Chemical element · reference page

56Ba

Barium

Barium is element 56 (Ba), located in period 6, group 2 and the s block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 6s2.

Atomic number56

Standard atomic weight137.327(7)

Group2

Period6

Blocks

Electron configuration[Xe] 6s2

Electronic structure

What controls its chemistry

The outer configuration ends in s². Removing two valence electrons produces a +2 ion, which is the characteristic ionic state of the alkaline-earth metals.

Compared with group 1 metals, the two-electron outer shell changes ionisation energies, bonding and reactivity. Their compounds commonly occur as oxides, carbonates, sulfates or halides rather than as free metals in nature.

Chemical forms

Compounds, ions and materials

The +2 oxidation state dominates. Oxides, hydroxides, halides, carbonates and sulfates are common families of barium compounds, although their solubility and thermal stability vary systematically down group 2.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 3.51 g/cm³. The melting point is 1 000 K (726.9 °C). The boiling point is 2 118 K (1 845 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where barium is found and used

In practice, barium is encountered in medical imaging equipment, drilling fluids and fireworks.

In practice

Examples of real uses

Medical imaging equipment

Barium is used in imaging agents, detector materials or the high-field magnets and specialised components of imaging systems.

Drilling fluids

Barium compounds with suitable density or chemistry are added to drilling fluids to control pressure and fluid properties in wells.

Fireworks

Barium compounds can produce characteristic colours, sparks or energetic effects when heated in pyrotechnic mixtures.

Periodic position

What its neighbours tell us

Barium is in period 6 and group 2. In the neighbourhood shown here, caesium is immediately to its left in the same period; strontium is above it and radium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationalkaline earth metal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 18 · 8 · 2

Density3.51 g/cm³

Melting point1 000 K (726.9 °C)

Boiling point2 119 K (1 845 °C)

Pauling electronegativity0.89

First ionisation energy502.8kJ/mol

Electron affinity13.95 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of barium is associated with Carl Wilhelm Scheele. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 137.327(7). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Barium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with barium depend on chemical form, dose and route of exposure. Pure barium, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.