Alkali Metal
Definition
The super-reactive metals of group 1: soft, shiny, and so eager to react that they are stored in oil to keep them away from air and water.
Technical definition
Alkali metals are the group 1 elements (Li, Na, K, Rb, Cs, Fr) with a single valence electron in an ns^1 configuration. They are soft, low-density metals with low ionization energies that readily form +1 ions and react vigorously with water to produce hydrogen gas and alkaline hydroxides.
Example
Dropping sodium into water produces fizzing hydrogen gas and enough heat to melt the sodium into a dancing ball, while sodium hydroxide turns the water basic.
Common confusion
Hydrogen sits above group 1 but is NOT an alkali metal. It is a nonmetal gas with very different chemistry.