Hydrogen. from the greek words "hydro" and "genes" meaning "Water-generator"
Hydrogen is the lightest element. It is by far the most abundant element in the universe and makes up about about 90% of the universe by weight. Hydrogen as water (H2O) is absolutely essential to life and it is present in all organic compounds. Hydrogen gas was used in lighter-than-air balloons for transport but is far too dangerous because of the fire risk (Hindenburg).
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen with a natural abundance in Earth's oceans of about one atom in 6,400 of hydrogen (~156.25 ppm). Deuterium accounts for approximately 0.0156% (or on a mass basis: 0.0312%) of all naturally occurring hydrogen in Earth's oceans, while the most common isotope (hydrogen-1 or protium) accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another.
The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen isotope, protium, has no neutron in the nucleus. The deuterium isotope's name is formed from the Greek deuteros meaning "second", to denote the two particles composing the nucleus.
Tritium is an isotope of hydrogen which has one proton and two neutrons . It emits beta particles only and has a half-life of 12.3 years. The maximum beta energy is 18 keV, and the mean beta energy is 6 keV. A beta particle with energy of less than 70 keV will not penetrate the dead outer layer of the skin. Therefore, tritium is not an external radiation hazard , but when taken into the body it becomes an internal hazard .