Answer: isotopes
Explanation: Isotopes are atoms of the same element having the same atomic number but different atomic masses. Atomic number of an element is the number of protons contained in the nucleus of an atom. Atomic mass is the sum of the protons and neutrons in the nucleus of an atom.
The difference in the atomic masses is due to the number of neutrons in the atoms. This isotopes have the same number of protons but different number of neutrons. The phenomenon whereby atoms of the same element have the same atomic number but different mass number is called isotopy.
