Barium (Ba). Diagram of the nuclear composition and electron configuration of an atom of barium-137 (atomic number: 56), an isotope of this element. The nucleus consists of 56 protons (red) and 81 neutrons (blue). 56 electrons (green) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Barium is an alkaline earth metal in group 2, period 6, and the s-block of the periodic table. It is extremely reactive and burns with a green flame. Barium compounds are used as X-ray contrast mediums for the digestive system.