Question

why the ionic character of the hydrides of alkali metals decrese down the group even though usually the ionic character of the metals increses down the group ?

Answer 1

Dear student

In this question, one has to compare the ionic character of alkali metal hydrides. Although as we move down the group, the metallic character increases because the size increases and thus the tendency to loose the electrons also increases.
But if we are asked to compare the ionic character of hydrides (LiH, NaH, KH, RbH, CsH) , we can not use the Fajan’s rule. This is because, here the anion is very small (H-) as compared to the cations( alkali metal cations)
​The ionic character is this case can thus be compared on the basis of Lattice energy.
Since Lithium ion and the hydride ion are approximately of same size, the charge density is high between the two, thus the ionic bond so formed will be very strong and hence more will be the lattice energy.
On coming down the group, the difference in the size of cation and anion increases, thereby making the ionic bond weaker, thus reducing the lattice energy.
Also, lesser will be the lattice energy, lesser will be the ionic character.
Thus, down the group, the ionic character of the hydrides decreases.

Regards