Shapes Of Molecules And Ions - Chemistry LibreTexts

The electron pair repulsion theory

The shape of a molecule or ion is governed by the arrangement of the electron pairs around the central atom. All you need to do is to work out how many electron pairs there are at the bonding level, and then arrange them to produce the minimum amount of repulsion between them. You have to include both bonding pairs and lone pairs.

First you need to work out how many electrons there are around the central atom:

• Write down the number of electrons in the outer level of the central atom. That will be the same as the Periodic Table group number, except in the case of the noble gases which form compounds, when it will be 8.
• Add one electron for each bond being formed. (This allows for the electrons coming from the other atoms.)
• Allow for any ion charge. For example, if the ion has a 1- charge, add one more electron. For a 1+ charge, deduct an electron.

Now work out how many bonding pairs and lone pairs of electrons there are:

• Divide by 2 to find the total number of electron pairs around the central atom.
• Work out how many of these are bonding pairs, and how many are lone pairs. You know how many bonding pairs there are because you know how many other atoms are joined to the central atom (assuming that only single bonds are formed).

  For example, if you have 4 pairs of electrons but only 3 bonds, there must be 1 lone pair as well as the 3 bonding pairs.

Finally, you have to use this information to work out the shape:

• Arrange these electron pairs in space to minimize repulsions. How this is done will become clear in the examples which follow.

Two electron pairs around the central atom

The only simple case of this is beryllium chloride, BeCl2. The electronegativity difference between beryllium and chlorine is not enough to allow the formation of ions.

Beryllium has 2 outer electrons because it is in group 2. It forms bonds to two chlorines, each of which adds another electron to the outer level of the beryllium. There is no ionic charge to worry about, so there are 4 electrons altogether - 2 pairs. It is forming 2 bonds so there are no lone pairs. The two bonding pairs arrange themselves at 180° to each other, because that's as far apart as they can get. The molecule is described as being linear.