The element phosphorus has five valence electrons. Valence electrons refer to the amount of electrons in the outer shell of an atom. If an orbital is not full of atoms, it is likely to try to fill or deplete it in order to become more stable.
Phosphorus is the 15th element on the periodic table of elements. It is incredibly reactive, which is why it is almost always found attached to other elements and compounds in rock. A single phosphorus atom has 15 electrons and protons, as well as usually having 16 neutrons. It is a very important element for life on Earth, found in both RNA and DNA.