An alkali is a chemical element in the first column ( group 1 , except the hydrogen ) of the periodic table of elements. The lithium ₃ Li, the sodium ₁₁ Na, the potassium ₁₉ K, rubidium Rb _37, the cesium ₅₅ Cs and francium ₈₇ Fr are alkali metals.

Very reactive, they are never in the elemental state in the natural environment, and react immediately in the presence of moisture, therefore they are kept immersed in mineral oil, such oil Vaseline.

The word comes from alkaline through the loan word alkali from the Arabic al-qily meaning the soda.

Properties

The alkali metals are silver in color (the cesium , however, has golden highlights), soft, low melting point and low density. They react readily with halogens to form ionic salts, and with water to form hydroxides strongly basic.

Element	Atomic mass ( u )	Temperature Melting	Temperature Boiling	Density (Kg / m 3)	Electronegativity ( Pauling )
Lithium	6,941	180.54 C	1 341.85 C	534	0,98
Sodium	22,990	97.72 C	882.85 C	968	0,93
Potassium	39,098	63.38 C	758.85 C	890	0,82
Rubidium	85,468	39.31 C	687.85 C	1 532	0,82
Cesium	132,905	28.44 C	670.85 C	1 930	0,79
Francium	(223)	26.85 C	676.85 C	1 870	0,70

Their electronic configuration is characterized by the presence of an electron n s ^a single, easily lost to find the configuration of noble gases in the previous period: the alkali metals are therefore very easily cations. They always have the ionization potential of the lowest period, the second ionization potential is very high, however, due to the electronic configuration of noble gas alkali cations.

Chemical element			Electronic configuration
No. 3	Li	Lithium	1s 2 2s 1
No. 11	Na	Sodium	1s 2 2s 2 2p 6 3s 1
No. 19	K	Potassium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 1
No. 37	Rb	Rubidium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 1
No. 55	Cs	Cesium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 1
No. 87	Fr	Francium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6 7s 1

The hydrogen with its electron lone 1s ^1, would normally belong to the group of alkali metals. However, the loss of this electron requires more energy than other elements of group 1. Hydrogen is not metal at normal temperature and pressure : it becomes metallic as very high pressures (see metallic hydrogen ).

Identification

Atomic Emission

The vapors of alkali metals (or their ions) excited by heat or electricity are known to emit characteristic colors. Thus the spectroscopy has seen its first steps to achieve, through the experiences of Bunsen and Kirchhoff. The colors are due to the fact that the atomic emission spectrum is a spectrum line , not a continuous spectrum of such black body , evidence of the nature quantum energy levels in atoms and ions. The color features are:

• Lithium : fuchsia intense in fact widely used in pyrotechnics,
• Sodium : intense yellow-orange; used in pyrotechnics and street lighting bulbs,
• Potassium : pale purple.

Reactions

Reaction with water

The alkali metals are known for their violent reaction with water. This violence increases as one descends the group order. Here is the reaction:

Alkali metal + water Alkali metal hydroxide + hydrogen

Example with sodium:

This reaction is very exothermic and is sometimes fire dihydrogen with a yellow flame. With potassium instead of sodium, the flame is lilac.

The reactions of alkali metals with water can be, depending on quantities used, very dangerous.

Reaction in the ammonia

The alkali metals dissolve in liquid ammonia giving blue solutions which are paramagnetic

Given the presence of free electrons, the solution takes more than the sum of the volumes of metal and ammonia. The free electrons make these solutions very good reducing agents.

Reaction with dihydrogen

By reacting with hydrogen, alkali metals form hydrides.

These hydrides are very unstable in solution, because of their very basic nature, and react with water to form hydrogen and hydroxides.

Reaction with dioxygen

By reacting with oxygen, alkali metals form an oxide, soluble in water. However, the reaction must be promoted by heating for example, if the peroxide or hydroperoxides (not oxide) is formed of alkali.

(Oxide formation)

(Peroxide formation)

(Formation of hydroperoxides)

Solvation of the oxide leads to the dissociation of the compounds and sodium oxide. The oxide ion is unstable in solution, and its basic character leads to the deprotonation of water:

Peroxides and hydroperoxides is dismutent into oxygen and oxide.

External Links

• IUPAC : Page of links to the periodic table
• IUPAC : Official Periodic Table of 22/06/2007

• Chemistry portal

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