Strontium (Sr). Diagram of the nuclear composition and electron configuration of an atom of strontium-88 (atomic number: 38), the most common isotope of this element. The nucleus consists of 38 protons (red) and 50 neutrons (yellow). 38 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Strontium is an alkaline earth metal in group 2, period 5, and the s-block of the periodic table. In elemental form, it is an extremely reactive metal that burns with a bright red flame and can ignite in powdered form.