Henry Cavendish is credited with discovering hydrogen in 1766 in London, though alchemist Paracelsus in the 1500s and Robert Boyle in 1671 did preliminary work on the subject. However, Cavendish identified hydrogen gas as a discrete substance. In 1783 Antoine Lavoisier provided the name hydrogen, Greek for water former.
Hydrogen's atomic number is 1, and its chemical symbol is H. Of all elements on the periodic table, hydrogen is the lightest. It is also the most abundant chemical substance in the known universe. Hydrogen gas is extremely flammable. A much rarer form of hydrogen, called deuterium, was discovered in 1931 by Harold Urey. It has double the mass of the regular form of hydrogen.