The nucleus contains both protons and neutrons. An element has a fixed number of protons but may exist with various numbers of neutrons. The sum of the protons and neutrons is the mass number. The two forms of helium are called isotopes of helium. Isotopes of an element have the same chemical properties but different weights.
Some elements have many isomers.
Naturally abundant isotopes exist around us because their half-lives are longer than the age of the earth. Most isotopes have short half-lives and must be produced in the laboratory to study or use. 60Co is used for radiation therapy of cancer patients. Over 3500 isotopes are known, and most are merely laboratory curiosities. In some cases more than one form of an isotope can exist.