Isotopes of hydrogen

Isotopes of hydrogen are hydrogen atoms with different numbers of neutrons. The principal isotopes are protium, deuterium, and tritium.

Protium: The hydrogen isotope with one proton and no neutrons; nearly all natural hydrogen is protium. It is the lightest and most abundant hydrogen isotope.
Mass number: The total number of protons and neutrons in an atomic nucleus. Hydrogen isotope names and symbols encode their different mass numbers.
Heavy water: Water in which ordinary hydrogen is replaced by deuterium. Its nuclear and physical properties make deuterium useful in reactors and experiments.
Deuterium–tritium fusion: A fusion reaction between deuterium and tritium that produces helium-4, a neutron, and energy. This reaction is attractive for fusion energy, but its neutrons activate reactor materials.
Deuterium: The stable hydrogen isotope with one proton and one neutron, commonly called heavy hydrogen. Its extra neutron makes it stable but roughly twice as massive as protium.
Nuclear stability: The tendency of an atomic nucleus to remain intact rather than undergo radioactive decay. Protium and deuterium are stable, whereas tritium decays.
Deuterium labeling: A method that replaces hydrogen atoms with deuterium to trace molecules or alter their properties. Deuterium can mark molecular pathways without changing an element’s identity.
Tritium breeding: The production of tritium from lithium through neutron reactions in a nuclear system. Fusion reactors must breed tritium because natural supplies are limited.
Tritium: The radioactive hydrogen isotope with one proton and two neutrons. It is the only naturally occurring hydrogen isotope that is radioactive.
Beta decay: Radioactive decay in which a neutron or proton changes into the other through emission of a beta particle. Tritium decays by turning a neutron into a proton and emitting an electron and antineutrino.

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