Selenium (Se). Diagram of the nuclear composition and electron configuration of an atom of selenium-79 (atomic number: 34), a radioisotope of this element. The nucleus consists of 34 protons (red) and 45 neutrons (yellow). 34 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. A solid non-metal, selenium is in group 16, period 4, and the p-block of the periodic table. Used in early semiconductors, it is a trace mineral required in the diet. The two most common allotropes are a black form and a red form.