b-tech applied science unit 1 chemistry ionic bonding okay we've talked already about what an ion is it's an atom molecule which has gained or lost one or more electrons now consider sodium sodium wants to lose an electron yeah it's got one electron in its outer shell it would much rather have a full outer shell so it wants to lose an electron chlorine wants to gain an electron it's got seven electrons in its outer shell if it gained an electron it would have a full outer shell so basically if they get together you get a chemical reaction happens and the sodium will give an electron to the chlorine yes and then the sodium will become a positive ion and the chlorine will become a negative ion a positive ion because in the atom and the sodium atom you've got the same number of protons and electrons so if it loses an electron then it has more protons so it's positive in a chlorine atom you've got the same number of protons and electrons if it gains an electron then it's a negative ion so n a plus c l minus sodium chloride and there's sodium chloride remember ionic compounds have got giant structures you get millions and millions of these positive and negative ions all stick together when it's a solid and you get a regular structure a lattice they're in rows and columns and layers a very regular kind of sodium chloride is actually cubic okay lots and lots and lots of ions arranged regularly giant structures in a lattice positive ions are called cations okay if you if it becomes a positive ion positive ion is called a cation now here's a sodium atom and i've drawn the electron in its outer shell as a dot we'll see what happens to that in a minute so remember metal ions lose electrons so if it's a metal it's going to become a positive ion metals become positive ions the charge will depend on what group the element is in if it's in group one or group two or group three for example sodium is group one so n a plus uh beryllium is in group two so that would be be2 plus aluminium is in group three so that would be al3 plus so you can figure out the charge if you know what group it's in transition metals can have different charges so uh iron for example is a transition metal it's in this middle bit of the periodic table and it can be fe2 plus or fe 3 plus now a dot and cross diagram if the outer shell is full then we don't draw anything if the outer shell isn't full then you can represent the electrons with either a dot or a cross so if you look at the dot and cross diagram for the sodium ion then it's just no dots or crosses because its shell is full now but with in brackets with a plus to show that it's has a positive charge you draw dot and cross diagrams for the ions of the following metals so figure out what group they're in and do a dot and cross diagram for each of them for the ions rather negative ions are called anions so positive is cations negative is anions non-metal atoms like chlorine they gain electrons they become negative ions you can figure out what charge they gain it's eight minus the group number so for example chlorine is in group seven so eight minus seven is one it will become cl minus these are polyatomic ions and you should learn them okay it's very useful you need to learn these the co3 two minus that's carbonate so4 two minus that's sulfate no3 minus that's nitrate and oh minus is hydroxide so you should learn these polyatomic ions what they're called and their charge as well their formula and their charge here's a dot and cross diagram for a fluorine ion f minus ion the cross represents the gained electron okay i mean strictly speaking that shell is full now so i don't need to draw it but uh there you go the cross comes from that came from something else like a hydrogen or whatever but it's in brackets with a negative because it's a negative ion so you draw dot and cross diagrams for ions of the following non-metals figure out what to find out what group they're in find out what their charge is do a dot and cross diagram for them strictly speaking the dot and cross diagrams when we do compounds they'll be a lot more important but you should be able to do them for the for the atom and the ion on its own what does the strength of an ionic bond depend on well consider this here are four different cases you'll notice that the ions are different sizes you'll notice that they have different amounts of charge now which bond will be strongest can you figure out which bond will be strongest and the answer is this one here this will be strongest for two reasons firstly because there's more charge you've got uh two plus and two minus so the attraction the electrostatic attraction will be bigger secondly because the ions are smaller and that means that the the average distance between the charges is smaller and so the force will be stronger okay the force gets weaker as you move further away so if the ions are closer together then the force is stronger which pair will be weakest which bond will be weakest and the answer is this one here why because it's only single charges there's only one plus and one minus and the ions are big so the the average distance between the charges is bigger so the the force the electrostatic attraction uh the positives and negatives attracting each other that force will be weaker so that's the strongest that's the weakest remember if the ions are smaller then the opposite charges are closer so the attraction is stronger yes if the the ions have more charge then the force attracting them is stronger so the bond will be stronger you could get this table in an exam this table shows the charges on different ions and it shows the ionic radius the size of the ions so you'll be able to figure out using this table whether the bond will be strong or weak here are some questions for you to do draw dot and cross diagrams for the first 10 elements and their ions what does the strength of an ionic bond depend on and explain why for each of the two factors explain why