Introduction

Hydrogen is the simplest atom in the periodic table: its nucleus is a single proton, orbited by a single electron. This makes it unique among all elements except helium, because there is no inner shell of electrons standing between the lone valence electron and the pull of the nucleus -- the valence electron sits directly in the nucleus's sphere of action.

Because that single electron is unpaired, an isolated hydrogen atom is reactive, and elemental hydrogen is never found as free atoms under ordinary conditions -- it pairs up into the diatomic molecule H₂, sharing electrons to complete a stable duet. Even so, dihydrogen gas is only a very minor constituent of the earth's atmosphere, since hydrogen is light enough that most of the free gas escaped into space over geological time. …