Isotopes are atoms of the same element that have different numbers of neutrons. This means that while they share the same number of protons and electrons (which defines the element itself), their atomic mass is different due to the varying neutron count.
For example, carbon has isotopes such as Carbon-12 and Carbon-14. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. This difference in neutrons causes variations in the atomic mass and can affect the stability of the atom.
Options like the number of electrons, protons, or atomic numbers would not correctly define isotopes. The atomic number remains constant for all isotopes of a given element, as it is determined by the number of protons, which do not change.