Isotopes are a group of atoms belonging to an element with the same atomic number but a different mass number. Such a group of atoms have the same number of protons in their nuclei but differ only in the number of neutrons.
For example, the Oxygen atom has three isotopes – 16O, 17O, and 18O. All three isotopes have 8 protons; however, the number of neutrons is 8 in 16O, 9 in 17O, and 10 in 18O.
Isotopes have a wide range of applications in medicine, agriculture, archaeology, mining, and power generation.