Question

How do you write the formula of an ionic compound?

Answer 1

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.