Shapes of Molecules

Molecular shape depends on Electron Pairs around the central Atom

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.

Electron pairs repel each other

Electrons are all negatively charged, so electrons pairs will repel each other as much as they can.

How much it is repelled depends on the the type of electron pair. Lone pairs repel more than Bonding pairs.

This means 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.

the shape of the molecule depends on the type of electron pairs surrounding the central atom as well as the number.

This way of predicting the molecular shape is known as ‘electron pair repulsion theory’.

You can use electron pairs to predict the shapes of molecules

You first have to know how many bonding and non-bonding electron pairs are on the central atom.

1) Find central atom. 2) Work out the number of electrons in the outer shell of the central atom. 3) The molecular formula tells you how many atoms the central atom is bonded to. From this you can work out how many electrons are shared with the central atom. 4) Add up the electrons and divide by 2 to find the number of electron pairs on the central atom. Remember to account for charge if its and ion. 5) Compare the number of electron pairs with the number of bonds to find the number of lone pairs. 6) You can use the number of electron pairs and the number of lone pairs and bonding centres around the central atom to work out the shape

Practise drawing these molecules

Linear molecules = 2 electrons pairs around central atom (180^o)

Trigonal planar — no lone pairs = 3 electrons around a central atom (120^o)

Tetrahedral — no lone pairs = 4 electrons pairs (109.5^o)

Trigonal pyramidal — 1 lone pair = 4 electrons pairs (107^o)

Nonlinear ‘bent’ — 2 lone pairs = 4 electrons pairs (104.5^o)

Trigonal bipyramidal — no lone pairs = 5 electrons pairs (90^o,120^o)

Octahedral — no lone pairs = 6 electron pairs (90^o)