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

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.

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

Therefore, Atomic mass = Number of protons + Number of neutrons

Or, 31 = 15 + Number of neutrons

Or, number of neutrons = 31 – 15 = 16.

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