Page 1 of 1

### Bond Angles

Posted: Wed Nov 14, 2018 10:31 pm
Why are some bond angles exactly 120 or 109.5 while others say they are slightly less than that?

### Re: Bond Angles

Posted: Wed Nov 14, 2018 10:40 pm
Molecules that have lone pair electrons on the central atom will have bond angles that are slightly less than the value they would be if the lone pair were a bonded atom. The reason for this is that repulsion of a lone pair of electrons is greater than the repulsion of a bonding pair, thus the lone pair forces the other bonded atoms to be closer together in order to be farther from lone pair (since lone pair electrons have greater repulsion). This compresses the bond angle between the atoms and thus the bond angles on a molecule with a lone pair of electrons on the central atom will have bond angles that are slightly less than the value you would expect based on the electron arrangement. Hope this was helpful!

### Re: Bond Angles

Posted: Wed Nov 14, 2018 10:50 pm
Bond angles are dependent on the number of lone pairs and bonding pairs around the central atom. Bonding pairs are less repelled by one another than from lone pairs.
When we take Methane for example, there are 4 bonding pairs around the carbon and to space themselves out maximally, they form a tetrahedral structure of 109.5 as the bond angle. When we take ammonia, there are 3 bonding pairs and 1 lone pair around nitrogen. It can be thought of as tetrahedral but since the lone pair causes more repulsion with a bonded pair than bonded pairs between themselves, the bonded pairs are pushed closer together (to increase the distance between lone and bonded pairs). Consequently, the bond angle reduces to around 106-107 degrees.

### Re: Bond Angles

Posted: Wed Nov 14, 2018 11:05 pm
The wide balloon effect of Lone Pairs may help you visualize why it pushes the bonds closer, thus decreasing the angles.