protio

Protio is a variant spelling sometimes used for protium, the most common isotope of hydrogen. In English-language science, the standard name for hydrogen-1 is protium, designated by the symbol 1H. The term protio is encountered in older literature or in some languages as a form of the same isotope.

The nucleus of protio contains a single proton and no neutrons, and it is accompanied by one orbiting electron in a neutral hydrogen atom. This makes protium the lightest and simplest nucleus among all elements. It has an atomic mass of about 1.0078 amu and a nuclear spin of 1/2. Protium is a stable isotope and accounts for the vast majority of hydrogen found in the universe.

In nature and laboratory use, protium behaves as ordinary hydrogen in chemical reactions, with isotopic substitution producing small kinetic isotope effects. The presence of deuterium (2H) and tritium (3H) alongside protium gives hydrogen a trio of isotopes used in a range of applications, including isotopic labeling, tracing, and studies of reaction mechanisms. Protium’s abundance is enormous compared with its heavier cousins; in the universe, most hydrogen is protium, and on Earth, water contains trace amounts of deuterium and nearly no tritium under normal conditions.

Etymologically, protium derives from Greek protos meaning “first.” The spelling protio appears mainly as a historical or language-specific variant. In contemporary English, protium is the preferred term for hydrogen-1.