The only stable isotope of hydrogen that has no neutron is protium. Protium is hydrogen-1, written as ¹H, and its nucleus contains one proton and zero neutrons. Because nearly all of its mass comes from that single proton, it is the simplest atomic nucleus found in nature.
Protium makes up more than 99.98% of naturally occurring hydrogen. Its electron arrangement is also the simplest possible for a neutral atom: one proton in the nucleus balanced by one electron. The word “protium” is used to distinguish ordinary hydrogen from its other named isotopes, deuterium and tritium.
Deuterium, or hydrogen-2, has one proton and one neutron and is stable. Tritium, hydrogen-3, has two neutrons and is radioactive, with a half-life of about 12.3 years. Heavier hydrogen isotopes, including hydrogen-4, can be produced briefly in laboratories but are highly unstable.
Protium is considered stable because proton decay has never been observed. Some grand-unified theories predict that protons might eventually decay, but experimental limits place any possible lifetime vastly beyond the age of the universe.