92.

Discuss the general trends in ionisation enthalpy and electropositive character of alkali metals.

Alkali metals possess lowest ionisation enthalpies in their respective periods. However, within the group, the ionisation enthalpies of alkali metals decrease down the group.

Reason: The atoms of alkali metals are largest in their respective periods, therefore, the outermost electrons, which are far away from the nucleus, experience a less force of attraction with the nucleus and hence can be easily removed. Decrease in ionisation enthalpy, on moving down the group, is due to the increase in the size of the atoms of alkali metals and increase in the magnitude of screening effect is by virtue of an increase in the number of intervening electrons.

Electropositive character: On account of their low ionisation enthalpies, these metals have a strong tendency to lose their valence electrons and thus change into positive ions. Consequently, alkali metals are strongly electropositive or metallic in character. As this tendency for losing electron increases down the group, the electropositive character increases.