in this video we're going to talk about molecular orbitals uh then we'll talk about sigma bonds and pi bonds in organic compounds first of all we want to go back to our atomic orbital story uh you have heard about the eisenberg heisenberg uncertainty principle in your general chemistry class it usually says that we cannot find the exact position of any electrons but the probability of finding electrons in certain regions of space can be found so that actually that give that gives birth uh the definition of orbitals uh what is orbital orbital is is a space where the electron like to occupy uh so those are orbitals each orbital has different shape than others so it means the probability of finding electrons in certain regions so they take a certain save and each orbitals are different for example if you have s orbital they're spherically shaped just like this this is the picture of one is atomic orbitals if it is a p orbital they are shaped like a dumbbell dumb dumbbells and just like this uh if you look at uh 1s and 2s orbitals orbitals against a spherical so you can see here this is pretty spherical uh and if you have two ace orbitals they are also spherical they have a node at the center so if you have higher energy orbitals usually they have the electron density located further from nucleus so therefore they're larger size than um lower energy orbitals p orbitals are shaped like a dumbbell shape as i said before so depending on which axis they are falling on so we can call it we are calling them accordingly if they are falling on x-axis we call 2px if they are falling on y-axis we call 2py if they are falling on z axis then we ca we call two pisa orbitals so now we know the the atomic orbitals for the s orbitals are p orbital those type of thing we know those are nothing but a kind of those are basically explaining some probability factor means the finding of electrons in certain regions in space so those are we call atomic those are we call atomic orbitals but when we try to draw the structure of molecules then we don't actually know how they're forming the those type of covalent bonds for example here let's say hydrogen gas this is we can begin with a simple example hydrogen gas is composed of two hydrogen atoms and they are bonded by single lines we call this a single bonds but just looking at this lowest structure of this hydrogen gas we cannot actually say how the orbitals form these bonds we don't know because we know the orbitals are where the electron uh are so so just kind of from this blue structure nobody can predict how the orbitals from these bonds and also we don't have any idea about how strong these bonds are we also don't know how the molecules will behave so at this point what we are going to do we are going to take a hell from quantum mechanics you will learn quantum mechanics uh in great detail in your pkm class at this point you can take it as given uh usually the molecular orbitals are based on quantum mechanics and molecular orbitals if we study then we should be able to pre answer all these questions here how the orbitals uh keep how the orbitals form the covalent bonds and you can predict how those type of bonds are strong whether they're strong or weak also you can predict their sum of the behavior so what is molecular orbitals so in quantum mechan the quantum mechanics usually tell us when you combine two atomic orbitals that leads to two molecular orbitals if we combine two s orbitals then they will form two uh molecular orbitals if you combine two p orbitals then they combine two they will produce two molecular orbitals all right any combination of two atomic orbitals they are always leads to form two molecular orbitals if you combine them in positively we call positive combination usually results a bonding molecular orbitals and if we combine them in a negative way we call negative combinations then we call antibonding molecular orbitals for example if you have one is atomic orbitals for one hydrogen if that combined with another one is orbitals of another hydrogen in constructive fashion means that if they are combining positively we call bonding combinations they form bonding molecular orbitals so in this case what you see the overlapped regions increased electron density so when they kind of couple together in a positive way bonding combination way then there is an increase in electron density around these two nucleus and if so this is recall bonding molecular orbitals if you have one is orbital from one hydrogens and one is orbital of another hydrogens if they are combining negatively we call negative combinations sometimes we call antibonding combinations then they form these orbitals we call antibonding orbitals and this is where the region there is a decrease electron density so what we are going to focus here is the bonding molecular orbital so now there is an increase in electron density around these two nucleus so the electrostatic attractions of this nuclei and the increased electron density usually give birth of covalent bonds for hydrogens so this is we call anti-bonding orbitals so this is bonding molecular orbitals for hydrogens what can one can expect one is orbitals another one is orbital they can do something like bonding combination from this bonding molecular orbitals and this is how they generate the covalent bonds between two hydrogen atoms so again the covalent bonds are generated from the electrostatic attraction of these two nucleus with the increased electron density that's overall call covalent bonds and this is also a driving force that that actually helped to explain why these two hydrogen atoms will combine together and from hydrogen gas h2 if you um also look at it here so they combine nick in a positive way uh so this type of combination we sometimes also call it end on collision uh end on overlap or hidden overlap and in android overlapper head and overlap there's an increase in electron density so this type of bond we call sigma bonds so the hydrogens and hydrogens the bond the covalent bonds they're forming between those two nuclei we call sigma bonds now the question is why the hydrogens are two hydrogen atoms are forming hydrogen gas the reason is if you have one is atomic orbitals of one hydrogens one is atomic orbitals of another hydrogen and if you combine them you see their energy level here this is the kind of energetics for hydrogen guys so what happened this is where the electrons energy for single hydrogen's atom uh one is orbitals and this is the energy level for uh one is atomic orbitals for another hydrogen atom and if you combine them positively then they form bonding molecular orbitals where there is an increase in electron density around this two nuclei and there's a decrease in electron density around these two nucleus we call anti-bonding molecular arbitrals now you can see here by combining these two atomic one is atomic orbitals you've generated the molecular orbitals and that bonding molecular orbitals now have those two electrons here so it means their energy level is lower here so therefore so so therefore when you mix those two so you are basically um saying this and these electrons are in the lowest uh or in the single bonding molecular orbitals which are lowest in energy energy axis and this is all about something close to 104 kilojoules per mole uh that means uh this is a favorable process uh when you lower the energy of the system it's a molecule always from you know try to go to the system where they can have a lower energy so that way they will feel more stable so this is why the hydrogens they hydrogen's atom when they combine so they form a molecular bonding molecule orbital and the electrons their atomic orbitals energy gets lower and that that's how they form the single bonding uh that's how they form these bonding molecular orbitals so there's no electron in antibonding orbitals so there are higher and higher in energy than bonding molecular attributes so these two electrons will prefer to go into the sigma bonding molecular orbitals molecular orbitals can also be formed from p orbitals um it's molecular orbitals uh so if they do a head-on overlap or end on overlap so they can generate like this type of molecular orbitals so this is like a sigma p and if they can also a molecular orbital from this hydro is overlap so if you this is one of the p orbitals this is one of the periodic tools if they overlap sideways so they can form these type of orbitals uh molecular orbitals this is we call pi bonding molecular orbitals and this is called sigma p molecular orbitals so when uh p orbitals they do handle overlaps and let's look at their energetics here so you can see this therapeutic energy level but when they combine uh constructively or positively they form these sigma bonding molecular orbitals and it looks like these and this is sigma antibonding molecular orbitals and again these two electrons will go to the sigma bonding molecular that's the lowest energy molecular orbitals when p orbitals they form uh molecular orbitals by sideways overlaps then then they form a pi bonding molecule orbitals and this is anti-pi bonding molecular orbitals so again the electrons these p electrons they can go into the pi bonding molecular orbitals so when a compound has a p when a compound has a carbon-carbon double bond that means the p orbitals they can form sigma bonds as well as they can also form a pi bond so pi bonds are formed by overlapping two p orbitals and sigma bonds are formed usually from the head-on collision a head and overlap or side by side for head-on overlap or end on overlap they generate the sigma bond as we saw the picture in the previous slide now those are all basically theory now the in in real life when you will be given a compound and you will be asked to determine um the sigma bond number of sigma bonds and pi bonds in a given compound and how do you do that so usually when you look at the molecule closely then if you see there is a single bonds single bonds are usually sigma bonds because those are formed from end on overlap or or head-on overlap so for this particular molecules methane carbon has four bonded with four hydrogen atom by single bonds so it has four sigma bonds if we are talking about this so carbon is doubly bonded with oxygen carbon um bonded with two hydrogen atoms so if you are talking about this bond here uh so we can see that this is a double bonds so this double bond is for has one sigma bonds and one pi bonds and if you want to count the total number of sigma bonds so it has this is one sigma bond sigma bonds so here is one single bonds and another bond is here we call pi bonds so total uh three sigma bonds and one pi bonds in this particular molecules if you're talking about this molecule you can see here this carbon-carbon triple bond so when you have a carbon-carbon triple bond means it can have one sigma bonds and two pi bonds and if you want to count this two sigma bonds um then it will be like three sigma bonds two pi bonds so the bottom line here is when you have a purely single bond those are sigma bonds when you have carbon carbon double bonds or carbon oxygen double bonds or carbon nitrogen double bond and the double bonds the two lines one is for sigma bonds another is for pi bonds if you have carbon-carbon triple bonds or carbon nitrogen triple bonds usually there is one sigma bonds and two pi bonds so one sigma bonds and two pi bonds always remember the sigma bonds are stronger than pi bonds so and also the bond length and bond strain follows in the decreasing water and carbon-carbon bond carbon carbon triple bond they are usually the shortest bond and also they're stronger than carbon-carbon double bonds so carbon-carbon double bonds are low uh longer than carbon-carbon single bond they're also stronger than carbon-carbon single bond again uh remember that when you have a single bond those are sigma bonds when you have pi bonds uh ones carbon-carbon double bonds are carbon nitrogen double bonds uh so then that would be one sigma bonds one pi bond if you have carbon-carbon triple bond that should be one sigma bond two pi bonds so this is how you have to determine the sigma bonds and pi bonds in a given compound so that's all for this video