Question

The bond angles in nh4+ and ch4 are same but nh3 has different bond angle. Why?

Answer 1

according to VSEPR (i.e., valance shell electron pair repulsion) theory, bond angle depends on no of bond pairs and lone pairs.

here, in NH4^+

bond pairs = 4

and lone pair = (5 + 4 - 1 - 2 × 4)/2 = 0 [ using formula, (Valence electron - n × 8 or 2n )/2 ,for hydrogen use 2n, otherwise use 8n ]

in CH4 , bond pairs = 4 but lone pair = (4 + 4 - 2 × 4)/2 = 0

here, you can see both species NH4^+ and CO2 have only bond pairs i.e., 4. so, these molecules are sp³ hybridised tetrahydral in shape. we know, bond angle of tetrahydral is 109.5°. hence, due to same geometrical shape, bond angle for both cases are same.

but in case of NH3,

bond pairs = 3 and lone pairs = (5 + 3 - 2 × 3)/2 = 1

of course, it is also sp³ hybridised but it is trigonal pyramidal in shape. so, bond angle of NH3 is ~107°

here it is clear that bond angles of CH4 and NH4^+ are same but NH3 has different bond angle.

eventually, you can say bond angle of NH3 is different due to present of one lone pair on it.