The mass number of an isotope of gold can vary depending on the specific isotope in question. Gold (symbol Au) has multiple isotopes, but the most common and stable isotope is gold-197.
The mass number of an isotope represents the total number of protons and neutrons in the nucleus of an atom. In the case of gold-197, it has 79 protons (since gold has an atomic number of 79) and 118 neutrons. Adding these together gives a mass number of 197 (79 protons + 118 neutrons = 197).
It's important to note that there are other isotopes of gold, such as gold-196, gold-198, gold-199, etc., with different numbers of neutrons and therefore different mass numbers. However, gold-197 is the most abundant and frequently referenced isotope of gold.