Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Barium was identified as a new element in 1774 but not reduced to a metal until 1808 with the advent of electrolysis.

Barium sulfate is used, in a purer form, as X-ray radiocontrast agents for imaging the human gastrointestinal tract. However, water-soluble barium compounds are poisonous and have been used as rodenticides.

Alkali Metals and Alkaline Earth Metals

The alkali metals (group 1) have very similar properties: they are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. The alkaline earth metals are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.

The alkali metals are a group (column) in the periodic table consisting of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).