Caption: An abstract diagram of the 3 main isotopes of the element hydrogen (H, atomic number Z=1). Note, atoms and nuclei do NOT look like these representations.

The isotope specifications:

1. protium (H-1, nucleus proton p, atomic number Z=1, neutron number N=0, stable, terrestrial mass fraction 99.9855 %). Note, protium has the simplest of all nuclei, a single proton.
2. deuterium (H-2, nucleus deuteron D, atomic number Z=1, neutron number N=1, stable, terrestrial mass fraction 0.0145 %). The deuteron is the second simplest of all nuclei consisting of a single proton and a single neutron.
3. tritium (H-3, nucleus triton T, atomic number Z=1, neutron number N=2, unstable: half-life 12:32(2) tropical years, trace).

Note, the term protium is seldom used and instead hydrogen (H) (or ordinary hydrogen (H)) is used as a synonym. Heavy hydrogen is sometimes used as a descriptive synonym for deuterium. The unstable tritrium radioactively decays to the stable daughter element (i.e., radioactive decay product) helium (He-3) by a β- decay (Wikipedia: Tritium: Decay). Various natural and artificial processes produce tritrium on Earth (Wikipedia: Tritrium).

Explication of isotopes:

1. The number of protons in the nucleus determines the element (i.e., chemical species) and its chemical reactions nearly entirely. This is because the electronic structure of an element follows from the electric charge of the nucleus nearly entirely.

2. An element is divided into isotopes which differ from each other in number of neutrons in the nucleus, and so in atomic mass.

   Chemically, isotopes are nearly identical since differing atomic mass has only a small effect on chemical reactions. The more massive isotopes have more inertia and that usually leads to slower chemical reaction rates.

   Note, isotope is sort of a tricky word to define since it is a relationship word like brother: all men are brothers, but NOT all men are brothers to each other.

   Mutatis mutandis, all atoms are isotopes, but NOT all atoms are isotopes to each other.

3. From nuclear physics perspective, isotopes of an element have very different properties: they can have very different nuclear reactions and degrees of nuclear stability.

   The most common isotope of an element is usually nuclearly stable (i.e., will usually NOT spontaneously radioactively decay to another daughter element). However, greater the difference neutron number of an isotope is from that of the most common isotope (here assumed to be stable), the less nuclearly stable it usually is and for a large enough difference, the isotope will be radioactive (i.e., will spontaneously radioactively decay to daughter element).

   Note, some elements have NO stable isotopes. For example, uranium (U, Z=92) has NO stable isotopes, but its longest half-life isotopes (Wikipedia: Uranium) are effectively stable over the time scale of human history.