Atomic number 56

Barium

Ba
+ phase
− phase

Shells Orbitals 6s

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56 electrons in 6 shells: 2, 8, 18, 18, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkaline earth metal
Period: 6
Group: 2
Block: s
Standard atomic weight: 137.33(1) Da
Electron configuration: [Xe] 6s²
Phase at 25 °C: solid
Melting point: 727 °C
Boiling point: 1845 °C
Density: 3.62 g/cm³
Electronegativity (Pauling): 0.89
Discovered: 1808, Humphry Davy; baryta told apart from lime by Scheele

Neighbours in the table

Cs La

Chemically, barium behaves much like calcium, Los Alamos notes, and the metal is soft and, when pure, silvery white. Its name comes from the Greek barys, heavy. Barite, the sulfate and one of the two minerals Jefferson Lab names as its commonest, weights drilling mud, and Jefferson Lab calls barium sulfate a good absorber of X-rays.

Barium sits in group 2 and period 6, with the configuration [Xe] 6s²: two electrons in the sixth shell outside a xenon core. Group 2 metals readily lose both, and OpenStax gives them a +2 oxidation state in compounds, a bookkeeping number assuming ionic bonds. In a larger atom the outer electrons lie farther from the nucleus, and OpenStax explains that a more distant electron is easier to remove. Barium’s first ionisation energy, the cost of taking off one electron, is lower than that of oxygen, polonium or lead, OpenStax says.

The group trend follows. OpenStax ties reactivity in group 2 to growing size and falling ionisation energy. Among magnesium, calcium, strontium and barium, it is barium that reacts hardest with water at room temperature. The products are hydrogen and barium hydroxide.

Where it is found

Air and water leave no free barium in nature: the metal combines with oxygen to give barium oxide and with water to give the hydroxide. Los Alamos finds it chiefly as sulfate and carbonate, the minerals barite, BaSO₄, and witherite, BaCO₃. Jefferson Lab puts its share of the crust at 4.25 × 10² milligrams per kilogram, and a litre of seawater holds 1.3 × 10⁻² milligrams.

Barite is mined on a large scale. The US Geological Survey (USGS) estimates world output in 2024 at about 8.2 million tonnes, not counting the United States, whose figure is withheld. India led with 2.6 million tonnes and China followed with 2.1 million. In the United States, three companies worked four barite operations, all in Nevada.

Impure barium sulfate had a reputation long before the element had a name. Stones of it found near Bologna, Italy, in the early 1500s glowed after being held in light, and alchemists thought them magical. Jefferson Lab adds that such stones, strongly heated with charcoal, keep glowing in darkness for up to six years. Los Alamos notes that impure barium sulfide phosphoresces, glowing after exposure to light.

Why it matters

Most barite is sold for its weight. Over 90 per cent of US barite sales go to weighting the fluids that drill oil and gas wells. The USGS reports that deeper wells have pushed up the barite used per rig. It has no large-scale substitute there. Because barite blocks X-rays and gamma rays well, it also goes into high-density concrete for radiation shielding.

Hospitals use the same property from the inside. Ultrapure barite serves as a contrast medium for X-ray and computed tomography (CT) scans of the gut. Jefferson Lab explains why this barium compound can be swallowed: the sulfate does not dissolve in water. Los Alamos states that every barium compound soluble in water or acid is poisonous. Haz-Map notes that barium carbonate, also insoluble in water, turns into soluble barium chloride in stomach acid.

The metal has narrower work. It serves as a getter, a material that takes up the last traces of gas inside vacuum tubes, and barium-nickel alloys go into spark plug wire. Barium titanate serves as a dielectric, the insulating layer, in capacitors, and barium sits in the superconductor YBa₂Cu₃O₇. Barium-137m, made when caesium-137 decays, has a half-life of 2.552 minutes by the evaluation of the International Atomic Energy Agency (IAEA), short enough for school half-life experiments.

Properties

Properties of barium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	56
Standard atomic weight	137.33(1) Da	Da	IUPAC/CIAAW 2024
Melting point	727 °C	°C	Los Alamos: 727 °C
Boiling point	1845 °C	°C	Los Alamos: 1845 °C
Density	3.62 g/cm³	g/cm³
Electronegativity (Pauling)	0.89
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²
Phase at 25 °C	solid

Safety

Barium metal carries the signal word Danger in PubChem’s classification under GHS, the Globally Harmonized System: in contact with water it releases flammable gas that may ignite on its own. The powder is pyrophoric, meaning it catches fire in air unprompted. The metal irritates the eyes, skin and airways, and about a third of notifiers also class it as toxic if swallowed and as causing severe burns. These hazards belong to laboratories and plants handling the metal, and the PubChem Laboratory Chemical Safety Summary among the sources is the sheet for it.

Questions

Is barium radioactive?

Natural barium is a mixture of stable isotopes, six by Jefferson Lab's count. Barium-138 makes up about 71.7 per cent. IAEA data mark two rare ones, barium-130 and barium-132, each about 0.1 per cent, with a possible extremely slow decay: barium-130 on a scale of roughly 10²¹ years, barium-132 at more than 3 × 10²⁰ years. The Atomic Mass Data Center lists nuclides from barium-113 to barium-154.

What colour does barium make in fireworks?

Green. OpenStax lists barium among the heavier group 2 metals that colour a flame and gives its colour as green, a band of the element's emission spectrum, the particular wavelengths its hot atoms give out. Jefferson Lab adds that barium nitrate, Ba(NO₃)₂, burns bright green and goes into signal flares and fireworks; Los Alamos names the chlorate as well among barium compounds that give colours in pyrotechnics.

Why is barium metal stored under oil?

Los Alamos says the metal oxidises very easily, so it is kept under petroleum or another liquid free of oxygen to shut out air; water and alcohol decompose it. Commercial metal ships the same way, in airtight steel drums under argon or paraffin oil. Water is the other threat: CAMEO notes that the reaction gives hydrogen and enough heat that the gas may ignite.

Who discovered barium?

Two chemists, a generation apart. Scheele showed that baryta, the oxide, was a separate substance from lime, according to Los Alamos. The metal itself came in 1808, when Humphry Davy, an English chemist, passed a current through molten baryta, BaO, as Jefferson Lab describes.

Compounds containing barium

BaSO₄ Barium sulfate 6 atoms in one formula unit of an ionic lattice.