Question

How would I go about creating a Lewis Dot structure for the Calcium Carbonate and Magnesium Hydroxide?

I know how to do it for any compound with two elements such as SF5, but I don't know how to approach it when there's a polyatomic ion involved and there are three elements.

Answer 1

For MgCO3 you write the Mg^2+ with a + sign then CO3^2-; i.e., write the lewis dot structure for two ions of Mg and carbonate. Same for Mg(OH)2. I would write the Mg ion and write two OH ions.
Mg is is just Mg:
Mg^2+ is Mg^2+.
Here is a site that will show you CO3^2-
http://en.wikipedia.org/wiki/Carbonate

You will need to add the other electrons but this shows the bonding.