Hydrogen

Groups 1 and 2 both belong to the s-block of the periodic table, because in each of their elements the last (highest-energy) electron added enters an 's' subshell. Hydrogen sits at the very top of group 1, but it is treated in its own dedicated section before the alkali and alkaline earth metals because, despite the resemblance, it differs from the rest of group 1 in several important respects. Hydrogen has the simplest possible atomic structure of any element: a nucleus carrying a single unit of positive charge (+1), surrounded by just one extranuclear electron. A hydrogen atom has very little tendency to lose this lone electron, but it readily pairs its electron with another atom's electron to form a covalent bond. Because of this, hydrogen is never found as isolated H atoms under …