Question

definition of isotopes
isotopes of an atom differ in
multiple choice
the locations of their electrons shells
their ability to form ions
their number of electrons
their atomic number
their atomic mass

Explanation:

Isotopes are atoms of the same element with the same number of protons (atomic number) but different numbers of neutrons. Since atomic mass is the sum of protons and neutrons, different neutron counts lead to different atomic masses. Other options: same number of protons (so same atomic number, eliminating "the numbers of their protons/electrons" and "their atomic number"); ability to form ions is not unique to isotopes; number of electrons can vary in ions but isotopes relate to neutrons/mass.

Answer:

The option related to "Their atomic mass" (the last option, assuming the full question is about isotopes: Isotopes of an atom differ in their atomic mass).