A = number of mass = 31 => number of protons + number of neutrons = 31
Phosphours, Z = atomic number = 15 = number of protons
Symbol of the element phosphorus = P
=> The symbol of the isotope is the symbol of the element, P, with the number of mass, 31, to the left, as a superscript, and the atomic number, 15, to the left, as a suscript.
