The alkali metals are a chemical series. They are the elements in Group 1 of the Periodic Table, excluding hydrogen: that is, lithium, sodium, potassium, rubidium, cesium and francium. They are all highly reactive and rarely if ever found alone in nature.

The alkali metals are silvery colored, soft, low density metals, which react readily with halogens to form ionic salts, and with water to form strongly alkaline (basic) hydroxides. These elements all have one electron in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose one electron to form a singly charged positive ion.

Hydrogen, with a solitary electron, is sometimes placed at the top of Group 1, but it is not an alkali metal; rather it exists naturally as a diatomic gas. Removal of its single electron requires considerably more energy than removal of the outer electron for the alkali metals. As in the halogens, only one additional electron is required to fill in the outermost shell of the hydrogen atom, so hydrogen can in some circumstances behave like a halogen, forming the negative hydride ion. Binary compounds of hydride with the alkali metals and some transition metals have been prepared.

Under extremely high pressure, such as is found at the core of Jupiter, hydrogen does become metallic and behaves like an alkali metal, see metallic hydrogen.

ca:Metall alcal� da:Alkalimetal de:Alkalimetalle es:Alcalino eo:Alkala metalo eu:Metal alkalino fr:M�tal alcalin gl:Alcalino ko:알칼리 금속 he:מתכת אלקלית nl:Alkalimetaal nn:Alkalimetall no:Alkalimetall ja:アルカリ金属 pl:Metal alkaliczny sl:Alkalijska kovina sv:Alkalimetaller zh:碱金属