Tutoring high school chemistry, ionic compounds are fundamental.  We’ll discuss how to write their formulas when radicals and involved.

In my Oct 20 post about the periodic table and ionic compounds, I mentioned the issue of metal vs nonmetal.  As I described in that article, it’s easy to tell for individual atoms.  However, what about something like NO₃^–?

The answer is that if the species has a negative charge, it is a nonmetal; if its charge is positive, it’s a metal.  Therefore, Cr₂O₇^2- is a nonmetal, even though Cr itself is a metal.  By that same reasoning, NH₄⁺ is a metal, even though N itself is a nonmetal.

Species like nitrate (NO₃^–) and ammonium (NH₄⁺) are called radicals or polyatomic ions. There are many of them; type “table of polyatomic ions” into your browser for lists.

To use a radical in a formula, you remember that the negative charges have to add up to the same as the positives.  If you need a multiple of the radical, you use brackets.  Here are some examples:

calcium hydroxide:  Look up “ion table” in your browser to find a list of common ions.  You’ll find calcium is Ca²⁺. Then, find (on the same table, or from a table of polyatomic ions) hydroxide. You’ll find it’s OH^–. The negative sign with no number means a charge of negative 1.

So, we have Ca²⁺ and OH^–.  Realizing positives must equal the negatives, we need two hydroxides (2x-1=-2).  So we have

calcium hydroxide:  Ca(OH)₂

By a similar process, you can assemble the following:

sodium sulphate:  Na₂SO₄

magnesium nitrate:  Mg(NO₃)₂

One important point to remember is that with a polyatomic ion such as NO₃^–, the 3 doesn’t mean there are three nitrates; it means there are three oxygens in one nitrate.  Therefore, if you need three nitrates – such as in the case of aluminum nitrate – you would put a three outside the brackets, as follows:

aluminum nitrate: Al(NO₃)₃

We have more to discuss about formulas of ionic compounds. Good luck with this installment.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.