Answer:
30
Step-by-step explanation:
A mass number indicates the total number of protons and neutrons. An element is defined by its number of protons, however neutrons can vary (this is what causes isotopes). By definition, iron (Fe) has 26 protons. Since the mass number is the number of protons + the number of neutrons, the number of neutrons is the mass number - the number of protons, in this case 56-26=30
A Fe2+ ion has a positive charge (but its number of protons can't change without it becoming a different element) so since each proton is +1 to charge and each election is -1, to get a +2 charge, there must be 2 more protons than electrons, so a Fe2+ ion has 24 electrons, but this does not affect the mass number or the number of protons.