To determine the mass number of an ion, we need to know the atomic number and the number of neutrons. However, knowing the number of electrons alone does not provide enough information to determine the mass number.
The mass number (A) of an atom or ion is the sum of the number of protons and neutrons in its nucleus. It is denoted as A = Z + N, where Z is the atomic number (number of protons) and N is the number of neutrons.
If you have additional information about the ion, such as the atomic number or charge, I can help you calculate the mass number.