Barium is a chemical element, having the chemical symbol Ba. Its atomic number (the number of protons) is 56. It has a standard atomic weight of 137.327 g•mol −1 and is a solid in its elemental form.
Barium is considered to be a member of the "Alkaline earth metal" class of elements. At a pressure of 101.325 kPa, it has a boiling point of 1,897 °C and a melting point of 727 °C.
Barium, usually as the sulfate, is opaque to X-rays, and is used as a contrast medium in examinations of the gastrointestinal tract. It is toxic if it escapes the tract, and can be extremely constipating, so barium enemas and barium meals are used less than in the past, when methods such as ultrasound and X-ray computerized tomography were not available. Nevertheless, its characteristic coating of mucosa can give views provided by no other modality.