How to Predict the Shape of a Molecule by VSEPR?

In 1957 “Valence shell electron pair repulsion (VSEPR) theory” was developed by Gillespie. This theory is quite successful in predicting the shape of a molecule and bond angles more exactly. VSEPR also uses electron pairs of central atom to predict the shape of a molecule like ​Sidgwick-Powell theory but, it gives equal importance to all electron pairs including bonding pairs and lone pairs.

Let’s take an example of NH₃ molecule. Its name is Nitrogen trihydride, it is known as ammonia. Draw the Lewis dot structure for NH₃. You will find 1 lone pair of N and 3 bonding pairs. Now you have to find a best possible arrangement for 1+3= 4 electron pairs. Yes, of course tetrahedral arrangement is best suited for them. But Lone pair raise objection on this arrangement. It says it must get privilege as it is exclusively owned by central atom while bonding pairs are shared between central atom and other atoms, and the judgment goes in favour of lone pair. When all 4 pairs are arranged in tetrahedral shape, lone pair gets freedom to dominate bonding pairs and it start pushing nearby bonding pairs and distort the tetrahedron. Lone pairs are like ghost because their presence influences the shape but they aren’t visible in the shape of the molecule. In tetrahedron 3 corners are occupied by bonding pairs and 4th place is occupied by lone pair. Because lone pair is not visible that’s why the NH₃ molecule looks like a pyramid.

Now let’s have a fresh look on H₂O molecule. In Lewis dot structure you must have spotted 2 lone pairs and 2 bonding pairs. Again you have 4 electron pairs to arrange in a shape. Obviously we try to arrange them in a tetrahedral shape. But this time situation is more difficult because central atom has 2 lone pairs. These ghosts are really aloof (unfriendly), they push bonding pairs as well as each other. So that the tetrahedron in which 2 corners are occupied by lone pairs and 2 by bonding pairs is distorted a lot. That’s why the resulting shape of H₂O molecule is bent shaped.

The VSEPR theory uses number of electron pairs of central atom to predict the shape of a molecule and also consider the presence of lone pairs. And it suggests that the repulsion between lone pairs-lone pair is greater than the lone pair-bonding pair which in turn greater than the repulsion between two bonding pairs. Thus the presence of lone pair distorts the ideal shape and results in decreased bond angle.

I hope you have understood the role of ghost lone pairs; then try to predict the shape of NF₃ molecule. It has 3 bonding pairs and 1 lone pair (you must have found similarity with the NH₃ molecule). We can arrange them in tetrahedron but lone pair will distort it and we will get a pyramidal shape like we have got in case of NH₃ molecule. But the shape of these two molecules are not identical, the angle of F-N-F is smaller than the H-N-H. Can you spot the difference between these two molecules? In the next post we will see how VSEPR solves this puzzle.

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