Isotopes are atoms of the same element having different atomic masses.
EXPLANATION
All atoms of the same element possess identical number of protons. However, they may have different number of neutrons.
This phenomenon is termed as isotopy and the atoms of the same element having the same number of protons but different number of neutrons are termed as isotopes.
The isotopes of the same element have the same chemical properties due to possession of identical number of electrons.
EXAMPLE
Hydrogen possess three isotopes which are; Protium, Deuterium and tritium.