Phosphorus (P). Diagram of the nuclear composition and electron configuration of an atom of phosphorus-31 (atomic number: 15), the most common isotope of this element. The nucleus consists of 15 protons (red) and 16 neutrons (yellow). 15 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. Phosphorus, a solid non-metal, is in group 15, period 3, and the p-block of the periodic table. Its compounds are essential for life. In elemental form it has two main allotropes, white and red phosphorus.