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^(+) + SO_4^(2-)

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 Na ion has a +1 charge and the SO_4 ion has a -2 charge. In order to give balance, we must have two Na ions to give an overall +2 with regards to Na: this, thus, neutralises the compound. Therefore, the formula is:

Na_2SO_4

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+ charge and the hydroxide ion (OH) has a 1- charge - as a result, the compounds in their individualised forms are:

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

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

Fe(OH)_3

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