The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Hydrogen (H), although nominally also a member of group 1, rarely exhibits behavior comparable to the alkali metals. This group lies in the s-block of the periodic table; all alkali metals' outermost electrons lie in an s-orbital. The alkali metals provide one of the best examples of group trends in properties in the periodic table, with elements exhibiting well characterized homologous behavior. For instance, when moving down the table, all alkali metals show decreasing electronegativity, increasing reactivity, and decreasing melting and boiling points. In general, their densities increase, with the notable exception that potassium is less dense than sodium.
All discovered alkali metals are naturally occurring, although francium is the second-rarest naturally occurring element, after astatine. All are highly reactive metals under standard conditions. Experiments have been conducted to attempt