with soluble barium salts must be avoided.
- Water reactivity: Barium metal reacts with water to produce hydrogen and barium hydroxide; fires require Class D extinguishing agents.
- Barium oxide and peroxide: BaO and BaO2 are strongly alkaline and oxidising, respectively; contact with skin or eyes causes chemical burns and requires immediate flushing with water.
Barium in the Real World
Real-World Uses
- Medical imaging contrast agent: Barium sulfate suspension is ingested or administered rectally as an X-ray and CT contrast agent for imaging the gastrointestinal tract; its high atomic number and complete insolubility make it safe for internal use while providing strong radiographic contrast.
- Fireworks (green flame): Barium nitrate and barium chlorate burn to produce a bright green colour in fireworks and signal flares, arising from barium emission bands at 505–535 nm in the visible spectrum.
- Oil well drilling mud: Barite (barium sulfate mineral) is ground and added to drilling fluids as a weighting agent to increase density, preventing formation fluids from blowing out during oil and gas well drilling.
- Vacuum tube getter: Barium metal evaporated inside electronic vacuum tubes reacts with residual oxygen, nitrogen, and CO to maintain the internal vacuum and prolong tube life; the characteristic silvery ring inside old vacuum tubes is the barium getter deposit.
- Ceramic capacitor dielectrics: Barium titanate (BaTiO₃) is the most widely used ferroelectric ceramic, serving as the dielectric in high-capacitance multilayer ceramic capacitors (MLCCs) in virtually every electronic device.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is barium used for?
Barium sulphate is used as a contrast agent in gastrointestinal X-ray examinations (the 'barium meal' or 'barium enema') because it is opaque to X-rays and coats the gut lining, making its shape visible. Barium sulphate is also used as a white pigment (permanent white or blanc fixe) in paints, plastics, and X-ray-shielding concrete. Barium carbonate is used in brickmaking and as a rat poison.
Is barium toxic?
Soluble barium compounds are toxic. Barium chloride, barium nitrate, and barium carbonate cause muscle weakness, cardiac arrhythmias, and hypokalaemia (low potassium) at toxic doses. Barium sulphate, however, is completely insoluble in water and the gut and is therefore non-toxic: this is why it is safe to swallow for X-ray procedures despite barium's general toxicity. The green colour in some fireworks comes from barium chlorate and barium nitrate.
How was barium discovered?
Barium was identified as a new element in 1774 by Carl Wilhelm Scheele, who found that heavy spar (barite, BaSO4) contained an unknown earth. The metal was isolated in 1808 by Humphry Davy through electrolysis. The name comes from the Greek 'barys', meaning heavy, reflecting barite's unusually high density for a non-metallic mineral.
Why is barium sulphate safe to drink when other barium compounds are toxic?
Barium sulphate (BaSO4) has an extremely low solubility in water: only about 0.0002 g per litre. Because barium toxicity depends on soluble barium ions entering the bloodstream, insoluble barium sulphate simply passes through the gastrointestinal tract without being absorbed. This allows radiologists to use it as a dense, gut-coating contrast agent with no systemic toxicity, despite the fact that soluble barium salts like barium chloride are quite poisonous.