Question: what is the mass number of an isotope?
Answer: The mass number of an isotope is the total number of protons and neutrons in an atomic nucleus. ... If the nucleus has 6 protons and 7 neutrons, then its mass number is 13. The symbol for an isotope lists the mass number as a superscript at the upper left. Thus, ₆¹²C represents a carbon isotope with mass number 12.