An atom of phosphorus is represented by the symbol ${}_{15}{}^{31}\mathrm{P}$.  Calculate the number of protons, electrons, and neutrons present in one phosphorus atom.

 The numbers of protons, electrons, and neutrons present in a phosphorus atom are 15, 15, and 16 respectively.

Solution:

The atomic number of phosphorus is 15. The atomic number of an element is equal to the number of protons and electrons present in an atom. Hence, the number of protons and electrons present in one phosphorus atom is 15.

Mass number of phosphorus is 31. Mass number of an atom is equal to the sum of the total number of protons and neutrons present in its nucleus.

Mass number     = Number of protons + Number of neutrons
                       31    =    15   +   Number of neutrons
               31 – 15  =   16
Number of neutrons = 16
Hence, the numbers of protons, electrons, and neutrons present in a phosphorus atom are 15, 15, and 16 respectively.