How does bonding affect molecular geometry?

1 Answer
Dec 5, 2015

The repulsion between the electrons in a bond affect the angle of deflection of the atoms in the molecule, changing the molecular geometry.


In a bond, there are two types of electron pairs, namely bond pairs and lone pairs. Bond pairs are pairs of electron that form bond between two or more atoms, while lone pairs are pairs of electron of an atom in a molecule that do not form a bond. Since electrons have like charges, they repel each other. So in a molecule, although the bond electrons hold the atoms together, they constantly repel each other and the neighboring electrons.

The bond electron pairs have a repulsion between themselves, and the lone electron pairs have a repulsion between themselves. There is also a repulsion between bond pairs and the lone pairs. When a bond forms, the repulsion between the electron pairs cause them to move as far away from each other as possible. This produces a unique geometrical pattern in the molecule.

So, if a molecule has only two electron pairs, like that in #CO_2#, the two bond pairs repel each other and move the farthest from each other. Two objects can only be the farthest away from each other if they are in a linear structure, so the bond angle in such molecules is #180^o# and the molecule has a linear shape. A molecule with three electron pairs, all of which are bond pairs like that in #BeCl_3#, will have an angle of deflection of #120^o#, as that is the only angle for which each electron pair is the farthest from each other, and the molecule will have a trigonal planar shape.

However, the presence of lone pairs can affect the deflection angle, and hence the shape, of a molecule. A molecule with three electron pairs, two bond pairs and one lone pair like that in #SO_3#, will have a deflection angle of #<120^o# and a V-shape. This is because the repulsion between lone pairs and bond pairs is greater than that between just bond pairs.