Hydrogen

Lightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen’s isotopes are:

  1. 1H (99.9885 %)
  2. 2H (0.0115 %), also called deuterium.
  3. 3H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth’s hydrosphere and atmosphere.