Explain the shape of ammonia molecule by using VSEPR theory.
The Correct Answer and Explanation is:
The shape of the ammonia (NH₃) molecule can be explained using the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to VSEPR theory, the shape of a molecule is determined by the repulsion between electron pairs in the valence shell of the central atom. In the case of ammonia, the central atom is nitrogen.
Nitrogen has five valence electrons. It forms three single covalent bonds with three hydrogen atoms, using three of its valence electrons. This results in three bonding pairs of electrons. The remaining two electrons form a lone pair. Therefore, there are four regions of electron density around the nitrogen atom: three bonding pairs and one lone pair.
VSEPR theory states that these electron pairs will arrange themselves as far apart as possible to minimize repulsion. Four regions of electron density adopt a tetrahedral arrangement to achieve this. However, the presence of a lone pair affects the molecular shape.
Lone pairs exert greater repulsive force than bonding pairs because they are localized closer to the nucleus of the central atom. This increased repulsion pushes the bonding pairs slightly closer together. As a result, the ideal tetrahedral bond angle of 109.5 degrees is reduced in ammonia. The actual H–N–H bond angle in ammonia is approximately 107 degrees.
Due to this lone pair, the molecular geometry of ammonia is described as trigonal pyramidal, not tetrahedral. The nitrogen atom is at the apex of the pyramid, and the three hydrogen atoms form the base. The lone pair occupies the space above nitrogen but is not visible in the molecular shape.
In conclusion, VSEPR theory helps predict that ammonia has a trigonal pyramidal shape due to the presence of three bonding pairs and one lone pair of electrons around the central nitrogen atom. The repulsion between these electron pairs shapes the final geometry of the molecule.
