2.2.2 (a) Ionic Bonding and Dot-Cross Diagrams

Your GCSE definition of Ionic Bonding is still find.

"Ionic Bonds are the electrostatic attractions between oppositely charged ions."

And your GCSE Dot-Cross diagrams are also fine.

The most commonly asked for is still Sodium Chloride (NaCl)

It will probably only earn you 1 mark now.

You will undoubtedly be asked to show "only outside shells".

Adding extra shells is more work, looks worse and gains you nothing.

The diagram above would gain you the marks because:

i) it shows the (+) and (-) charges of the Cation and Anion.

ii) It is clear that one of the electrons on the Chloride ion has come from the Sodium atom.

But how do you know what to draw?

Sodium has Atomic Number 11 (so has electronic configuration 2.8.1)

Or it is in Group 1 so its electronic configuration must end with a 1

Either way it needs to lose 1 electron to have a full outer shell (or a complete octet).

Losing negative electrons must make you a positive Cation.

Losing one electron must make you a 1+ ion.

Chlorine has Atomic Number 17 (so has electronic configuration 2.8.7)

Or it is in Group 7 so its electronic configuration must end with a 7

Either way it needs to gain 1 electron to have a full outer shell (or a complete octet).

Gaining negative electrons must make you a negative Anion.

Gaining one electron must make you a 1- ion.

So, the diagram shows the Sodium atom losing the only electron in its outer shell, the Chlorine atom adding one electron to the seven it already has, and the charges of the two ions formed with Sodium having an empty outer shell and the Chloride having a full outer shell.

Sodium atoms still need to lose one electron each for the reason set out above.

But Oxygen atoms need to gain two electrons.

So we need two Sodium atoms for each Oxygen atom as below.

The electron transfers are obvious, the charges are correct, the outer electrons on the ions formed are also correct.