The difference between isotopes for the same element will be found in the atoms'
a. number of protons.
b. number of electrons.
c. number of neutrons.
d. charges.

The mass of an atom comes from the protons and neutrons that are found in the nucleus. The number of protons is the atomic number of an element. To find the number of neutrons, subtract the atomic number from the mass of an atom. So, if an atom is missing a neutron or has an extra neutron, the atom is called an isotope. An atom is still the same element if it is missing an electron. That is also true for isotopes. They are the same element but their number of neutron differs.