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 are rarely found in elemental form in nature.
The alkali metals are silver-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.
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