An atom is the smallest indivisible particle of matter, i.e. Something that cannot be divided consisting of protons, electrons and neutrons; protons and neutrons are present in nucleus. While Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons—the number of protons plus neutrons.
