Even though P has its octec complete.in a covalent bond the two atoms are said to share 1 electron to form a single covalent bond.Now following this analogy ,if P atom bonds with 4 different C atom to complete its octec which makes it non reactive or inert gas configuration.The atomic number (P= 15 number of protons) is charge in the nucleus but the number of electrons that P effectively holds is (2,8,4) charge of outer shells is 14.So the net charge of the atom is (+15+(- 14) =+1).I hope this helps.