Discovered in 1766, the hydrogen atom is the simplest atom that can possibly exist. The most common isotope is composed of a single proton and an electron. There is relatively little hydrogen gas in the earth's atmosphere, but there are plenty of hydrogen atoms in compounds like water. Consider that every molecule of water in all the seas, lakes and streams contains two hydrogen atoms, hydrogen is one of the ten most abundant elements on the earth.

Hydrogen is a light colorless gas, which has no smell or taste when pure. It burns explosively in air or oxygen to form water, H₂O. It combines directly with nonmetals to form compounds. With reactive metals such as lithium, sodium, and calcium, hydrogen forms metal hydrides. These decompose in water and liberates hydrogen gas. The cation H⁺, is characteristic of acids in aqueous solution.

The principal industrial sources of hydrogen are:

• electrolysis of aqueous solutions of sodium chloride, table salt.
• reaction of carbon with water to form carbon monoxide and hydrogen gas,
• cracking processes in oil refineries, and
• reaction of methane, CH₄ (in natural gas), or other simple hydrocarbons with steam.

There are three isotopes of hydrogen:

1. ¹H, occurs in nature at 99.9985% abundance,
2. ²H, or D, also known as deuterium or heavy hydrogen occurs in nature at 0.015% abundance,and
3. ³H, or T, also known as tritium, are found only in trace amounts.

The most common compound is deuterium oxide, D₂O, or heavy water. This name is appropriate because the deuterium atom is twice as heavy as ¹H. Since deuterium occurs naturally, in every 7000 molecules of ordinary H₂O, there is one D₂O. Heavy water can be prepared by prolonged electrolysis of ordinary water. Approximately 100,000 gallons of water have to be carefully electrolyzed to produce a single gallon of pure heavy water. Considering the cost of the electrical energy involved in such a process, heavy water is generally regarded as a scarce commodity. Heavy water is a suitable and convenient moderator in nuclear reactors.

Tritium, ³H, is extremely rare in nature. It occurs in ordinary water, but only in portions of one atom for every 10¹⁸atoms of ¹H. Tritium is more effectively produced by nuclear reactions than by separations from water by electrolysis.