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0:00 how do atoms really make molecules why doesn't oxygen have a double bond why don't they teach you this stuff at school and what on Earth's going on here keep watching to find out before we start this video is just an introduction to molecular orbital Theory but it is still going to be more fullon than my other videos so buckle up and for those of you that want a more detailed academic introduction to molecular orbital theory in chemistry I'll tell you about a great resource at the end of the video okay ready let's go at school our first explanation of how atoms make molecules comes from Lewis diagrams and
0:46 the octet rule very simply atoms can overlap their circles and fill their principal veence Quantum shells by sharing electrons this works really well and you can understand a lot of chemistry with just this simple model in fact organic structure diagrams are just simplified Lewis diagrams this idea of pairs of atoms sharing electrons is known as veence Bond Theory more advanced courses introduce you to the concept of three-dimensional Atomic orbitals I've made a video about that and I really recommend checking that one out if you're not already confident about Atomic orbitals now these Atomic
1:31 orbitals can combine with each other to make the shapes and geometries we need to explain the shapes of molecules we see and they are called hybridized orbitals now these hybridized orbitals can overlap with orbitals from another atom to give a space of high electron density between the nuclei and that's our chemical bond importantly these overlaps only occur between adjacent atoms and we consider bonds to only occur between Pairs of atoms at a time that's great but there are many things it doesn't explain as a student I could never understand for example why some atoms break the octet rule and others don't and it's not very good for making
2:18 predictions for example with aromaticity and sometimes it's just plain wrong for example the bonding in oxygen gas so what's uh better explanation well it's something called molecular orbital Theory this is a different explanation for how atoms make molecules there certainly is some overlap with veence Bond Theory but thinking of it as a different and complementary theory makes everything a lot easier to follow so what is the big difference in molecular orbital Theory we imagine that all the atomic orbitals across all the atoms in the molecule overlap to make bigger orbitals that
3:03 extend over the whole of the molecule so let's take a look at how that works and of course we are going to start with hydrogen gas because it's the simplest molecule in H2 the veence bond model is simple the atoms overlap their 1s orbitals share their electrons forming a single Bond whoopee now molecular orbital Theory gives us a similar picture but with slightly different consequences we can imagine the orbitals are threedimensional oscillations oscillations of what well probability density but let's just stick to calling it an oscillation for now but where the orbital is oscillating the
3:49 most is where we are most likely to find our electrons now oscillations in quantum mechanics are nothing that we can visualize in our real world so I'm going going to make some analogies in a simple wave phase just means is it pointing up or is it pointing down so now let's say that our two orbitals are either oscillating in phase and interfering constructively or out of phase and interfering destructively anything in between is not a standing wave so this gives us two possibilities for making our combined molecular orbitals when they're in Phase the resulting molecular orbital extends over the whole of the molecule we call this orbital a bonding orbital because it
4:34 holds the atoms together now remember an orbital is not a container of any kind a cloud is a better way to imagine it and this hard surface just represents a certain value of probability of finding an electron so if I want to change that probability I can just move the slider to the left or to the right and it just makes the shell grow bigger or smaller and we can make an analogy of that bonding orbital with this oscillating band where the band oscillates the most is where we find the greatest electron density okay so that's not so different to our normal veence Bond Theory so far but now look what happens when the two Atomic orbitals have opposite phases now
5:23 they cancel each other out where they overlap and we end up with more electron density outside the two nuclei what this does is pull the nuclei apart and it makes an antibonding orbital that breaks up the molecule it's literally the opposite of a chemical bond that's something that we absolutely don't have in veence bond Theory now it looks like we have two orbitals but we don't they are two loes of the same orbital but they have different phases a bit like the P orbitals on atoms we can visualize it as being similar to this ribbon vibrating with a node in the center and secondly because it's one orbital there
6:08 is only space for two electrons there and thirdly remember that although this picture shows a solid shape the orbital is better thought of as a cloud we can find electrons crossing the Gap in the middle but we're much more likely to find them somewhere inside the solid shape so let's take a closer look and see how it all works firstly we started with two Atomic orbitals and they overlap to give us two molecular orbitals that's a fundamental rule in molecular orbital Theory however many Atomic orbitals we start with is the number of molecular orbitals we finish with knowing that at the start makes
6:53 everything so much simpler later it also removes one of the big problems with veence Bond Theory expanded octets and expanded 18 electron tets with molecular orbital Theory we don't need to find ways to explain how certain atoms get too many electrons instead we can see that all of the atoms bring along more than enough orbitals for the electrons that they have next up a crucial aspect of orbitals is how much potential energy electrons have when they occupy them potential energy comes from pulling apart things that are attracted to each other or by pushing together things that
7:39 repel each other so our electrons will have less potential energy the closer they get to positive charges since the two electrons sitting in the bonding orbital are close to two nuclei at the same time they will have less potential energy than electron sitting in the antibonding orbital where they're only close to one nucleus at a time and further away from it anyway now remember that systems with less potential energy are more stable because they have less energy to do things like break apart that's one half of what stability is furthermore the antibonding orbital has a node in the middle a space with little
8:25 to no electron density having more nodes for given size is often associated with higher potential energies for now we'll just say that those nodes give orbitals a shape that keeps electrons further from the nucleus so we put all this together and see that bonding orbitals have electrons with less potential energy than when they are in separate atoms and much less than when they are in the antibonding orbital and remember that electrons will always always drop into the lowest energy orbital that they can but if some UV light of just the right wave length hits the molecule it can excite one of the electrons to the
9:11 anti-bonding orbital the low energy electron gluing the nuclei together is now cancelled out by the high energy electron pulling them apart and there is no longer any benefit to the atoms staying together so the atoms will fly apart because of entropy which is a f pancy way of saying that they will just go everywhere that they can go and UltraViolet like breaking up chemical bonds is exactly why it's so dangerous to bags of chemicals like us but how does all this work with helium our other first row element in veilance bond Theory we simply say that helium has a full principal Quantum shell so it doesn't do chemistry but molecular orbital Theory explains this in a
9:58 different way we have the same atomic orbitals as hydrogen but now we have four electrons to fill our two molecular orbitals that means two drop into the bonding orbital that pulls the nuclei together but two are left in the anti-bonding orbital that pulls them apart so now there is no incentive for the atoms to stay together and they'll simply fly apart now we say that helium doesn't do any chemistry but actually it does and not only that it did the first chemistry ever in the history of the universe if we overlap a helium atom with a positive hydrogen ion a proton we only have two electrons there isn't much
10:43 benefit for the electron spreading themselves over two nuclei instead of one so it falls apart very easily but we have detected this molecule in space and it's very likely to have been the first ever molecule even before H2 so that's the basics but what is it that makes molecular orbital Theory so special well let's start by looking at dioxygen O2 now I'm not going to go into details of which atomic orbitals make which molecular orbitals I just want to draw your attention to the top of our little energy diagram here as we drop electrons into the molecular orbitals we still have two electrons
11:30 left over after filling up all the bonding orbitals but the next orbitals are antibonding orbitals that both have the same energy so those electrons won't sit together in the same orbital they'll sit in the two orbitals separately just like when you're on a bus you won't sit next to a stranger if there's an empty seat just across the aisle and that means that according to molecular orbital Theory oxygen gas has two unpaired electrons in other words it's a double radical which is completely different to veence bond Theory which predicts a double bond with two lone pairs on either oxygen atom so which one's right well it's molecular orbital
12:18 Theory we can tell because the unpaired electrons don't just give oxygen a faint blue color they also make it slightly magnetic now diatomic molecules are fairly simple so let's extend our oxygen molecule by one atom to make ozone and after that we'll have a look at some really kooky examples but before I forget if you click on the like button it'll help me make more videos thank you now in Ozone our s orbitals and our P orbitals link up to make molecular orbitals that run up and down the whole molecule for example example we can get this lovely tube shaped bonding orbital
13:04 now as we make the different combinations of atomic orbitals we get nodes of low electron density like this one that links both the outside atoms directly to the central atom but with a space right in the middle but it's the anti-bonding orbitals that really show us the fun of molecular orbital Theory this is the antibonding orbital version of the tubular bonding orbital notice how it has nodes all over the place but particularly between the atoms meaning there is empty space with virtually no electron density between the atoms now but now we've got more than two atoms we can also see non-bonding orbitals these
13:50 are molecular orbitals that aren't very different in energy compared to the separate Atomic orbitals and that's because the constru constructive and destructive interference basically cancel each other out we can see from this orbital for example that electron density sitting directly above and below the atoms doesn't really do anything to pull them together or apart and I'm going to repeat this this is one single molecular orbital with only two electrons with lobes on different atoms vibrating in different phases electrons will cross the space between them but only fleetingly and if you're interested in the details of how Atomic orbitals combined to make these molecular
14:36 orbitals I've put a link in the description so now we've seen how molecular orbitals extend over a whole molecule let's have some fun Can you spot what molecule this one is it is of course cyclohexane once again this is one single orbital with only two electrons in it so now how about this molecule can you guess what it is before I show you that's right it's Benzene now Benzene is directly related to cyclohexane it's just lost six of the hydrogens and sure in veence bond theory that means it should be like hexane but with some double bonds but it's not a bit
15:22 different it's completely different it's far more stable than it should be it doesn't do double bond chemistry and all the bonds are the same length instead of having different lengths for single and double bonds not to mention the weird stuff it does when we analyze it with spectroscopy this is a phenomenon called aromaticity and it's actually much more complicated than the textbooks tell us so let me know in the comments if you want a dedicated video but for now veilance Bond Theory doesn't do a great job of explaining why Benzene is aromatic while cylo OCTA tetraene isn't but in molecular orbital Theory we can
16:08 see that Benzene and other aromatic molecules have all their electrons in bonding orbitals while related non-aromatic molecules are also populating the anti-bonding orbitals the symmetry of benzene makes it a great molecule for visualizing how molecular orbitals work as before this is not literally what's happening but it helps us understand the importance of symmetry and what the phases mean if we look at this bonding orbital we can represent it as one big red balloon or lobe stretching right across the molecule well that balloon represents a single standing wave doing something like this if we look at this bonding orbital
16:55 however we get balloons between all the carbon atoms in other words it's holding the carbon skeleton together but the loes all have alternating red and blue colors those colors are saying that three of the lobes have one phase and the other three have the opposite phase what does that mean well it's something like this knowing about this symmetry right at the start means that we can combine it with the rule of atomic orbitals making the same number of molecular orbitals to really narrow down the possibilities when we're trying to calculate what molecular orbitals could be made now I should say here that theoretical chemists don't actually care
17:42 about the shapes of these orbitals they're only interested in numbers like what the energy differences are or the number of unpaired electrons which is a shame because they miss out on treats like this non-bonding orbital for Benzene what a beauty chemist designing molecules however are more interested but even then they're mostly interested in the combined effect of all the orbitals in other words where are the areas of high or low electron density where are or aren't electrical charges going to show up on a molecule the one time designers do care about the shapes of orbitals are for what we call the
18:27 frontier orbitals which is where most of the chemistry is going to happen electrons with the highest potential energy are the least stable ones Things fall off the top of a pile right so one of the places that chemistry will happen on a molecule is the molecular orbital that has electrons with the highest potential energy and this orbital is called the highest occupied molecular orbital or homo for short now electrons use usually move into empty orbitals so we're also interested in the empty orbitals of a molecule with the lowest potential energy because that's the easiest one for electrons to get into and this orbital is called the lowest
19:13 unoccupied molecular orbital or lumo for short so for example in a Lewis acid base reaction electrons from the homo orbital of one molecule will be injected into the lumo orbital of another molecule to make a new chemical bond chemists will change the atoms around these homo and lumo orbitals to make their reactions faster and more efficient or to make the products more stable when we design catalysts on the other hand we'll be making fine adjustments for a kind of dance The Catalyst might need a low energy lumo to entice the reactants in and disrupt
20:00 their stable molecular orbitals while at the same time keeping a high energy homo to make sure the complex is unstable enough for the products of the reaction to get free and getting that balance right is every bit as difficult as it sounds want a video about designing catalysts let me know in the comments another important point is that the difference in energy between the homo and the lumo in the same molecule represents the smallest amount of energy to excite an electron from one orbital to another within a molecule in electrically conducting molecules we want that energy to be as low as possible so electrons can flow easily
20:47 from occupied orbitals to empty ones and in insulators we need the opposite situation but it's the in between cases where the fun stuff happens organic LEDs use this homo lumo Gap by using an electric field to excite an electron from the homo to the lumo of a molecule the electron drops back down again almost immediately and when it does it loses that extra energy as a photon of visible light so chemists have to design those molecules very carefully to finally tune that energy gap and get just the weight wavelength we want for a particular phone display for example all right then so molecular orbital theory
21:34 is cool and better than veilance Bond Theory and Lewis diagrams but if it really is so wonderful and better and fancy pancy why don't we just start there at school why do we still learn all the other stuff well very simply as so often in science we have to make a choice between usefulness and accuracy for most people people who only study chemistry at school Lewis diagrams are fine for understanding the basic rules behind how atoms make molecules but at University we need a deeper understanding and so we move deeper into veence Bond Theory with hybrid orbitals it's more complex but you can still do it in your head and yes molecular
22:21 orbital theory is much more accurate it makes better predictions and explains more phenomena than veence Bond Theory but for anything more complicated than small molecules you need a computer to calculate the properties properly using veence Bond Theory I could have talked about all of these molecules using only a white board but most importantly bance Bond Theory allows us to use our qualitative brain our visual imagination and that's important because so much of chemistry is an imaginary V visual science so theoretical chemists typically work in tandem with synthetic chemists to get the best Synergy of the
23:07 two ways of thinking so the different approaches to molecular theory each have their own advantages and disadvantages and so in practice we use the best tool for the job that said veence Bond theory is making a comeback in the theoretical world too there are some things it explains better than molecular orbital Theory but that discussion is for another day now at the beginning of the video I promised a resource for an academic course on molecular orbital Theory so I've put a link in the description to a great YouTube channel called Chad's prep his lessons are great so check it out if you want to dig deeper and why not tell him I sent you so what do you think have you
23:54 heard of molecular orbital Theory before were you ever conf confused about it would you like to see how it's used in practice let me know in the comments but why not hang around for the bonus extras I showed you the vibrating ribbon as an illustration of how phases work in hydrogen but there's a more accurate image if we use a two-dimensional sheet and imagine that it extends to the end of the universe now we can see that the oscillation doesn't magically stop at the end of the molecule but extends outwards in all directions basically forever but there's a point relatively close to the nucleus where that oscillation is no longer significant and
24:42 that's what we call a single molecule the next and probably most important point is that when we get down to the size of atoms and especially electrons it simply isn't possible to imagine them clearly we can call them particles or call them waves but they're not truly like either of those things so we need to make models using our qualitative imagination or mathematics and those models will always be flawed in some respect so what we have to do is to choose a model that works best for our purpose some are simpler some are more accurate but none of them is universally better in chemistry we usually talk
25:28 about electrons and orbitals as being separate things but that's not really true in quantum mechanics the orbital represents the combination of the wave nature of an electron and its particle nature and no no one can really explain what that means in qualitative terms so electrons as particles sitting in oscillating spaces called orbitals is simply the most useful model for chemistry but if you have a good way of explaining molecular orbitals I'd love to see it in the comments see you next time
