Shapes of Molecules

Do you know what shape a H₂O molecule is? How about CH₄? You may have correctly guessed that CH₄ is tetrahedral (tetra meaning four).

This blog will hopefully show you how to correctly work out the shapes of molecules and ions, including those you may have never heard of!!

If you read my ‘Ionic and Covalent Bonding’ blog, you should know that the outer electrons of atoms are found in pairs in orbitals.

Remember that electrons have a negative charge so when an electron pair come together they will repel each other (like charges repel, unlike charges attract).

Because the electrons repel each other, they also want to be as far apart as possible from each other. This forms the basis of what is known as the electron pair repulsion theory which states that:

The following table shows how to work out the shapes of molecules that contain between 2 and 6 pairs of electrons in their outer shell. I also recommend drawing a dot and cross diagram of the molecule first, counting the number of electron pairs then referring to the table below:

Worked examples

Ø  Molecules with lone pairs of electrons

When trying to figure out the shape of a molecule that has a lone pair of electrons, an important thing to remember is that lone pairs repel more than bonding pairs.

See below:

It may not look like it but all the above molecules have 4 electron pairs around the central atom so they are all based upon the tetrahedral shape. However, the second molecule has only 3 bonded pairs and the 4^th pair of electrons is known as a lone pair which we can not see, so we say the shape is trigonal pyramidal and not tetrahedral.

A rough guide when working out the bond angle is that each lone pair pushes a bonded pair 2º closer together.

As I mentioned earlier, I recommend drawing a dot and cross diagram of the molecule first so you know for sure how many lone pairs you have.

Worked example