Alkali Metals
Li, Na, K, Rb, Cs and Fr belongs to IA group. Oxides of Li, Na, K, Rb and Cs dissolve in water giving strong alkalies. So, these elements are known as alkali metals.
General electronic configuration is ns1, they are members of s block.
Occurrence of Alkali Metals
These elements are highly reactive and do not occur in free state. All the alkali metals are silvery white, soft and light metals.
General Properties
Size of the atoms: Atomic Radii
The alkali metal atoms have the largest atomic radii in their respective periods. These radii go on increasing on going down the group.
Density
Alkali metals are light metals having low densities. Densities of alkali metals increases from lithium to caesium The density of potassium is lesser than that of sodium contrary to the expectation. This is probably because of the abnormal increase in atomic size on moving from Na (186 pm) to K (227 pm). Hence potassium is lighter than sodium. Lithium is the lightest known metal.
Melting and Boiling Points
Melting and boiling points of alkali metals are quite low and decrease down the group.
Ionisation Energies
The first ionisation energy of alkali metals is the lowest amongst the elements in their respective periods. The alkali metals show great tendencies to lose the only s-electron present in their valence shells after which they acquire stable noble gas configuration.
The first ionisation energies of elements decrease on moving down the group. The second ionisation energies of all the alkali metals are very large because when one electron is lost from these elements the resulting ions acquire noble gas configurations which are very stable and have high effective nuclear charge.
Action of Air
Alkali metals are so reactive that they tarnish rapidly when exposed to air because of the formation of oxides, hydroxides and ultimately carbonates at the surface.
Action of Oxygen
Alkali metals when heated with oxygen or excess of air form oxides the nature of which depends upon the nature of alkali metals.
Reaction with Water
The alkali metals reacts vigorously with water by liberation of and large amount of heat.
Action with Hydrogen
Alkali metals combine with hydrogen at about 673 K forming ionic hydrides. These ionic hydrides have high melting points.
Action with Halogens
Alkali metals combine readily with halogens (X2) to form ionic halides. All halides of alkali metals (except LiF) are highly soluble in water.