b tech applied science unit 1 chemistry covalent bonds we did ionic bonds in the last video this is all about covalent bonds so here are two chlorine atoms uh and they've got seven electrons in their outer shell that rather have eight so what they do is they get together and they share a couple of electrons okay and here is a dot and cross diagram for a chlorine molecule cl2 okay you see the dots are from one atom the crosses are from another atom and then we have this bond in the middle and that is a covalent bond ionic bonding is swapping covalent bonding is sharing ah nice okay and this is a dot and cross diagram here's another one this is uh an ammonia molecule nh3 so a nitrogen and three hydrogens and here's our dot and cross diagram so the hydrogens are happy because they've got two electrons now and the nitrogen is happy because it's got eight so nh3 that's ammonia this is ch4 which you should know is methane and here's our dot and cross diagram you need to be able to draw these diagrams dot and cross diagrams so ch4 this is important because the the shape of it is called a tetrahedron and in organic chemistry tetrahedrons are important organic is basically with carbon hydrogen and oxygen um tetrahedron ch4 is a tetrahedron now look at this we have two oxygens and a carbon and the oxygen has got six outer shell electrons it wants another two the uh the carbon has got four outer shell electrons it wants another four so what happens is we get this we get double bonds so they share four electrons there's four electrons being shared there and that is a double bond and you can also get triple bonds as well so this is carbon dioxide co2 and here we see some double bonds this is a special type of bond which you need to know about it's called a dative covalent bond and the interesting thing here is that the electrons come from the same atom so the electrons in that bond that's the dative bond there those electrons have come from the nitrogen but the the hydrogen's electron has been lost and so the whole thing is positive okay and this is an ammonium yeah nh4 plus it's an ammonium uh ion okay and this special type of bond a dative covalent bond they're not very common and if you're going to learn an example learn this one what do the strength of covalent bonds depend on well they'll depend on the distance between the nuclei and that's called the bond length and basically they'll be stronger if the if the nuclei are shorter covalent bonds are stronger than long covalent bonds okay so it depends on the bond length the shorter the bond length the stronger the bond and then multiple bonds like double bonds are stronger than single bonds and triple bonds are stronger than double bonds etc so it depends on the bond length and it depends on whether it's a single or a double or a triple bond here are some questions for you to have a go at if you're in my class this would be a homework so draw a dot and cross diagram to show the bonding in the following so h2s and oh minus a nitrate no3 minus has this structure and included in there is a dative bond what is a dative bond also known as a coordinate bond what is a dative bond and then lastly what affects the strength of a covalent bond this isn't the end of the video because there's something else i want to talk about look at this this is a hydrogen fluoride molecule so you've got a hydrogen atom and a fluorine atom and they're sharing a couple of electrons but they're not going to share them equally okay the the fluorine is going to get a bigger share of these electrons if you like the electrons are going to spend a lot more time hanging around the fluorine atom than they are the hydrogen atom why because the fluorine if you're an electron i'd rather be around the fluorine because it has a lot more protons so it's a lot more attractive to the electron so the electrons are going to spend a lot more time hanging around the fluorine and so what you'll find is that that end of the molecule will be negative compared to the other end of the molecule electronegativity is the tendency of an atom to attract a bonding pair of electrons the more electronegative an atom is then in a bond it will get a bigger share of the electrons okay and we can give electron negativity a number yes and looking at this you'll see that the the biggest electronegative negativity is your fluorine yeah that is the most attractive to electrons in a bond it only applies if you're actually in a bond so looking at the periodic table here what trends can you see what trends in electronegativity can you see and can you explain these trends don't worry about the transition elements okay we'll just worry about groups one two up to group seven really we don't worry about group zero because they don't form any bonds really okay so they don't take part in bonding so forget about them we're not interested in them and basically what's going to happen is that the more protons an atom has then the bigger its electronegativity will be so if we work our way across a period yes for instance lithium beryllium boron carbon nitrogen oxygen fluorine you see that the electron negativity gets bigger and that's because the number of protons is increasing then if we go down a group basically what's happening is that there are more and more shells hydrogen has got one shell lithium has got two sodium has got three and the more shells there are that means that the nucleus is further away and so it will be less attractive so electronegativity gets less as you go down a group and then it gets bigger as you go across a period from left to right and you should recognize that trend and be able to explain it as well this is a chart which might turn up in an exam and it's electronegativity for different elements against atomic number you should be familiar with this chart and studying it you should see that it agrees with what we just talked about on the last slide yes if you look from sodium up to chlorine the electronegativity is getting bigger but then as we go down a group the electronegativity gets smaller what kind of bond we get when do we get a covalent bond uh when do we get an ionic bond and it's all about the difference in electricity if you've got two atoms looking at the diagram on the left if there's no difference between electronegativities then they're going to share the electrons equally they'll get an equal share and we will get a pure covalent bond uh the electrons shared equally now if there is a difference a significant difference in electronegativity then what you'll find is that one of the atoms gets a bigger share of the electrons for example in hydrogen fluoride the the fluorine atom gets a bigger share and we end up with what we call a polar molecule if the difference is between 0.4 and 2 we get a polar molecule and that means that one of the atoms gets a bigger share of the electrons and we end up with a molecule which has a negative end and a positive end okay it's a polar molecule it has a dipole which basically means it has a negative end and a positive end and then lastly if the difference in electronegativity is greater than two we get an ionic bond and that's basically because one of the atoms gets the electrons all of the time okay if the difference in electronegativity is greater than 2 you get an ionic bond so a pure covalent bond a a polar molecule a dative bond if you like and then an ionic bond not dative wrong word sorry this is an important example of a polar molecule and that's h2o the electronegativity of the oxygen is bigger than the hydrogens it gets a bigger share of the electrons and so you end up with a negative end and then these two positive hydrogens okay and this is a polar molecule h2o and we'll see in another video that these are very important okay h2o if you want to watch another video a good video about covalent bonds then i recommend this one yeah it explains covalent bonds very well a bit simple but entertaining