Shapes of molecules

Molecules and molecular ions come in loads of different shapes. The shape depends on the number of pairs of electrons in the outer shell of the central atom.

Bonding pairs and lone pairs of electrons exist as charge clouds.

A charge cloud is an area where you have a really big chance of finding an electron pair. The electrons don't stay still - they whizz around inside the charge cloud.

1. Electros are all negatively charged, so it's pretty obvious that the charge clouds will repel each other as much as they can.

2. This sounds straightforward, but the shape of the chrage cloud affects how much it repels other charge clouds. Lone-pair charge clouds repel more than bonding-pair charge clouds.

3. So the greatest angles are between lone pairs of electrons, and bond angles between bonding pairs are often reduced because they are pushed together by lone-pair repulsion.

Lone-pair/lone-pair > Lone-pair/Bonding-pair > Bonding-pair/Bonding-pair

4. This is known by the long-winded name 'Valence-Shell Pair Repulsion Theory'.

5. This means that the shapes and bond angles of loads of molecules can be predicted.