Determining the formula of an ion

Please could I have help on how to work out the formula of nitrate (III) ion and nitrate (V) ion? Thanks

Please could I have help on how to work out the formula of nitrate (III) ion and nitrate (V) ion? Thanks

The formula of nitrate (III) ion, also known as nitrite ion, is NO2-. It has one nitrogen atom and two oxygen atoms. The nitrogen in nitrite ion has a +3 oxidation state.

The formula of nitrate (V) ion, also known as nitrate ion, is NO3-. It has one nitrogen atom and three oxygen atoms. The nitrogen in nitrate ion has a +5 oxidation state.

To remember the difference between nitrite and nitrate ions, you can use the following mnemonic:

"Nitrite has two, nitrate has three. One less O, NO2; One more O, NO3."

Let’s start by considering relevant oxidation states:

Nitrate (III): suggests that nitrogen is in +3 oxidation state.

Nitrate (V): suggests that nitrogen is in +5 oxidation state.

Oxygen is (almost) always in the -2 oxidation state.

You should remember that the overall charge on an ion is the sum of the oxidation states of all the constituents. In the case of both the nitrate (III) and nitrate (V), the overall charge is -1 and since there is only one nitrogen per ion:

-1 = (+3) + (n)(-2) ==> n = 2 in nitrate (III), so it is NO2^-

-1 = (+5) + (n)(-2) ==> n = 3 in nitrate (V), so it is NO3^-

Note that you are expected to memorise the formulae of some common ions at A level, including both of these ones. Working out the formulae isn’t always easy and can be time consuming.