The lightest chemical element; its symbol is H. Hydrogen normally consists of a single electron in orbit around a nucleus made up of a single proton. It is usually found as a gas and has several uses as a fuel.
Note: Hydrogen atoms are combined to form helium atoms in fusion reactions in stars and in hydrogen bombs, which release huge amounts of energy. Hydrogen also burns rapidly, producing water as it combines with oxygen (see H<sub>8</sub>O and oxidation).
Note: For a time, hydrogen was frequently used to fill blimps and dirigibles because of its extremely low weight. In 1937, however, the hydrogen in the dirigible Hindenburg caught fire, and many of the passengers and crew were killed. Since that time, helium has been widely preferred to hydrogen for use in airships; it is not as buoyant (see buoyancy) or cheap as hydrogen, but, being an inert gas, it does not burn.
Note: Because there is so much hydrogen in stars, it is by far the most abundant element in the universe.