 Another aspect of ionic compounds is incorporating polyatomic ions into ionic compounds. On the board, I've listed a few of the most common polyatomic ions that you'll run across. The polyatomic ions are a group of non-metals that are covalently bonded together but have an overall charge, so that they are still ions. To carbonate, its formula is CO3 with a 2-minus charge, sulfate SO4 with a 2-minus charge, phosphate PO4 with a 3-minus charge, nitrate NO3 with a 1-minus charge. All of these are negative ions. The one positive ion is ammonium, which is NH4 with a plus 1 charge. These ions can be incorporated into ionic compounds exactly the same way that we used metals and non-metals to make ionic compounds. The charges must balance each other so that you have a neutral compound. So, if you were going to write the formula for magnesium carbonate, first step when you're given a name is to write the appropriate ions for each. Magnesium is a group to a metal, so it forms a 2-plus ion. Carbonate. Since this is a polyatomic ion, we look at the chart and look up its formula. You'll probably have most of them memorized by the end of this course. CO3 2-minus. Again, you are looking at charges. 2-plus, 2-minus, they cancel each other out. They're balanced already, so you need just one magnesium and one carbonate group, so MGCO3. If your charges were not equal, we would use the same crisscross process that we used previously. So, if we had something like sodium phosphate, sodium is a group 1A metal. This has a plus one charge. I'll put the one just so we remember. Phosphate is a group, and you can tell by the ending. It has an ATE ending. That indicates that it's a group. Phosphate has a formula of PO4 3-minus. Notice that our charges are not the same. So, we're going to have to balance that, and we'll use the crisscross method. Crisscross the number of the charge to the opposite ion. It becomes the subscript in your formula. So, we need three sodiums, but only one group of the phosphate, PO4. Notice that I didn't change the subscript that is part of that formula for phosphate, PO4. We leave that 4 there. So, that would be the formula for phosphate. Another example, let's say we have magnesium nitrate. Magnesium group 2A. So, it has a 2 plus charge, nitrate NO3 with a minus one charge. I'll put the one there. Since there are different charges, we have to crisscross. We need one magnesium. We need two nitrate groups. MG, we only need one magnesium ion. Since we need more than one group of the nitrate, we're going to put the formula for nitrate in parentheses and put the new subscript outside the parentheses. The parentheses hold that group together. The subscript outside the parentheses tells you how many of that group you need. So, this would be the correct formula for magnesium nitrate. From the formula, we can determine the name as well. So, I'll put some formulas on the board and we will name them. I'll put some formulas on the board and we will name them. AGNO3. AG is the symbol for silver. NO3, that is a group and it is nitrate. So, we use the name nitrate. We don't change any endings. This is the name that we use. So, this would be silver nitrate. CU3PO4. The first element is copper. The second portion, PO4, is the polyatomic group phosphate. So, we write its name. Now, something special about copper. It is a transition metal. So, it requires a Roman numeral to indicate its charge. To figure out the charge, we have to do the reverse crisscross. Our subscript for copper is 3. Crisscross up, that means the phosphate had a 3 minus charge. How many groups of phosphate do we have? We only have one group. So, if you wanted to put this in parentheses, there would have been a 1 here. That is the subscript that you crisscross, the 1. So, this was a 1 plus charge. This means that this was copper 1 phosphate. The subscript that is part of the formula for that polyatomic group is not the subscript you use for crisscrossing. That is the formula. The formula never changes. It is the subscript that is outside of that formula that you would use to crisscross to determine the charge of your metals. So, I've given you examples of how to write formulas from names and how to write names from formulas. And you should be able to write formulas and names for any ionic compounds at this point.