Three naturally occurring isotopes of hydrogen: protium, deuterium and tritium.

Isotopes are atoms that have a similar atomic number but have a different number of neutrons in the nuclei and thus a different mass number. They have nearly similar chemical properties but can have different physical properties. Some isotopes can be radioactive, then we speak of a radioisotope. There are 254 known stable isotopes and more than 3,000 radioisotopes.

For example, in the fission reaction in nuclear power plants, the uranium isotope 235 is used.

The fusion of two isotopes of hydrogen, deuterium and tritium, leads to the release of large amounts of energy, and these two isotopes are the fuel of the future for thermonuclear fusion power plants.