The atomic number of phosphorus (P) is 15. The atomic number of an element tells you the number of protons in the nucleus of an atom of that element. The number of protons in an atom is equal to the number of electrons in a neutral atom of that element. So, a neutral atom of phosphorus has 15 electrons.

The number of valence electrons in an atom is equal to the number of electrons in the outermost energy level of that atom. The outermost energy level of phosphorus is the third energy level. The third energy level of phosphorus has five electrons. So, a neutral atom of phosphorus has five valence electrons.