In the methane molecule (CH₄), the bond angle between the hydrogen atoms is 109.5 degrees. This angle is a result of the tetrahedral geometry around the central carbon atom, which has four valence electrons that form single bonds with four hydrogen atoms, each contributing one valence electron.

When examining ammonia (NH₃), the nitrogen atom, which is in group 5A, has five valence electrons. Three of these electrons are used to form bonds with three hydrogen atoms, while the remaining two electrons form a lone pair on the nitrogen. The presence of this lone pair affects the molecular geometry, leading to a trigonal pyramidal shape.

The lone pair exerts a repulsive force that compresses the bond angles between the hydrogen atoms. Consequently, the bond angle in ammonia is approximately 107.3 degrees, which is smaller than the 109.5 degrees observed in methane. This decrease in bond angle is a key concept in understanding molecular geometry and the influence of lone pairs on bond angles.