Question

How do you arrange `NH_4^+, BF_3, H_2O, C_2H_2`, in increasing order of bond angle?

Answer 1

Determine Geometry of each.
`NH_4^+ => theta_b = 109^o`
`BF_3 => theta_b = 120^o`
`H_2O => theta_b = 105^o`
`C_2H_2 => H - C-= C - H => theta_b = 180^o`

`H_2O < NH_4^+ < BF_3 < C_2H_2` about both carbon centers.

Explanation:

Geometry of HOH => `AX_2E_2` => Bent angular from an `AX_4` Tetrahedral Parent. The 2 free pair repel the 2 bonded pair to compress the bond angle from `109^o` in a tetrahedron to `105^o` => Bent Angular Geometry.

Geometry of `NH_4^+` is `AX_4` tetrahedral => `theta_b = 109^o`

Geometry of `BF_3` is `AX_3` Trigonal Planar `AX_3` => `theta_b = 120^o`

Geometry of `C_2H_2` is linear through two carbon centers.
=> `H - C-= C - H => theta_b = 180^o`