The HNH angle value is higher than HPH, HAsH and HSbH angles. Why?

Explanation

Calculation of bond angle:
(a). The concept of EN has been found to be very useful to explain the variation of bond angle.
For example, for `AB_x` type molecule (A) is the central atom, B are the atoms attached with atom A and x is the number og B atoms), the `B-A-B` bond angle is changed with the change of EN of central atom or with change of size of central atom.
Rule (i): If the EN of central atom decreases or/and size increases bond angle decreases.
Bond angle `(B-A-B)propEN` central atoms and
`prop(1)/("Size of central atom")`
In all hydrides of group 15 elements the central atom is `sp^3-`hydridised Three of the four `sp^3` orbitals form three `A-H`,`sigma`- ponds while the fourth contains the lone pair of electrons.
Since the `(lp-lp)` repulsions are larger than the `(bp-bp)` repulsions therefore in `NH_3` the bond angle decreases from `109.28^@` to `107.5^@` Down the group `(darr)`, the electronegativity of tbhe central atom goes on decreasing as a result bond pairs of electrons lie away and away from the central atom. In other words, force of repulsion between the adjacent bond pairs goes on decreasing and consequently the bond angles keep on decreasing from `NH_3` to `SbH_3` as shown in table. Bond angle order is
`NH_3(107.5^@)gtPH_3(93.3^@)gtAsH_3(91.8@)`
Change of bond angle of hydrides of groups 15 with change of EN and size of central atom.