The atomic number is the number of protons in the atom. The atomic number of phosphorus is 15 so the number of protons in the atom is also 15.
31p, 32p and 33 p are the isotopes of phosphorus, there are number of neutrons are changes in the isotopes where the number of protons are same in all isotopes. so, there are 15 protons in each of the isotope.
