# How do you write the formula of an ionic compound?

Jun 6, 2014

What you want to do is make the compound neutral.

Let's take the following example:

$N {a}^{+}$ + $S {O}_{4}^{2 -}$

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 $N a$ ion has a $+ 1$ charge and the $S {O}_{4}$ 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:

$N {a}_{2} S {O}_{4}$

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:

$F {e}^{3 +}$ and $O {H}^{-}$

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

$F e {\left(O H\right)}_{3}$

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