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Properties of Alkaline Metals

 

Alkaline Metals, also called Group IA elements of the periodic table show certain properties that differentiate them from other elements.

The alkaline metals consist of six elements, with one electron in the outermost shell: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

They are the most reactive of metals due to the ease with which the one outermost electron is removed to form a unipositively charged ion, which is isoelectronic (i.e. of the same electronic configuration) with the noble gases. Hence, they are not found free in nature.

M → M+ + e-

Properties - They react vigorously with water, liberating hydrogen gas and forming hydroxides that are highly soluble.

2M + 2H2O → 2(M+ + OH-) + H2

For this reason, metals of this family are known as alkali metals.

- They react in similar fashion with alkanols to give alkoxides.

- They react violently with acids.

- They have high electrical conductivity due to the presence of metallic bonding which allows free movement of bonding electrons.

- They are strong reducing agents due to their readiness to lose their outermost electrons.

- All but lithium combine readily with water on contact with air to form normal oxide (containing the O2- ion). Lithium forms a normal oxide when heated with oxygen.

- The oxides react vigorously with water to form the corresponding hydroxides. M2O + H2O → 2(M+ + OH- )

- They form peroxides (containing the O2 2- ion). Except lithium, the peroxides of the metals are formed by heating the metal with oxygen.

2M + O2 → M2O2 Lithium peroxide can be obtained as a precipitate by adding hydrogen peroxide to a solution of lithium hydroxide.

- The peroxides react with water to give hydrogen peroxide.

- Na, K, Rb, and Cs also form superoxides, MO2 , which contain the O2 - ion.

- They form ionic hydrides with hydrogen at high temperatures.

2M + H2 → 2M+H-

- They also combine with the halogens to form alkali-metal halides.

2M + X2 → 2M+X-

- All alkali-metal salts are colorless, except those that contain colored anions, such as yellow Na2CrO4 and purple KMnO4.

- Their salts dissolve in water with dissociation into their component ions.

- The force holding a crystal of an alkali metal together is due to the attraction between the metal ions and the electron cloud. Since this is not a strong force, the structure is not a tightly packed one. Consequently, all the alkali metal, except lithium are so soft that they can be easily cut with a knife, and they are the least dense of all metals. lithium, sodium and potassium are less dense than water Lithium is the lightest metal known. - They have low melting points which decrease with increasing atomic number (see table below).

 

Element

Density (g/ml) at 20 oC

Melting Point oC

Boiling Point oC

Li

0.53

180

1326

Na

0.97

97.5

889

K

0.86

63.4

757

Rb

1.53

38.8

679

Cs

1.90

28.7

690

 

   

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