ionic compounds so recall that compound is made up of more than one type of element if you just have one element it's called an element if you combine more than one type of element though you can get a compound if those elements when they combine they exchange electrons and so one of them loses electrons the other one gains electrons we know that ions are going to end up being formed if those ions form a solid crystal we call it ionic compound now i suppose you could have a really high temperature and you could have a liquid ionic compound doesn't necessarily have to be a solid but for all the ones we're going to see they are going to be solids formula units are concepts we use to describe ionic compounds and really understanding what a formula unit is is necessary to understand what ionic compound is so the formula of an ionic compound is a ratio and that's the key term there's a ratio of its ions and we call this ratio we put it down to the simplest whole numbers and we call this ratio a formula unit and again that's the key point about ionic compounds is that when we're talking about them we use their their their chemical formula that is a formula unit so for example sodium chloride common table salt we know has the chemical formula nacl the formula unit for sodium chloride is nacl now it is really really important to understand this is not saying that sodium chloride is made up of one sodium atom and one chlorine that is incorrect that's the common misunderstanding with ionic compounds so people would think oh sodium chloride the formula definitely is nacl and so they'd think okay it must have one sodium and one chlorine atom but that is incorrect that's the wrong way to think about ionic compounds that is not how ionic compounds are that's not how sodium chloride is instead what you need to think about is you need to consider that this formula is the lowest whole number ratio it is a ratio of one sodium which is an ion not an atom an ion to one chloride ion that is the lowest whole number ratio that's not saying there's one of them that's saying for every one sodium there is there is one chloride so think of it as one gazillion sodium ions however much that is a whole bunch but there whatever amount there is of sodium ions there is an equal amount in this case because it's a one to one ratio there's also going to be if there's one gazillion sodium ions there will also be one gazillion chloride ions that is the key to understanding how ionic compounds actually are so when you go to picture them do not picture them as their formula would make you think you should be picturing them instead picture them as they actually are as this ionic crystal where you have a particular ratio whatever it happens to be whatever the format tells you it is for certain chloride it's a one to one ratio and that's how you want to think of them there is a a certain amount of sodium ions and a certain amount of chloride ions that are equal to each other and it is a whole bunch of them so really when you see the formula you should think okay there is sodium ions notice that the formula doesn't have charges in it but they are there so remember that they're there and then think of a little subscript n denoting some number and then there is chloride ions also present in the same number as sodium ions so that's really how you should be thinking of the formula for any ionic compound they are charged and the ratio of them is denoted by how many there happen to be leaving you to think of them as having a giant three-dimensional that doesn't look like a cube three-dimensional crystal structure lots and lots and lots of those ions in a particular whole number ratio so the properties of these ionic compounds are going to be based on the fact that they are in these crystal lattice structures um lattice means repeating units so they're in this three-dimensional crystal lattice repeating unit structure and then all of their properties are going to be based on that that's why it's so important understand how they actually are they do not look like little molecules they are these big repeating patterns of ions so each ion is going to be surrounded by many oppositely charged ions so for example we can see in this one here we have a sodium ion and sorry sodium ion here and a chloride ion there sodium with a positive charge chloride with a negative charge and it's not just one sodium and one chloride um and this is a one to one ratio here so we have lots of sodium ions lots of chlorides are just present in a one to one ratio but if you look at this chloride ion here we'd see that um let's get this a little bit easier to see it is surrounded by a number of different positively charged ions plus there's one behind it there's one in front of it right um so this one negative ion has six positive ions surrounding it and again that that is how you understand the properties of ionic compounds as you think of them as repeating crystals these three-dimensional particular ratioed a particular ratio of ions in the three-dimensional pattern as a crystal lattice structure so this leads them to have a high melting point remember melting is essentially being able to get the particles to move around relative to each other so if they're frozen as a solid they're fixed and then when you melt them they become free so they can actually move relative to the other particles around them in an ionic crystal though if you imagine like this sodium here again it's it's surrounded by these chlorine ions and this is just a two-dimensional slice of the crystal there's another plane in front of it and behind it as well and so they're they're they're held in place really really well by those oppositely charged particles so this attraction makes it hard to free these ions and therefore it's really hard to melt ionic crystals same property same cause for this particular property they are known to be quite hard not the same thing as strong don't don't confuse those at the same thing strength is a is a another type of thing think in terms of hardness as the ability to scratch them and so ionic crystals are really hard to to scratch if you tried scratching an io crystal you'd be trying to move the ions away from their relative position and again they're surrounded by absolutely charged particles large oppositely charged ions so doing that is going to be difficult and therefore ionic crystals end up being quite hard and just make sure in your head you're thinking that's not strong that means hard to scratch as hard as they are to scratch they are also brittle and that's why you don't want to think of them as being strong you want to think them as being hard to scratch but they are also brittle because if you do happen to move the ions it takes a bit of force but if you do happen to do it the spot where they want to be they're no longer in so for example if you imagine this crystal here i think i don't know the red ones are positive imagine and then the blue ones are negative so everyone's happy in this arrangement here you've got a positive surrounded by negatives everything's great but if you then to move some of these ions over a little bit just move them over the space of one atom which is not much at all then you end up with these positively charged ions being next to other positively charged ions and that is going to cause them they're the same charge that's going to cause them to repel each other and therefore the crystal will actually break apart and that's what it means to be brittle a lot of ionic compounds are going to be soluble in water now there's lots of exceptions but the general trend is that ionic compounds tend to be soluble in water the ions themselves have charges and water also has charges to it they're not ionic charges they are partial charges so we we label them slightly different but they are charges and so therefore the charged water molecules are going to be attracted to the ions and the ions to the charged water molecules and therefore the water is able to actually get in and break up the structure fairly easily because of that opposite attraction of charges conductivity with these ones if the ions are free to move and remember that the way to do that would be by melting them which would be really difficult or easier to do you could get a charged substance to get in there and start pulling the crystals apart one ion at a time by dissolving them in water either way melting or dissolve the ions would be free to move and as long as they're free to move they will be able to move a electrons from one place or another carry a current which is known as being conductive this does not work when they're solid so when they're solid remember they're stuck in their their positions they can't move very easily um so so they do not conduct electricity as a solid but if you put them in water the ions are able to move around we're able to get electrons from one place to another they will be conducted or if you were to melt them very difficult to do but they would also be conductive if you melted them now in terms of ionic bonding we can look at how these ions are going to be coming together based on how many electrons the atoms that make up these ionic compounds how many electrons these atoms want to gain or lose so recall atoms are going to gain or lose electrons because that is the way in which they become stable they can gain a full outer shell become isoelectronic with their nearest noble gas this is what they're trying to do to become stable this is their goal in life the ratio in which these ionic compounds are made so again sodium chloride is a one to one ratio of sodium to chlorine but if you had magnesium chloride it would be a one to two ratio so what determines this ratio is depending on who's making them up how many electrons they want to lose or gain so we can see how this works by using our lewis dot diagrams to essentially make track the electrons and where they go from one atom to the other so if we try this first with lithium let's do a lithium a little bit darker there lithium and so if we were to draw a lewis dot diagram it's got three electrons total it's got two in the inner shell and one in the outer shell one valence electron so when we do a lewis dot diagram we only draw that one electron and then if we have a chlorine atom we've got a total of seven valence electrons that we've got to put on here and so there is our lewis dot diagram for chlorine so in order to become stable what lithium is going to want to do is transfer its one electron over to chlorine so if lithium and chlorine were to react and do realize we're talking a chlorine atom here the name for chlorine gas is also called chlorine confusingly enough so in this case we are not looking at a chlorine molecule we are just looking at a single atom of chlorine so here is lithium and if it loses its one electron it would then have the next shell down the inner shell would be full if chlorine gains that one electron it would then have eight in its outer shell that would be a full outer shell so they'll do that and then our lithium becomes an ion so when we're doing lewis dot diagrams we want to put it in square brackets and we can put one plus or just plus that's good too and our chloride let's stick with the same colors our chloride is now an ion as well so i'm going to draw in its original electrons and i'll throw in a different color to show the difference between them if you can see that maybe a bit darker now i can't say that very well at all um either way it is going to have a charge of one negative though just to clarify these electrons belong to the original diagram so to show the bonding with an ionic compound you essentially have to do that in two steps you'd say okay here's the original setup with the atoms as they are and then here is how they end up after they have transferred their electrons so this ends up with one lithium to one chlorine and so our formula for lithium chloride is licl one lithium one chlorine now realize this is not a a a group of two though right this is happening so what we really mean is one gazillion lithium ions and one gazillion chloride ions are going to combine on a crystal and that's how we get lithium chloride all right slightly harder magnesium again two valence electrons it's in column number two then we have chlorine over in column number 7 again so it has 7 valence electrons and in this case here magnesium can transfer one electron to chlorine and then chlorine chlorine's happy right it's not going to take anymore because it only wants it that's it that's it for full shell so magnesium though is not happy it hasn't gotten rid of both of its electrons so you're going to need to have another chlorine atom involved to get magnesium to become stable so magnesium wants to lose two electrons chlorine only wants to gain one therefore you need two chlorines so then you would end up with your magnesium ion with a charge of two plus that has lost two electrons and then your chloride ion it has gained an electron i'm going to draw this one blue so we can actually see it this time put it in square brackets it has a charge of negative one but there are two of them so if you'd like you can write that out twice so you know there's two of something by drawing it twice um chemistry we get kind of lazy and say i don't want to write that twice i'm just going to put a 2 in front of it and a a large number 2 in front of a symbol tells you that there is two of them um not necessarily connected together so you can draw both of them separated out or you can put this large full sized number in front of the ion to say that there are two of them if you put a 2 underneath that's a subscript we use that to say something else we say that's when they're connected to each other in this case they're just two of them so we just put a large number two in front all right with this last example we have aluminum and it is in column 13. so it has three valence electrons around it and we are combining it let's do oxygen here one actually that's the other proper order two three four five six so in terms of transferring aluminum can transfer one two over to oxygen and and now this oxygen has eight it's happy it's done but aluminum is not it's got one more to do one more to transfer so we're to need another oxygen one two three four five six and that aluminum can then transfer its electron there and now it's happy great but this second oxygen is not happy so we gotta have another aluminum and it is the same as the first one it's got three valence electrons um and it can transfer its one of its electrons oh there we go to that oxygen and now this oxygen's got eight it's happy but there's aluminum second aluminum is not happy so we need another oxygen one two three four five six column 16. and now this aluminum can transfer its two electrons to that oxygen this oxygen's happy that aluminum is happy and this is why aluminum oxide is found in a ratio of two aluminum ions each one has lost three electrons so it has a charge of three plus and three oxygen let's draw in their lewis structures here each oxygen has gained two electrons so their charge is negative two so you find aluminum oxide and a ratio of two aluminum ions to every three oxygen ions now realize the formula for aluminum oxide we would write it as a subscript a little bit easier to read so we would say there are three aluminums and two oxygens oh did that wrong two aluminums and three oxygens and this is the formula for aluminum oxide we can see these are the two aluminum ions and these are the three oxide ions the name changes to ide for the negatively charged anion so this is aluminum oxide two aluminums three oxide ions and again it's not saying that that's all there is it's saying that this is made up of a giant three-dimensional crystal and in there there's a ratio of two aluminums to three oxygens but it's like two gazillion aluminums each one with a charge of plus three and three gazillion oxide ions each one with a charge of negative two so key take away make sure you're thinking of ions correctly think of them as this crystal lattice structure huge huge numbers of ions the formula does not tell you how many there are it tells you the ratio of the ions present in that ionic compound