structures ⇒ Refer image,

here, the single lone pair causey lone pair bond pair repulsion, for this steric effect, the bp-bp repulsion decreases and the hydrogen atoms come closer to each other. So the molecule loses the identical bond angle (109°28) and the bond angle becomes 107°.

NH₄⁺ ⇒ Hybridisation = Sp³

structure, Refer image,

here, the single lone pair is involved in bonding with a more hydrogen atom and hence, the structure is a perfect tetrahedral.