Deuterium is hydrogen — specifically an isotope of it. Ordinary hydrogen has one proton and no neutron; deuterium has one proton and one neutron, making it roughly twice as heavy. It behaves almost identically in chemical reactions, which is why the difference shows up as weight rather than as different chemistry.
The core difference
Not two elements. One element and one of its isotopes.
- hydrogen (protium) — one proton, no neutron. Around 99.98% of natural hydrogen.
- deuterium — one proton, one neutron. About 0.02% of natural hydrogen.
- tritium — one proton, two neutrons. Radioactive, and very rare.
Heavy water
Water made with deuterium instead of ordinary hydrogen is heavy water (D₂O). It is about 11% denser, and ice made from it sinks in ordinary water. It is used as a neutron moderator in some nuclear reactor designs, which is why its production has historically attracted strategic attention.
Why hydrogen has named isotopes at all
Hydrogen is the only element whose isotopes have their own names. The mass difference between them is proportionally enormous — doubling and tripling — where for heavier elements adding a neutron changes the mass by a few percent, so the isotopes behave near-identically and no separate name is warranted.
Frequently asked questions
Is deuterium a different element from hydrogen?
No. It is an isotope of hydrogen — same element, same single proton, with a neutron added.
What is heavy water?
Water in which the hydrogen is deuterium (D₂O). It is around 11% denser than ordinary water, and deuterium ice sinks.
Why do hydrogen isotopes have their own names?
Because the proportional mass difference is uniquely large — doubling or tripling — where for heavier elements an extra neutron barely changes behaviour.