Now the formula unit represents the empirical, what we call the simplest ratio of ions in an ionic solid that combine to give a neutral charge. We're going to say here in reality an ionic solid doesn't exist as an ionic pair. So it's not just a positive ion and a negative ion combining. It's not that ionic pair, but instead it's a crystal lattice. Now, a crystal lattice is a 3D arrangement of several cations and anions together that form a stable pattern. So for example here, we have the ions of sodium ion and chloride ion. They have opposing charges which makes them attracted to one another. So that's why they're kind of combined together. In reality, though, the ionic compound is not just a single positive ion and 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 and one of these negative ions, pull them out, that would represent the formula unit, which is the simplest portion of my ionic solid. So here it would be sodium chloride which would be NaCl. So just remember, when we're talking about ionic pairs, that's just a 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 lattice.
Writing Formula Units of Ionic Compounds - Video Tutorials & Practice Problems
A Formula Unit represents the simplest ratio of ions used to make an ionic compound.
Examining Formula Units
Writing Formula Units of Ionic Compounds Concept 1
Video transcript
Writing Formula Units of Ionic Compounds Concept 2
Video transcript
Here we have the rules for writing ionic compounds. Now step 1, we write the ions involved in the compound from the provided name. Step 2, you use these ions to write the formula of the ion compound. So if we take a look here, we have aluminum, nitride, and barium phosphate. Alright. So we're gonna say aluminum is in group 3A. Because of that, its charge is plus 3 or 3 plus. So that's why you write it as Al3+. Nitride, ide would mean that it is a nonmetal, and it had its ending changed to ide. Nitrogen is in group 5A, so its charge is 3 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'll 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 33. Okay. So the numbers are 33, they cancel out to combine the elements. So since they both have 3 in their charges, they just cancel out. So aluminum nitride will be AlN. Now barium phosphate. Barium is in group 2A, so its charge is 2+. Phosphate is one of our polyatomic ions. It's one of our common tetraoxides. Its formula is PO43- Now, when the numbers in charges are different, here this is 2 and this is 3, then they crisscross. They crisscross to combine the elements. So what I mean by crisscross is that the 2 from here would come here, and the 3 from here would come here. That would give me, if we look, it'd give me Ba3(PO4)2. So that would represent barium phosphate. These are the things you must do when given the name of an ionic compound. Write out its ions and then look to see, are the numbers in 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 would be our two formulas for these ionic compounds. Now that we've gotten down the basics, we'll move on to some problems and put what we just learned into practice.
Writing Formula Units of Ionic Compounds Example 1
Video transcript
Provide the formula unit for the compound formed by the following ions. So we have magnesium ion and sulfate ion. Alright. So remember, when the numbers in the charges are the same, they just simply cancel out. So Mg2+ SO42-, just combine the Mg with the SO4. The charges completely cancel out, so ignore the 2, ignore the positive negative charges. So it'd just be MgSO4 as our newly created formula unit for our ionic solid.
Provide the molecular formula for the following compound:Sodium dichromate
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