[Music] hi and welcome back to three sighs lessons co uk by the end of this video you should be able to determine the formula of an ionic compound using the charges on ions in the last video we looked at the charges on metal and nonmetal ions in this video we're going to use these charges to work out the formula of ionic compounds here's the key rule which you need to learn in an ionic compound the charges on the ions have to cancel out to leave an overall charge of zero I'm showing you three ionic compounds here and you can see that all of these have no overall charge no ionic compounds have an overall charge so we're going to see how to work out the formula of an ionic compound let's start by looking at a simple ionic compound which is sodium chloride this contains the sodium ion and a closed and the chloride ion Cl - don't forget that you'll be given the charges on any ions that you need in an exam question so how do we work out the formula of sodium chloride if we look at the ions we can see that the sodium ion has a single positive charge and the chloride ion has a single negative charge this means that one sodium ion can cancel out the charge on one chloride ion so the formula of sodium chloride is NaCl and here it is if we had two sodium ions then the compound would have an overall 1 plus charge which I'm showing you here under this is not allowed similarly if we had two chloride ions than the compound have an overall one negative charge and again that's not allowed here's another ionic compound sodium oxide let's work out the formula of this compound as we've seen the sodium ion is na plus the oxide ion is o2 minus remember that the charges have to cancel now this means that we need two sodium ions to cancel out the charge on one oxide ion so the formula is na2o here's one for you to try this is magnesium iodide the magnesium ion has a two plus charge on the iodide ion has a 1 minus charge you should pause the video now and work out the formula of magnesium iodide okay in this compound we need to iodine ions to cancel out the 2 plus charge on the magnesium ion so the formula is mg i2 we're going to look at a couple more examples to make sure that you really get this here's the next one this is lithium carbonate the lithium ion has a charge of 1 plus M the carbonate ion has a charge of 2 minus pause the video now and work out the formula of lithium carbonate okay as we said the lithium ion has a one plus charge the carbonate ion has a two minus charge this means that we need to lithium ions to cancel out the charge on one carbonate ion so the formula of lithium carbonate is Li 2 Co 3 ok the next compound is calcium hydroxide the calcium ion has a two plus charge on the hydroxide ion has a 1 minus charge pause the video now and work out the formula of this compound okay we need two hydroxide ions to cancel out the charge on one calcium ion so the formula of calcium hydroxide is ca o h2 now there's a problem here if we place this small 2 directly to the right of the O H then this means that we have two hydrogen atoms more than two hydroxide ions we've got to put brackets around the hydroxide to show that the little two multiplies the whole hydroxide by 2 so this is correct whereas this is wrong ok here's a final example for you to try this is magnesium nitrate the magnesium ion has a two plus charge on the nitrate ion has a 1 minus charge pause the video now and work out the formula of magnesium nitrate because the magnesium ion has a two plus charge we need two nitrate ions to cancel this out however we cannot simply write a 2 next to the nitrate ion like this this would not mean that we've got 32 atoms of oxygen which is clearly 1 so in this case again we need to use brackets like this now I should point out that in an exam you'll not see any question any more difficult than the ones I've shown you here you'll find plenty more questions on ions and working out the formula by on a compound in my revision workbook which you can get by clicking on the link above okay so hopefully now you should be able to determine the formula of an ionic compound using the charges on ions [Music]