Shapes of Molecule and Ions

These slides revise how to work out the shape of molecules that contain single bonds only. 

We can determine the shape of the molecule using Valence Shell Electron Repulsion Theory (VSEPR). This is done by placing bonding pairs of electrons in such a way as to cause the least repulsion between them.

You can determine the number of electron pairs using dot cross diagrams.

Firstly, work out how many electrons there are around the central atom:

• Determine the number of electron in the outer shell of the central atom
• Add one electron for each bond being formed
• Allow for any ion charge on the molecule

The work out the bonding and lone pairs:

• Divide by 2 to find the total number of electron pairs
• Determine how many bonding pairs there are by how many other atoms are bonded to the central atom
• Any pairs that aren't bonding pairs are called lone pairs

Finally, arrange the electron pairs to minimise repulsion

There are not many examples of this. One is BeCl2. It forms two bonds and has no lone pairs as Be is a group 2 element. The shape of this molecule is described as linear.

Examples could be BF3 or BCl3. Borong is a group 3 element so forms 3 bonds without leaving any lone pairs. This shape shown below is trigonal planar and has bond angles of 120o.

There are many examples for 4 electron pair molecules.

A simple example of this could be methane:

CH4 has no lone pairs, so all atoms are equally spaced around the central atom.

The molecule is described as a tetrahedral and has bond angles of 109.5o.

Ammonia (NH3) is also a 4 electron pair molecule, this time it has 1 lone pair. As lone pairs are more repulsive that bonding pairs, this alters the bond angles, making them smaller than those of the tetrahedral. This shape is described as pyramidal.

H2O is also a 4 electron pair molecule, containing 2 lone pairs. The large repulsion creates a bent molecule with a bond angle of 104o.

An example would be PF5, 5 equally spaced atoms creates a trigonal bipyamid with bond angles of 90o and 120o.

When 6 bonding pairs are formed (eg: SF6) the shape of the molecule is octahedral with 90o bond angles.

XeF4 also has 6 electron pairs, but 2 of those are lone pairs. Arranging the lone pairs to have the least repulsion makes a shapes described a square planar.