Working out the formula of ionic chemicals (GCSE Chemistry)

This post is mostly for students taking GCSE chemistry, but it may be helpful for AS-level students struggling a bit balancing equations.

One standard challenge is when you’re given a reaction but not the formulae of all the chemicals in it, and told to balance it.

Often, you need to make sure you’ve correctly worked out the formula of ionic chemicals in the reaction. Ionic chemicals include all the compounds made of metals and non-metals at GCSE chemistry, like iron oxide, copper carbonate and calcium sulphate. Here’s how to do that.

1) Get the facts

We’ve written a well-formatted list of the ions that make up these ionic chemicals-you can download it as a one-page pdf with some extra advice here. Ions are particles-often atoms-that have gained or lost electrons.

Ions listRemember what forms what: metals always form positive ions, non-metals form negative ions, with the exception of the hydrogen and ammonium ions, formula H+ and NH4+.

Charges are written in superscript: which means they’re above the line of text. (The numbers in subscript are numbers of atoms or ions.)

2) Remember that all chemicals have a total charge of zero.

You need to balance the charges on chemicals until the charge of the + ions and the – ions are equal.

Then, don’t put any charges in the final formula of your chemical.

Sometimes, this is simple.
Formulae of chemicals 1
For sodium chloride, the charges are just +1 and -1. A one-one ratio gives a neutral chemical.

Sometimes, it’s simple, but the charges aren’t just +1 and -1. Barium oxide is like that:

Formulae of chemicals 2In this case, +2 and -2 add to give 0.

When the two ions have different charges, you need to balance their numbers to get to a total charge of 0.This is calcium chloride, which often comes up as it gets made when you react marble chips with hydrochloric acid:
Formulae of chemicals 5

Some ions contain more than one atom inside them. Carbonate and sulphate ions are common examples of that:
Formulae of chemicals 3Sometimes, it’s hard and you need to plan out the formula. You need to find the first number you can get to on the times tables of the + charge and the – charge: there, the charges will balance and you’ll have a neutral-charge chemical:

Formulae of chemicals 4 Iron (III) oxide, like in rust, has a similar formula, Fe2O3.


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