Question

Why does NH3 have a larger bond angle than PH3?

Answer 1

`"NH"_3` has a bond angle of about `106.67^@`, while `"PH"_3` has a bond angle of about `93.3^@`, according to CCCBDB. Here are their Lewis structures, to-scale with bond angle and bond length:

My (sequential) rationale is that

1)

Nitrogen atom is smaller than phosphorus atom (one less quantum level), so the `"N"-"H"` bond length is shorter than the `"P"-"H"` bond length (`"N"` would have a harder time reaching `"H"` if at the `"P"-"H"` bond distance).

(Actually, they are `"1.012 Å"` and `"1.421 Å"`, respectively.)

2)

As a result, the bonding electrons are on the average closer to the central atom's electron density, thus moving the `"H"` atoms closer together in `"PH"_3` compared to in `"NH"_3`.

We know that `"H"` atoms have a finite radius, so when the bonding distance is shorter between `"H"` and the central atom, each `"H"` electron cloud becomes closer to the other (on the average).

That induces charge-charge repulsions which push the `"N"-"H"` bonds apart, increasing the `"H"-"N"-"H"` bond angle relative to the `"H"-"P"-"H"` angle.