A Level Chemistry Exam Question-need help!

So the question says write the formula of aluminium nitrate(V). From this I assumed the oxidation state of aluminium is +5. But the answer in the mark scheme is Al(NO3)3. Can anyone please explain how?

So the question says write the formula of aluminium nitrate(V). From this I assumed the oxidation state of aluminium is +5. But the answer in the mark scheme is Al(NO3)3. Can anyone please explain how?

The oxidation state always refers to the variable element immediately preceding it. In this case the nitrogen of the nitrate ion.

Nitrogen can take many oxidation states, but in the form of an oxyion it is usually either (III) or (V).

In the nitrate(V) ion, NO₃^- , the nitrogen is in the (V) oxidation state.

In the nitrate(III) ion, NO₂^-, the nitrogen is in the (III) oxidation state.

Adding on to the above, it is well worth looking at the group your metal is in to deduce whether it is sensible for it to potentially have a +5 oxidation state.

Aluminium is in group 3 (or 13), so if it were to lose 5 electrons and form the Al^5+ ion, you would have to remove two electrons from the next shell down, which isn’t realistically achievable.

At least at A level, you can safely assume that group 3 (or 13) elements form compounds where they assume the +3 oxidation state.