How does electronegativity affect the Shape of a Molecule?

I hope you have understood the role of ghost lone pairs; how they distort the ideal shape of a molecule. Who do you think stands firm against the lone pairs and put a stop to their action? Bonding pairs guard the shape of a molecule and resist any changes made by lone pairs. Think of a situation when these guards are shifted away for some reason, what do you think will happen? Obviously lone pairs will perform their evil doing uninterruptedly. Let’s see a few examples where similar situation happens.

In NF₃ molecule, central atom has 3 bonding pairs and 1 lone pair (like NH₃ molecule). We can arrange them in tetrahedron but lone pair will distort it and we will get a pyramidal shape as we have got in case of NH₃ molecule.

Now focus on NF₃ molecule. Here N is bonded to F. F belongs to the 17^th group which has largest electronegativity and F is the most electronegative element of the periodic table. That’s why when N is bonded to F, F wins the war over bonding electrons and gets maximum share of it.

Here F is the culprit who shifts bonding electrons towards itself thus weakens the defense against ghost lone pairs. In this situation lone pair succeeds to distort the shape a lot. That’s why the angle of F-N-F is found 102°30′ which is much smaller than expected.

While in NH₃ molecule, the electronegativity difference between N and H is comparable. That’s why bonding pairs are present on their guarding posts to guard the shape of the molecule. As a result the lone pairs could cause very little distortion. The angle of H-N-H is found 107°48′ which is just a little smaller than the expected (109°28′).

This similar situation happens in case of H₂O and F₂O molecule. I am sure you will be able to figure it out which one has the smaller angel? Yes certainly angle of F-O-F will be smaller than the angle of H-O-H.

After the VSEPR theory scientist were able to predict the shape of a molecule successfully but they didn’t stop there. They wanted to know what happens inside the atom when it is bonded to another atom and how a molecule is formed. To explain it, Linus Pauling proposed the Valence Bond Theory (VBT) which gave the orbital picture of the formation of a molecule.

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