Question

Explain the shapes of Methane, Ammonia and Water molecules using VSEPR theory.

Answer 1

VSEPR theory uses the number of bond pairs and lone pair of electrons present around the central atom in a molecule is to determine the shape of the molecule. The theory is based on the principle that the bond pairs and lone pairs of electrons around a central atom should be placed as further away as possible as electrons repel each other. Also, repulsion between two bond pairs is the least and between two lone pairs is the maximum.
The first step In determining molecular geometry by VSEPR theory is to determine the Lewis structure, number of bond pairs and lone pair of electrons areound the central atom. This gives the type of molecule and the molecular geomtry associatedwith the type of the molecule.

1. Methane, CH₄

C has 4 valence electrons
H has 1 and so for four H atoms, the number of valence electrons = 4
Total number of valence electrons = 8
4 C-H bonds form 4 bond pairs and use all the 8 valence electrons.
So, the molecule has C as central atom which forms 4 bonds one with each H atom.
4 bond pairs are present in the molecule and type of molecule is AB₄ (central atom A surrounded by 4 bond pairs)
4 bonds pairs having the maximum possible bond angle can be arranged in space in a tetrahedral geometry.

2. Ammonia, NH₃

N: 5 valence electrons
H: 3 X 1 = 3 valence electrons
Total valence electrons = 8 (4 pairs)
Bond pairs = 3 N-H
Electrons present as bond pairs = 3 pairs
Electrons present as lone pair = 1 pair

So, the type of molecule is AB₃X (central atom A surrounded by 3 bond pairs B and one lone pair X)

The best way to have 4 electrons pairs around a central atom is tetrahedral. The molecular geometry is determined by position of atoms and lone pair is not considered. So, an AB₃X​ type of molecule has a trigonal pyramidal shape.

3. Water , H₂O

O: 6 valence electrons
H: 2 X 1 = 2 valence electrons
Total valence electrons = 8 (4 pairs)
Bonds = 2 O-H
Electrons present as bond pairs = 2 pairs
Electrons present as lone pair = 2 pair

So, the type of molecule is AB₂X₂ (central atom A surrounded by 2 bond pairs B and 2 lone pairs, X)
An AB₂X₂ type molecule is bent shaped.