Phosphorus has 15 electrons. With an atomic number of 15, phosphorus also has 15 protons. Phosphorus has five electrons in its outermost energy level and forms many compounds with other elements.
Phosphorus is a nonmetal that is fairly reactive and usually shares electrons with other elements. This element is never found free in nature. Phosphorus is extremely poisonous and corrosive; it can spontaneously start burning when exposed to oxygen.
A number of compounds contain an ion of phosphorus called phosphate, which consists of one atom of phosphorus and four atoms of oxygen. Phosphate ions are negatively charged and can combine with electropositive elements, such as calcium.