Chemical bonding part 25

Added: 2026-05-11

[00:00:00]now we have already seen the vspr theory and we have seen the lewis concept now vspr theory has got limitations what is that vcpr theory could say only the shape of a simple molecules not about the odd electron pairs not about the complex molecules and it could not uh its what you can say its properties it could not say all the properties it is so what theoretical we can say that some of the properties which was put forward by vsapn theory was more or less it was theoretical it is theoretical in nature theoretically we can know about explain the formation of the molecule we can explain the properties of the molecule theoretically so it was not convincing so to say not for all the molecules i am saying repeatedly i am saying its not for all the molecules some of the molecules its not so convincing that particular one about the ah that is the properties about the nature about the reactivity about the reactions it was not so convincing all right so the next it came here your balance bond theory due to this reason the volume balance bond theory came into being because first of all as i said you were not able to explain the uh what you can shape up all the molecules very simple molecules we could explain by a vsapm theory we could not explain the properties or the characteristics of all the molecules because theoretical experiment it was completely a theoretical explanation it did not have any practical practical capability means it was not guided by any practical verification so to say for that reason i don't say it is rejected of course but it is not accepted for all the couples so hence came the valence bond theory the valence bond theory was first of all it was given by hitler and london hitler and london hitler and london gave this particular theory hitler and london but they said a very simple sentence it is that the presence of one or more unpaired electrons in the valence shell of an atom is the cause of chemical bonding the presence of one or more electrons in the valence shell of an atom all right means i want i will explain with help an example like let us say hydrogen atomic number one so just one is one so here you can say one unpaired electron is there you can see that all right now we are having let us say chlorine atomic number 17 so one is two two is two two p six three s two three p five so we are always interested with the outermost subshell because innermost shell or subshell is fully fit in almost so three p five one two three four five you should fill it up in this manner always make it a practice that you should fill it up in this way first unpaired okay you'll fill it up then after that the pairing is going to take place if at all any electron is left so you can see here one ampered electron is there one unpaired electron and here also one third electron is there now this is the cause of chemical bonding between hydrogen and chlorine that is what it was said by hitler and london the presence of one or more unpaired electrons in the valence shell of an atom is the cause of chemical bonding the presence of one or more now here one unpaired electrons here one suppose if you take oxygen you will be getting two unpaired electrons you'll be seeing it all right so that is the presence of one or more unpaired electrons is a cause of chemical bonding it is a very simple sentence given by hitler and london but it was the postulates i should i i mean to say it was not given by hitler and london the postulates of this particular valence bond theory that is v b d we can say that the postulates was not given by him it was given by polling and slater calling and slater they gave the postulates all right they gave us the postulates means they took it forward rather i should say because they gave us just one sentence but they took it forward they now what you can say study the hitler and london's balance bond theory and they that the same sentence they made different points and they explain the theory in terms of points that is known as the postulates now before moving to the postulates we will understand little bit about the valence bond theory then we will understand the postulates it will be easy for us fine so now next is what i said the presence of one or more unpaired electrons is the cause of chemical bonding that was said by hitler and london but it now always remember that whenever an atom approaches another atom there are four forces which always acts upon it listen carefully whenever any atom approaches any other atom now both atoms should have unpaired electrons mind it you will you will not say here suppose neon atom is approaching hydrogen atom nothing will happen what is what is going to happen nothing because neon the outer moisture is fully filled now where the bond formation is capable of taking place we will talk about that particular atom only here that means we need to talk something rational you can say that okay fine so what i said that is basically basically two forces are acting upon it one is nothing but the force of repulsion between the two atoms and another is the force of attraction between the two atoms but there are some other forces also two more forces are also being there which i am going to talk about talk about here okay otherwise when one atom approaches another atom suppose these two atoms are there they are approaching so they have to overcome the repulsive forces first then they will what you can say they will undergo bond formation and hence they will have the attractive forces so basically we are having attractive forces and repulsive forces suppose one atom is another atom is b so there will be repulsive forces between them because outer motion both contains electrons in it they have to overcome the repulsive forces first then after that they will undergo bond formation and hence they will have attractive forces again and again i am repeating this because these are the two major forces apart from that there is a force of attraction even noted down also there is a force of attraction between the nucleus of atom a and the electrons of atom b nucleus of atom a and the electrons of atom b third over now fourth there is a force of attraction between nucleus of what you can say that is electron atom b and the electrons of atom a again there is a force of again attraction between the nucleus of b and electrons of a first i said nucleus of a electrons are b now i am saying nucleus of b and electrons of a and other two forces are that is the force of attraction between a and b and the force of repulsion between a and b we can surround clear or we can say the force of repulsion between two electrons and force of attractions between the electrons and the nucleus we can see that here that is we can say that so now in this way these are the four basic forces again i repeat force of attraction force of repulsion between the two atoms force of ah vertical attraction nucleus of atom and electrons are atom b then force of attraction between nuclear nucleus of atom b and electrons of atom a all right this you need to understand it now we will understand this thing graphically all right which is very very important look here and understand it look here so here i am taking potential energy in the y axis and in the x axis i am taking inter nuclear distance now let us say the two hydrogen atoms i will be taking it it can be any two items but i am taking the example of hydrogen atom and i am going to explain you it will be better let us take an example so what will happen is that two hydrogen atoms are at infinite all right distance from each other they are infinite now they are coming closer now when they are coming closer let us say the two hydrogen atoms is here somewhere all right it is here somewhere the hydrogen h plus h i will say that it is here somewhere now when they will come closer what will happen is that they are coming closer so what is happening that is repulsive forces is decreasing and attractive forces is increasing all right listen carefully repulsive forces is decreasing and attractive forces increasing so when the attractive forces is taking place energy is released energy is being released okay when the attractive forces is taking place that is energy is released so initially what you find is that that is when the atoms are approaching in this way so you will find here the repulsive forces is there now when the repulsive forces will decrease attractive forces is going to increase energy is what happened you can see the energy is decreasing initially the energy is here after that energy remains constant initially then after that when the attractive forces starts increasing energy is decreasing all right energy decreases do you think it is going to decrease infinitely no it is going to decrease to a certain extent all right let us say it decreases here let us say it decreases here then after that again it increases again it increases now in case of hydrogen atom what is this value all right what is this distance but in case of hydrogen atom from where we have started we have started with what we can say arbitrarily we take energy as zero you know that in atomic structure we have taken arbitrarily when the atoms are at infinite distance arbitrarily the energy is taken to be s zero we are taking the energy to guess zero we can say that now here when the energy is released initially the energy remains same because it has to overcome the repulsive forces then after that when the attractive forces is taking place because atoms are coming closer you can see internal distance is decreasing internuclear distance is decreasing you can see it here decreasing energy is also decreasing by energy is decreasing what i said when the atoms are coming closer it will overcome the repulsive forces and attractive forces is going to decrease when the attractive forces is going to take place energy is being released all right now what is this the lowest energy it is going to act at a minus 433 kilojoules per mole we can say that minus 4 33 kilojoules per mole energy is in kilojoules per mole potential energy i'm writing it so minus 433 we are having it in the graph we can say that all right now what is this distance this particular distance is known as nothing but the bond length all right this particular distance is known as a bond length let me represent it by d o let us say here d o represents what the inter nuclear distance between the two hydrogen atoms i have taken hydrogen atoms internuclear distance between the two correct so what happens is that beyond this now this is internal distance this is a distance between the two nucleus which is represented as d go and in case of hydrogen this is your 74 picometer in case of hydrogen this distance is your 74 pico meter we can say that which is otherwise known as bond length now if you want to decrease further can you decrease further no why because when the two atoms they are coming closer closer closer energy is released then after that it is at the minimum but you can say energy got it and the bond length is maximum but still if you want to bring closer what will happen again it will start repelling but will repent because now the nucleus of both the atoms listen carefully now the nucleus of both the atoms after attaining this energy still you want to bring closer so it is not possible why the nucleus of both the atoms both are hydrogen atoms now they will repel one another due to repulsion again energy is increasing all right so this is the lowest energy where the atom will be stable correct but the atoms will be stable these are lowest energy because when repulsion is there unstability is there when attraction is there stability is there so here what you will find is that the attractive forces and the repulsive forces that balance each other initially there was repulsive forces now after that repulsive forces decreases attractive forces increases here the interactive and the repulsive forces balance each other and that particular what you can say the bond length is known as that nothing but known as a d o which is known as the bond length of a given molecule we can say that now beyond that if we want to decrease the bond length it's not possible why because again the repulsive force is going to increase why because the nucleus of the two atoms will repel one another so hence energy is again it is increased for that particular purpose now this particular energy this one this is known as what it is called as bond energy which is known as our bond enthalpy let me write bond energy you do not know what enthalpy it is called as a bond energy this is that all right so higher is the energy released more stable is the bond remember it higher is the energy release more stable is the bond because for dissociating that bond again the same amount of energy is required isn't it so here how much amount amount of energy is released same amount of energy will be required to break the bond again for that purpose higher is energy released more stable is the bond all right and to again break that particular bond which is called as the bond dissociation energy will also be higher so if the energy release is higher then energy absorbed to there the bond is also higher we can say that okay so in this way the formation of a molecule takes place here i have taken hydrogen molecule you can take any other atoms here i have taken the formation of hydrogen molecule two hydrogen atoms i have taken you can take any other atom and you can explain the formation of this the basic concept will be this much that you will be having the minimum that is the energy which the boundary is the two atoms is going to attain is known as the bond dissociation energy here in case of hydrogen is this much this is a bond energy this much it is bond enthalpy and this is your bond length in case of hydrogen it is your 74 picometer all right initially we take arbitrarily uh when the atom is at infinite distance on the nucleus so uh that is um at infinity both atoms then the energy is zero then after that when they come closer the energy decreases right the energy is decreasing so to say clean up so this is your bond length this much and this is your bond we can say energy we consider that the bond dissociation energy this is how the formation of that is the bond takes place or the formation of molecule takes place according to the valence bond theory i should say that now i will explain this one in another manner i am going to explain it i am going to explain another matter all right i hope you understood this particular graph so now in another manner is that is the forces which i said you that forces i am going to explain now always remember that what i said when the attractive forces and the repulsive forces balance each other just now i said that is the molecule said to be stable when the repulsive forces is more than attractive forces it is unstable got it clear or at the most attractive forces can also be more than the repulsive forces then also it is stable clear either attractive force is equal to repulsive forces or attractive forces can be greater than repulsive forces then the molecule will be stable but if repulsive forces will be more than attractive forces the molecule will be unstable definitely if repulsive forces is more it is unstable they are going to repel one another if they are going to move far apart from one another but when they come closer then the attractive forces is taking place the stability takes place right fine so this with this concept i am going to explain the formation of hydrogen molecule now you know hydrogen has got one electron and one proton in its nuclei what you can see in its atom so this is one hydrogen atom and this is another hydrogen atom which contains one proton in its nucleus this another atom contains one proton in its nucleus and it has got one electron in its shell this hydrogen atom gets one proton one electron this hydrogen atom gets one proton and one electron so what are the forces it is possible here now you can see we can have this forces this is your what repulsive forces i am writing it as r again this is what this is your what you can say repulsive forces i am writing again it is as r here let me write it here it is better so it is r clear so we are having repulsive forces between two electrons and repulsive forces between the two nucleus now you see here we are having an attractive forces between this atom nucleus and the electron and here also we are having an attractive force between this atom and that of the electron we are having it at the same time as it is an electron nucleus and its electron cloud we can also have now this one let me put a dotted line it will be better so as to differentiate between the attractive and the repulsive forces so to say let it be the dotted line so here also we are having some amount of some amount of attractive forces here also we are having it some amount of attractive forces between this proton and electron and this proton and electron all right so here how many attractive forces we are having it we are having four attractive forces how many repulsive forces we are having it we are having two repulsive forces an attractive forces is more than the repulsive forces energy is released and hence the formation of molecule takes place hence the formation of molecule takes place and results in the formation of molecule you can see a molecule is being formed we can say that okay in this way the molecule formation takes place all right clear so not only there should be unpaired presence of unpaired electrons but also the energy should be lowest because energy is highest even if the molecule is formed it will not be stable it will be unstable in nature but here hydrogen molecule is being stable we can say because this is one hydrogen atom and this is on hydrogen atom and it gives rise to a hydrogen molecule which is known as h2 molecule here also the question may come that by h3 and h4 is not possible the answer is very simple in one hydrogen atom only one unpaired electron is there in another hydrogen atom another unpaired electrons is there so they combine to form h2 there is no other unpaired electron present which will combine with another unpaired electron of hydrogen to form h3 understood let me repeat once again in one hydrogen atom there is one unpaired electron in another hydrogen atom there is another unpaired electron so they will come closer to each other they will overlap with one another to form h2 h2 only one one ampered electron is there is there any another unpaired electron present which will combine with another unpaired electron of hydrogen no so hence as there is no unpaired electron present then there is no question of formation of h3 or h4 molecules for that purpose h3h4 molecule does not take place only h2 molecules takes place okay thank you