VSEPR Theory - Practice Questions & MCQ

VSEPR Theory

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate bond angles around a central atom of a molecule from the estimation of the number of bonds and lone pairs of electrons in its Lewis structure. 

The main postulates of VSEPR theory are:

• The actual shape of molecule depends upon the number of electron pairs (bonder or non–bonded) around the central atom.

• The electron pairs tend to repel each other due to their negative charge.

• Electron pairs arrange themselves in such a way that there exists a minimum repulsion between them.

• The valence shell is considered as a sphere with the electron pairs placed at distance.

• A multiple bond is treated as if it is a single electron pair & the electron pairs which constitute the bond as a single pair.

• The repulsive interaction of electron pairs decreases in the order as mentioned below:

Lone pair (lp) – Lone pair (lp) > Lone pair (lp) – Bond pair (bp) > Bond pair (bp) – Bond pair (bp).

• Double bonds cause more repulsion than single bonds, and triple bonds cause more repulsion than a double bond. This repulsion decreases sharply with increasing bond angle between the electron pairs.

Let us understand VSEPR theory using a gaseous BeF₂ molecule. The Lewis structure of BeF₂ as shown in the figure, there are only two electron pairs around the central beryllium atom. With two bonds and no lone pairs of electrons on the central atom, the bonds are as far apart as possible, and the electrostatic repulsion between these regions of high electron density is reduced to a minimum when they are on opposite sides of the central atom, thus the bond angle is 180°.

The BeF₂ molecule adopts a linear structure in which the two bonds are at maximum distance from each other and maintain an angle of 180°.

As given in the table below, two regions of electron density around a central atom in a molecule form a linear geometry, three regions form a trigonal planar geometry, four regions form a tetrahedral geometry, five regions form a trigonal bipyramidal geometry, and six regions form an octahedral geometry.