What is gold’s mass number?

Mass number is equal to the number of protons and neutrons in the nucleus of an isotope’s atom. . Gold has only one stable isotope which has 79 protons and 118 neutrons in its nucleus. The mass number of that isotope is therefore 197.

The most stable of the other isotopes of gold with a half life of 184 days has 79 protons and 116 neutrons and therefore a mass number of 195.