Ionisation energies is one of the first new things you learn in AS level and it is also an examiner's favourite.  Silly mistakes can be avoided if candidates learn the definition of ionisation energy fully.

Mistake 1:  Missing the state symbols for an ionisation equation

You may get a 2 mark question asking you to write an equation for the first ionisation energy of an element (let's say sodium).

This answer will only get you 1 mark out of 2:

Na --> Na+ + e-

The mistake here is that the candidate has missed out the state symbols for sodium.  'How should I know what the state symbols are?', I hear you say.  For that, we need to go to the definition of first ionisation energy:

The energy required to remove one mole of electrons from one mole of atoms/molecules in a gaseous state.

So the state symbols have to be (g) because of the definition of first ionisation energy.

The correct equation that gets 2 marks:

Na(g) --> Na+(g) + e-

Remember:  A question asking you for an ionisation energy equation might be worth two marks.  If it is, one mark will be for the correct equation and the second mark will be for the state symbol, so remember that you need them.

Mistake 2:  Removing more than one electron in an equation for ionisation energy.

A question may ask you for an equation for an ionisation energy of an element greater than 1.  As an example, lets say the question is asking for the second ionisation energy of sodium.

This is not the correct answer:

Na(g) --> Na2+(g) + 2e-

Why is it not correct?  It is not correct because ionisation energies measure the energy required to remove 1 mole electrons at a time.

This means that the second ionisation energy of an element is:

The energy required to remove one mole of electrons from one mole of +1 ions in a gaseous state.

We could write the ionisation energy for any number.

The nth ionisation energy of an element is:

The energy required to remove one mole of electrons from one mole of an element/molecule with a charge of n-1 in a gaseous state.

The correct answer is:

Na+(g) --> Na2+(g) + e-

Mistake 3:  Thinking that ionisation energy gets higher as the electrons get furthern away from the nucleus.

This mistake comes from the thinking that most of us have hard wired in our brains.  When one thing gets bigger, the other thing gets bigger.  Inverse proportions come harder to us than direct proportions, it seems.  Ionisation energy gets smaller as the electron gets further from the nucleus.

As a summary, here are the things that affect ionisation energy:

1)  The distance between the nucleus and the outer electron:  It takes less energy to remove an electron that is further from the nucleus.  This is because the positive nucleus pulls the negative electron towards it (opposites attract) but it has less influence over a larger distance, like most things.  Distance from the nucleus has more effect on ionisation energy than the number of protons in the nucleus (See below).

2)  The number of electrons between the outer electron and the nucleus:  It takes less energy to remove an electron with more  electrons between it and the nucleus.  These electrons 'block' the attraction of the nucleus.  Here is an analogy:  If you are at a talk and you are sitting in the back row with lots of people between you and the speaker, you will find it harder to see or hear them through the crowd than if you were sitting on the front row.

3)  Whether the electron is paired or alone:  It takes less energy to remove a paired electron.  This is because electrons have the smae charge and so they repel.

4)  The size of the positive charge in the nucleus (i.e the number of protons in the nucleus):  It takes more energy to remove an electron with a bigger positive charge in the nucleus.  This is because there is a stronger attractive force between the protons and electron.  Distance from the nucleus has more effect on ionisation energy than the number of protons in the nucleus (See below).  This means that caesium has a much lower first ionisation than lithium - even though it has more protons, the outer electron of lithium is closer to the nucleus.

Mistake 4:  Thinking that ionisation energy gets lower as you remove more electrons from a substance.

This mistake is made probably because people are thinking in direct proportions again.  As you remove electrons from a substance, there are fewer electrons remoaining to 'share' the available positive charge.  For example, after we have removed one electron from lithium, there are two electrons to share 3 positive charges.  This also means that the electrons are pulled closer to the nucleus.  The change goes up by a few hundred for each electron, but several hundred to thousands for an electron in a new energy level as it is closer to the nucleus.

For more in depth information of how much attraction an electron 'feels' look up Slater's Rules.