The number of neutrons in barium depends on what isotope of barium it is. The most common types of barium are barium-138 and barium-137, which have 82 and 81 neutrons respectively. Barium also has 56 protons and 56 electrons, and it has an atomic mass of 137.33 atomic mass units.
Barium has 16 different isotopes, but only seven of them are stable. All forms of barium are toxic, but patients can be given barium sulfate to distinguish the stomach and intestines on an x-ray, as it is insoluble. Barium is only found in nature in combination with other elements. Humphrey Davy first produced barium metal in 1808 by electrolyzing barium hydroxide.