The average mass of a single phosphurus atom in grams is found by dividing the atomic mass (in uma or grams / mole) by Avogadro's number, 6,02 * 10^23 atoms / mol. So, search for the atomic mass of phosphorus in a table. It is 30.974 g/mol. Then, the average mass of a single phosphorus atom is (30.974 g/mol) / (6.022 * 10^23 atoms/mol) = 5.143 * 10^-23 g/atom