The alkali metals are a series of chemical elements comprising Group 1 (IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). (Hydrogen, although nominally also a member of Group 1, very rarely exhibits behavior comparable to the alkali metals). The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group.

The alkali metals are all highly reactive and are never found in elemental form in nature. As a result, in the laboratory they are stored under mineral oil. They also tarnish easily and have low melting points and densities. Potassium and rubidium possess a weak radioactive characteristic (harmless) due to the presence of long duration radioactive isotopes.

The alkali metals are silver-colored (caesium has a golden tinge), 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, i.e. cation.

Hydrogen, with a solitary electron, is usually placed at the top of Group 1 of the periodic table, but it is not considered 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.

Alkali metals have the lowest ionization potentials in their respective periods, as removing the single electron from the outermost shell gives them the stable inert gas configuration. Their second ionization potentials are very high, as removing an electron from a species having a noble gas configuration is very difficult.

Series of alkali metals, stored in mineral oil (note "natrium" is sodium.)

Alkali metal + water ? Alkali metal hydroxide + hydrogen gas

With potassium as an example:

2K (s) + 2H₂O (l) ? 2KOH (aq) + H₂ (g)

Trends

The alkali metals show a number of trends when moving down the group - for instance, decreasing electronegativity, increasing reactivity, and decreasing melting and boiling point. Density generally increases, with the notable exception of potassium being less dense than sodium, and the possible exception of francium being less dense than caesium.

Alkali metal	Standard Atomic Weight (u)	Melting Point (K)	Boiling Point (K)	Density (gˇcm?3)	Electronegativity (Pauling)
Lithium	6.941	453	1615	0.534	0.98
Sodium	22.990	370	1156	0.968	0.93
Potassium	39.098	336	1032	0.89	0.82
Rubidium	85.468	312	961	1.532	0.82
Caesium	132.905	301	944	1.93	0.79
Francium	(223)	295	950	1.87	0.7

Reference material

• "Physics Update." Physics Today June 1996: 9.

• "Visual Elements: Group 1 - The Alkali Metals." Visual Elements. Royal Society of Chemistry. <http://www.chemsoc.org/Viselements/pages/data/intro_groupi_data.html>.

See also

• Lithium
• Sodium
• Potassium
• Rubidium
• Caesium
• Francium
• Hydrogen

External links

• Science aid:Alkali metals A simple look at alkali metals
• Atomic and Physical Properties of the Group 1 Elements An in-depth look at alkali metals

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