Shapes of molecules

In a nutshell

The shape of a molecule depends on the electron pair arrangement around the central atom. The number of pairs of electrons around the central atom influences the arrangement.

Electron pair repulsions

Electrons repel each other in an atom.

The amount of repulsion will depend on the type of electron pair:

Lone pair/lone pair has the largest repulsion angle
Lone pair/bonded pair has the second largest repulsion angle
Bonded pair/bonded pair has the smallest repulsion angle

The shape of a molecule will depend on the number of electron pairs as well as the type of electron pair.

Electron pair repulsion theory

You can predict the shape of a molecule using electron pairs.

procedure

1.	First, find the central atom.
2.	Work out the number of outer shell electrons in the central atom using either the periodic table or a dot and cross diagram.
3.	Use the molecular formula to find the number of electrons that are shared with the central atom.
4.	Count the the number of electrons and divide by two to find the number of electron pairs. *Tip:* Remember the charges on ions.
5.	Compare the number of electron pairs with the number of bonds to find any lone pairs. *Tip:* A double bond will count as two bonds. A triple bond will count as three bonds.
6.	Use the number of electron pairs, number of lone pairs and number of bonding centres around the central atom to find the shape of the molecule.

Shapes of molecules

There are several different shapes that molecules can adopt.

shape	electron pairs around central atom	bond angles	example
Linear	Two electrons pairs	$180\degree$	$CO_2$
Trigonal planar	Three electron pairs No lone pairs	$120\degree$	$AlCl_3$
Non-linear/bent	Three electron pairs One lone pair	$119\degree$	$SO_2$
Tetrahedral	Four electron pairs No lone pairs	$109.5\degree$	$CH_4$
Trigonal pyramidal	Four electron pairs One lone pair	$106.7\degree$	$NH_3$
Non-linear/bent	Four electron pairs Two lone pairs	$104.5\degree$	$H_2O$
Trigonal bipyramidal	Five electrons pairs No lone pairs	$90\degree$ $120\degree$	$PCl_5$
Seesaw	Five electron pairs One lone pair	$87\degree$ $102\degree$	$SF_4$
Distorted T	Five electron pairs Two lone pairs	$87.5\degree$	$ClF_3$
Octahedral	Six electron pairs No lone pairs	$90\degree$	$SeCl_6$
Square pyramidal	Six electron pairs One lone pair	$81.9\degree$ $90\degree$	$IF_5$
Square planar	Six electron pairs Two lone pairs	$90\degree$	$XeF_4$