 In this video, let's learn how to write the formula for some ionic compounds. Now, for the first one, we have to think about the formula of zinc chloride. And the main idea of writing the formula of an ionic compound is that we need to keep in mind that the total charge on the compound of the final ionic compound, it has to be zero. The total charge is always zero. And what is total charge? Well, total charge is the charge that comes from cations plus the charge from anions. In this ionic compound, we have zinc chloride. So zinc would be the cation because it's the metal and chloride would be the anionine because it's the non-metal. Now, if we think about the charge that a zinc ion has, we can have a look at the Roman numerals which is written inside the brackets. So this is two. So the zinc ion really looks, it's Zn plus two. And the chloride ion, we can have a look at the periodic table. So if we bring, if we have a look at the periodic table, we can see that chloride lies right over here. This is group 17. And that means that it has to gain one more electron to form a complete octet. The chloride ion would be Cl. It would be Cl minus. This would be Cl minus. Now, the total charge has to be zero. So if we have a look at the two ions in the picture, if we take one Zn ion, so this is one into plus two. And if we take two chloride ions, two into minus one, the charge on the chloride ion, this becomes equal to zero. So this means that one ion of Zn plus two ions of chloride will be required to make the total charge of this ionic compound to be zero. So the formula becomes ZnCl2. Okay, let's have a look at some more. Now we have sodium sulfate. So again, the total charge on the ionic compound has to be zero. Total charge is charged from cations plus charge from anions. So here the cation would be sodium. What is the charge that a sodium ion has? Well, we can have a look at the periodic table again. So sodium really lies over here. This is group one, which means the sodium ion is Na plus. And sulfate ion is SO4 minus two. It's a polyatomic ion with a charge of negative two. And we can see that sodium has one plus one, sulfate has minus two. So if we take two sodium and one sulfate, the charge should come out to be zero. So the charge from cation, if we take two cations with plus one charge and if we add it to one and nine with a minus two charge, this will come out to be zero. So that means the formula would be Na2SO4 because we are taking two sodium ions. Okay, let's have a look at one more. Here we have ammonium nitrate. Now both of these are polyatomic ions. And the cation for ammonium, it is NH4 plus. It has plus one charge. And nitrate is Na3 minus. It has a negative one charge. Now the total charge on the ionic compound has to be zero. And in this case, the charge on cation is plus one. Charge on the anion is minus one. So we can take one of each. So this would be just one. If we take one of plus one and if we add it to just taking one of minus one, this will come out to be equal to zero. So the formula becomes NH4NO3.