The hydrides are ionic.

For the rest of the Group

15 hydrides, the lone pair is not

readily available for bonding.

The centers are sterically hindered due to short bond lengths and it is difficult to coordinate to the lone pair.

There are fewer valence

electrons available.

It resembles both alkali metals and halogens

Its reactivity is similar to halogens.

Ionization enthalpy of hydrogen resembles that of alkali metals.

Loss of an electron from hydrogen atom results in a nucleus of very small size as compared to other atoms or ions. 

tritium

deuterium

protium

hydrogen

LiH < NaH < KH < RbH < CsH

LiH > NaH > CsH > KH > RbH

LiH > NaH > KH > RbH > CsH

LiH < NaH < CsH < KH < RbH

-1

0

+1

-2

Hydrogen and halogen only resembles in its first letter

Its tendency to lose an electron to form a cation

Its tendency to gain a single electron in its valence shell to attain stable electronic configuration

Its low negative enthalpy value

0.65

1.65

1.0

1.70