Now the formula unit represents the empirical where we call the simplest ratio of ions in an ionic solid that combined to give a neutral charge, we're going to stay here in reality. And ionic solid doesn't exist as an ionic pair. So it's not just a positive ion in a negative ion combining. It's not that ionic pair, but instead it's a crystal lattice. Now, Crystal Lioness is a three D. Arrangement of several cat ions and ions together that form a stable pattern. So for example, here we have the ions of sodium ion and chloride ion. They have opposing charges is what which makes them attractive to one another. So that's why they are combined together. In reality though, the ionic compound is not just a single positive ion in a single negative ion, it's a collection of all of them together. This helps to make my crystal lattice. So in reality this is my ionic solid, it's a bunch of positive ions and negative ions combined together. Now if we were to take one of these positive ions in one of these negative ions pull them out that there would represent the formula unit, which is the simplest portion of my ionic solid. So here it would be sodium chloride, which would be n a C L. So just remember when we're talking about ionic pairs, that's just the formula unit of an ionic compound, it is not the true representation of that ionic solid. The true representation is a collection of those positive and negative ions together, which form a crystal last.

here we have the rules for writing ionic compounds. Now, Step one. We write the ions involved in the compound from the provided name step to use these ions to write the formula of the Ionic compound. So if we take a look here, we have aluminum nitride and berry, um, phosphate. All right, so we're gonna say aluminum is in group three A. Because of that, it's charge is plus three or three plus. So that's why you write it as a all three plus nitride. I would mean that it is a non metal, and it had its ending change to ID. Nitrogen is in group 58 so his charge is three minus. Now, if you don't quite remember the charges associated with the different groups of the periodic table, make sure you go back and take a look at my videos on the periodic table charges. Okay, that will help you see all the elements of the periodic table. And what are the common types of charges based on the location of the element. Now, for this first one, when numbers in charges are the same, So here the numbers are three and three. Okay, so the numbers are three and three. They cancel out to combine the elements. So since they both have three in their charges, they just cancel out its aluminum nitride will be a little end now, Barry, Um, phosphate barium is in group two way. So it's charges two plus phosphate is one of our Polly atomic ions. It's one of our common tetra oxides. Its formula is peel for three minus. Now, when the numbers in charges air different here, this is to when this is three. Then they crisscross. They crisscross to combine the elements. So what I mean by crisscross is that the two from here would come here and the three from here would come here. That would give me if we look it. Give me be a The three came down here. Now this, too, is for the entire Polly Atomic ion for both phosphorus and oxygen. So because it's for both of them, I have to put it in parentheses, p 04 and then the little too. So that would represent barium phosphate. These are the things you must do when given the name of an Ionic compound right out its ions. And then look to see are the numbers and the charges the same. If so, they cancel out. Are they different? If so, crisscross them. Now, doing this crisscrossing motion, we don't have to worry about crisscrossing the charges. Okay, so you're only crisscrossing the numbers. Doing this type of action takes care of the charges on them on their own, so just ignore them. Okay, So these will be our two formulas for these ionic compounds. Now that we've gotten down the basics will move on to some problems and put what we just learned into practice.