Hydrogen is a gas element which has an atomic number of 1 and an atomic weight of 1.0079. It combines with oxygen to form water (H20) and is present in all organic compounds. A few types of bacteria can metabolize atmospheric hydrogen (H2). Hydrogen gas itself is not poisonous, but when it mixes with air it can easily ignite or explode. Hydrogen was discovered by Henry Cavendish in 1766 and was named by Lavoisier. There are two main isotopes of hydrogen: deuterium (2H) and tritium (3H, which is radioactive and is used in some glow-in-the-dark paints and as a tracer in biological studies).