Hydrogen

Hydrogen is the chemical element of atomic number 1. Its symbol is H, it has an atomic weight of 1.008, its specific gravity at 32°F (0°C) is 0.0000899, and it melts at -434.7°F (-259.3°C). The boiling point of hydrogen is -423.2°F (-252.9°C), just above absolute zero. Boiling liquid hydrogen is the coldest substance known, with the exception of liquid helium. At room temperature, hydrogen is a colorless, odorless, tasteless gas. It consists of two stable isotopes of mass numbers 1 and 2.

Hydrogen is "number one" among the chemical elements. That is, it is the element whose atomic number is one. Its atoms are the simplest and lightest of all. A hydrogen atom contains only one electron, and it has a nucleus that consists of nothing but a proton. (A small percentage of hydrogen nuclei also contain one or two neutrons; see below.) In the periodic table, it is in a class by itself; there are no other members of its exclusive "group." It is usually placed at the top, all by itself.

Hydrogen's name is a clue to its most important position among the world's elements. It comes from the Greek hydro, meaning water, and genes, meaning born or formed. Hydrogen is a substance that gives birth to water (with a little help from oxygen). The name was coined in 1783 by the French chemist Antoine Lavoisier (1743-1794) in honor of the fact that when hydrogen burns in air it reacts with oxygen to form water, H₂O.