Answer:
Isotopes are atoms of an element that have the same number of protons (Z) and electrons but different number of neutrons and, therefore, different mass numbers (A).
For Iron (Fe) the following is true:
Isotope | A | Z | Protons | Neutrons | Electrons
54Fe | 54 | 26 | 26 | 28 | 26
56Fe | 56 | 26 | 26 | 30 | 26
57Fe | 57 | 26 | 26 | 31 | 26
58Fe | 58 | 26 | 26 | 32 | 26
In each case, the number of protons is the same (Z=26) and number of electrons is the same, but the number of neutrons is different, which results in different mass numbers (A) for each isotope.