How do you write the formula of an ionic compound?

1 Answer
Jun 6, 2014

What you want to do is make the compound neutral.

Let's take the following example:

Na^(+)Na+ + SO_4^(2-)SO24

We need to balance the charges, the easiest way to balance this charge is by looking at the overall charge of the ions involved. The NaNa ion has a +1+1 charge and the SO_4SO4 ion has a -22 charge. In order to give balance, we must have two Na ions to give an overall +2+2 with regards to Na: this, thus, neutralises the compound. Therefore, the formula is:

Na_2SO_4Na2SO4

If you're asked to balance an ionic compound such as Iron(III) Hydroxide, write down the formula. We know that Fe (Iron) has a 3+3+ charge and the hydroxide ion (OH) has a 1-1 charge - as a result, the compounds in their individualised forms are:

Fe^(3+)Fe3+ and OH^(-)OH

In order to balance this, we need to add brackets around the hydroxide ion to give:

Fe(OH)_3Fe(OH)3

This balances the charges and, thus, an accurate ionic formula has been given.