The mass number of an element is the total number of protons and neutrons in its nucleus. For phosphorus, the mass number is 31.
Phosphorus has an atomic number of 15, which means it has 15 protons in its nucleus. The most common isotope of phosphorus is phosphorus-31, which has 16 neutrons. Therefore, the mass number of phosphorus is calculated as follows:
Mass Number = Number of Protons + Number of Neutrons
Mass Number = 15 + 16 = 31
This makes the mass number of phosphorus 31.