Final answer:
Isotopes are alternate forms of an atom with identical numbers of protons but varying numbers of neutrons; they have the same atomic number but different atomic masses.
Step-by-step explanation:
Two elements with the same number of protons but a different number of neutrons are called isotopes. Isotopes are alternate forms of an atom that have the same atomic number, due to the same number of protons, but they differ in their atomic mass because they have a different number of neutrons.
This difference in neutron number does not affect their chemical properties, meaning isotopes exhibit the same chemistry. However, their physical properties, such as mass and nuclear stability, can vary significantly. A well-known example of isotopes can be seen in hydrogen: protium with zero neutrons, deuterium with one neutron, and tritium with two neutrons.