isotopes of hydrogen (protium, deuterium, tritium)
/ protium = PROH-tee-um; deuterium = doo-TEER-ee-um; tritium = TRIT-ee-um /
Most elements have isotopes that differ by only a few percent in mass, so the variants behave almost identically. Hydrogen is the dramatic exception. Its three isotopes — protium (ordinary H, one proton), deuterium (D, one proton plus one neutron), and tritium (T, one proton plus two neutrons) — differ by factors of two and three in mass. When the heavier isotope is twice or three times the weight of the lightest, even chemistry starts to notice.
All three have the same single electron and so the same chemical valence, but their nuclei differ. Protium (H-1) makes up about 99.98 percent of natural hydrogen. Deuterium (H-2) is a stable, rare isotope; water made with it, D2O, is the famous heavy water, denser than ordinary water and used to slow neutrons in some nuclear reactors. Tritium (H-3) is radioactive, decaying by beta emission with a half-life of about 12 years; it is made in reactors and used as a tracer and in self-luminous signs. Because the mass difference is so large, deuterium bonds break more slowly than protium bonds — the kinetic isotope effect — which chemists exploit to probe reaction mechanisms.
These isotopes matter far beyond a textbook curiosity. Deuterium labeling lets chemists trace exactly which hydrogen atom moves in a reaction; NMR spectroscopy routinely uses deuterated solvents; and deuterium-tritium fusion is the reaction that powers the hope of fusion energy and the hydrogen bomb. The honest caveat is that for almost all other elements, isotope substitution barely changes chemistry — hydrogen is special precisely because it is the lightest element, so a single neutron is a big relative change.
Heavy water, D2O, looks and tastes much like ordinary water but is about 11 percent denser, freezes and boils a few degrees higher, and slows neutrons well — which is why some reactors use it as a moderator.
A single extra neutron per atom is enough to give heavy water noticeably different physical properties.
Isotopes are not different elements: protium, deuterium, and tritium are all hydrogen, with identical proton count and electron arrangement. Only the neutron number — and hence the mass and nuclear stability — differs.