barium 138 is the most common stable isotope of barium. It is a member of group 2 and period 6 on the periodic table.

The nucleus of an atom of barium contains 56 protons and 82 neutrons (the number of electrons in the neutral atom is 56 + N – Z = A). The arrangement of the protons and the number of electrons in each shell determines the element’s chemical properties.

There are a variety of stable isotopes of barium, including the long-lived radioactive primordial isotope, barium-130. This isotope decays by double electron capture with a very long half-life of more than 4 x 1021 years.

This isotope is used to produce the gamma-ray reference source, Ba-133, and also to study photon scattering phenomena. This isotope is also used in brachytherapy, a treatment for cancers.

Another important use of barium is the production of ceramic superconductors, such as lanthanum barium copper oxide or yttrium-barium cuprate. These materials are extremely sensitive to temperature and have high thermal conductivity, which makes them useful in a range of electrical applications.

Compounds of barium include barium sulfate (BaSO4), which is an insoluble powder that occurs naturally in the mineral barite; and barium chloride, which is a colourless liquid and soluble in water. Crystalline barium fluoride (BaF2) is transparent to a wide spectrum of light, and is used to make optical lenses for spectroscopy.

Volatile barium compounds, such as barium nitrate and barium chloride, impart a green colour to a flame, making them useful in pyrotechnics and fireworks. Barium carbonate (BaCO3) is used to produce special glass, primarily for radiation shielding in cathode-ray and television tubes.