Answer: Neutrons
Step-by-step explanation:
The isotopes of a particular element can be distinguished from one another by the number of neutrons in each atom. Isotopes are variants of an element that have the same number of protons in the nucleus but different numbers of neutrons. This difference in the number of neutrons causes a variation in the mass of each atom. For example, hydrogen has three isotopes—namely, protium (1H), deuterium (2H), and tritium (3H). Protium contains one proton and one electron, whereas deuterium contains one proton, one electron, and one neutron. Tritium contains one proton, one electron, and two neutrons. All three isotopes have the same atomic number (1), but they differ in their atomic mass.
The number of electrons in each isotope of an element will always be the same since an atom of any element always has the same number of electrons as it has protons. Therefore, the number of electrons is not a distinguishing factor between isotopes.