Question

The ionisation energy of alkali metals decreases from Li to Cs because :

• A. the atomic size increases from Li to Cs
• B. the distance between nucleus and outermost orbital decreases from Li to Cs
• C. electropositive character decreases down the group
• D. melting point decreases from Li to Cs

Answer 1

The correct option is A the atomic size increases from Li to Cs

The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs. This is because the force of attraction between nucleus and the outermost electron is reduced due to intervening of inner electrons. On account of reduced force of attraction, the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge. hence the atomic size increases and it is much more easier to remove electron in gaseous state by providing heat or irradiation with light.