 Hello. Yeah. Can you hear me? Okay. So, next we are going to see the octet rule. So, what is octet rule? Write down, first of all, atoms forms, write down, atoms forms covalent bond, covalent bond in order to, in order to attain noble gas configuration, noble gas configuration and for this, each atom should have, should have eight electrons in the valence shell, in the valence shell. Okay. For example, HCl, CO2, O2, CH4. In fact, all hydrocarbon follows this rule. All hydrocarbon follows this rule. Yes, eight electron in the valence, in the valence shell. Okay. However, there are only few elements which follows this rule. Okay. Majority of the molecules does not follow this rule. Okay. That's why we have a lot of exception in this rule. Okay. So, if you see this HCl, if I explain you this one, hydrogen never forms octet. It can form duplet. One or more than one also possible, covalent bond. Okay. So, you see HCl, hydrogen and chlorine, right? So, we have one electron here and one electron here. So, for hydrogen, that duplet is complete. Chlorine, if you see, has three lone pairs on it and head bonding electron. If you count the number of electrons, only covalent. See, ionic is the transfer of electron. One atom has to lose the electron. Other one has to gain the electron. Right? So, there we have exchange of electron. Right? Hence, in that case, octet rule we don't consider because one atom has to lose, other one has to gain. Here, we have sharing. So, sharing takes place between the two atoms in order to gain octet for both atoms. So, one atom, if it is losing, other one, if it is gaining. Right? So, in that case, it is ionic bond. Right? And octet rule we do not define for ionic bonds. Okay. It is not possible. So, as you see, the octet of chlorine is complete and hydrogen, we have duplet of this. Okay? CO2, another example, carbon 4 double bond C, double bond O. For carbon, the number of electrons you count, it is 1, 2, 3, 4, 5, 6, 7, 8. Carbon has 8 electron, oxygen has two lone pairs on it. So, the octet of oxygen is also complete. So, all these molecules you see, all the elements will have its octet complete. Right? There are majority, like the most of the compounds in fact, does not follow octet rule but still they form the compound and those compounds are stable also. That's why all these compounds will study under exception of octet rule. Write down exception of octet rule. The first one, molecules write down like this, lesser than 8 electron, 8 electrons in the outer shell, 8 electrons in the outer shell. Means we have molecules in which the central atom has lesser than 8 electrons in the outer shell but still the molecule is stable. Right? It does not follow octet rule but still the molecule forms. For example, you see, we have BF3, we have ALCl3, H2, etc. The BF3 molecule if you see, the structure of BF3 is this. The structure of ALCl3 is also very much similar and H2 we know already at single bond H. All these molecules you see, the number of electrons for this boron in this compound, we have 6 electron, 2, 4, and 6. Here it is again 6 electron, 2, 4, and 6 and here it has 2 electron. So there are molecules in which the central metal atom, central atom has less than 8 electron but still the molecule is stable. Right? This is the exception of octet rule. Clear? I didn't get your shit. It is balanced as in this molecule, it forms ALCl3. We use this as a catalyst in the reaction but it is also possible that ALCl3 goes under dimerization and forms AL2Cl6. But yes, ALCl3 exists, it is a strain. We use ALCl3 as a Lewis acid in organic reactions. That is why it is, yes, that is why it is an exception. But yes, dimerization is also possible and forms AL2Cl6. For which one? A structure we will discuss, Shiddish. We will discuss VSEPR theory. There you will understand the structure. Okay, let's not go over there. Structure-related things. We will discuss that. Okay, this is the first exception of octet rule. The second one you write down, less than 8 electron, one more thing I forgot to write here, less than 8 electron it's possible and this kind of compound, we call it as, this kind of compound, we call it as hypovalent compound, less than 8 electrons, hypovalent molecule. And when we say less than 8 electrons, I'm talking about the central atom. We'll calculate the, to count the number of electrons for central atom, aluminium, boron. Here we do not have that central atom. That's why I'm calculating the 6 electron for aluminium here, for boron here. Okay, second point. In case of, second you write down like this, less than, sorry, more than 8 electrons, more than 8 electrons end up in outer shell, 8 electrons in outer shell. Right on, there are molecules, there are molecules which contains more than 8 electrons, molecules which contains more than 8 electrons, but still they are stable. For example, you see, example is SF6. We have PCL5, sulphate, iron, SO4, 2-minutes, etc. SF6, if you look at the molecule, the structure, sulphur has 6 chlorine atom attached here, PCL5 has 5 chlorine, all these molecules you see, the number of electrons, this sulphur has, if you count, each bond has 2 electrons, there are 6 bond, there are 12 electrons for the sulphur atom. For phosphorus, we have 8 electrons, so 10 electrons, so we can see there are molecules which has more than 8 electrons, but still they are stable. This kind of molecules which has more than 8 electron, but stable, we call it as hypervalent molecule. Hypervalent, if you understand this, how do we calculate the number of electrons here? How do we get 12 electrons or 10 electrons? Please tell me, all of you. Yes, 2 electrons in one bond. D shell, that's also right. Okay, why these molecules shows more than 8 electrons, because they have D orbitals available. Sulfur belongs to third period, phosphorus belongs to third period and third period elements, we know they have vacant D orbital, hence they can expand their octet and we call it as expanded octet. That's why you see mainly the P block of second period, those elements only shows or follows octet rule, like boron also we have seen some exceptions in boron, mainly we have carbon, nitrogen, oxygen, chlorine. These are the few elements which follows octet rule, mainly other elements can easily expand their octets and can show expanded octet. Because of the availability of D shell, they can have more than 8 electrons easily. Next, write down the third exception here we have. In case of odd number of electrons, in case of odd number of electrons in outer shell, these molecules we also call it as odd electron molecules. For example, we have NO, we have NO2, CLO2, all these molecules are odd electron molecules, because they have odd number of electrons, you see this, nitrogen has 7, oxygen has 8, 15 electrons, odd electrons, 7, oxygen has 8 into 2, 23 electrons, again odd electrons, 7 plus 8 into 2, 23 electrons, again odd number, sorry, it is 17, 33 electrons, which is again odd number of electrons. So, since we have odd number of electrons, it does not follow octet rule. Yes, outer shell has 7 only, I am just calculating the total number of electrons, how do you find out the molecule is an odd electron molecules or not. So, it is the total number of electrons I have taken. This is 7, 8, 7, 8 into 2 like that. Yes, that's right, outer shell for chlorine has 7 electrons only. Yes, president, tell me. The last exception we have is in case of, in case of transition elements compound, means compounds of transition element. Now, in this one you see, the example we have transition element compound, we call it as coordination compound. For example, K4FE CN6, this compound we call it as coordination compound or we also call it as complex compound. There are many differences between the coordination compound and the simple compound, like NACL, MG, CO2, etc. Those things we are not discussing now because we'll discuss this in grade 12, you have a chapter called coordination compounds only. They will discuss in detail of all these things. But here you see the metal here that is iron, it takes electron from 6 cyanide. So, obviously it has more than 8 electrons, 6 cyanide ion we have and that's why it does not follow the octet rule. So, in this four cases, the octet rule is violated. Only second period element follows octet rule. In that also there are few elements like boron and all that does not follow octet rule. So, this theory, these are the drawbacks in this particular theory of octet rule. That's why we require a new theory of bonding and that is nothing but valence bond theory, MOT molecular orbital theory and other things which we will discuss later. Before going into that, first we'll see the Lewis dot structure. This structure we draw based on octet rule only. Lewis dot structure. There are rules which you need to follow here. This structure shows the bonding between the atoms of a molecule and the lone pair of electron. Next slide down, rules for Lewis dot structure. Just we need to follow the rules in order to draw the Lewis dot structure. The first one, write down count the number of valence electrons. The first step is to find out the number of valence electrons. Write down the first rule, count the number of valence electrons. First of all, write down the rules. I'll dictate it. Just a second. Hold on. Just a second, guys. Hold on. Otherwise, I would have shown you the rules. Not a problem. I wanted to show you the rules. It's not there. Anyways, Lewis dot structure. The first step, we'll find out the number of valence electron, valence electron. We don't count the total electron. Valence electron only becomes. Second point, add one electron for each negative charge and subtract one for each positive charge. Add one electron for each negative charge and subtract one for each positive charge. Add one electron for each negative charge and subtract one for each positive charge. Third point, choose the central atom. Choose the central atom. Generally, the least electronegative atom is the central Atom generally the least electronegative atom is the central atom then same third point next line write down Hydrogen can never be the central atom. Okay. All of you have written till here. Please tell me Because this is important. We'll do the examples based on this Ha so I'll go slowly over here, right so that all of you can write it down Otherwise, you won't understand the example Could you repeat the second point? Yes, the second point is add one electron for each negative charge and Subtract one for each positive charge Now fourth point you write down. Now in third point you have written in the third point next line you write down place the outer atom around the central atom place outer atom around the central atom and Insert a pair of electron Insert a pair of electron between the central atom and the outer atom place the outer atom around the central atom and Insert a pair of electron and insert a pair of electron Between the outer atom and the center line. This is the third point All of you have written please tell me no, this is the Third third point only in third point. I have given three points here three different lines. Okay now the next point Okay, or whatever number you have given follow the same. Okay Let it be follow the same but be alert right don't mix this here in there. Okay next point is write down place that remaining electron on the outer atom place the remaining electron on the outer atom Unshared electron place the remaining electron on the outer atom as Unshared electron in order to complete their octet in order to complete their octet Starting from the most electronegative one. Okay. I'll repeat this one again Place the remaining electron on the outer atom as unshared electron to complete their octet Starting with the most electronegative element next point Place the remaining electron on the central atom Place the remaining electron on the central atom on the central atom Next point you write down hydrogen should have Maximum two electrons more than two electrons for hydrogen is not possible. So hydrogen should have Maximum two electrons Beryllium should have four Beryllium should have four Boron and aluminium should have six electron Beryllium should have four and Boron and aluminium Should have six electron Beryllium should have four Beryllium should have four Boron and aluminium should have Six electron on an aluminium. These are the rules we have to follow in order to draw the Lewis dot instruction Mean is this? Okay. Now in this There are certain few points that you have to take care of Next write down and this point all of you write down carefully if the central Atom has more than eight electrons If the central atom has more than eight electrons Then it's atomic number should be more than eleven two three more points we have if the central atom Has more than eight electrons Then it's atomic number should be more than eleven next point is very important. Okay Central atom has central atom has less than seven electrons central atom has less than seven electrons then move an Unshared pair of electron then move an Unshared pair of electron from the outer atom the outer atom to form a Double or triple bond a double or triple bond With the central atom I'll repeat if the central atom has less than seven electrons less than seven electrons Then move an unshared pair of electron an unshared pair of electron on the outer atom In the outer atom a double or triple bond to form a double or triple bond With the central atom point calculate the formal charge Calculate the formal charge on each atom formal charge on each atom So the formula for formal charge is V minus minus you s is the Shared pair of electron of electron V is the number of valence electron number of valence electron and u is the number of number of unshared Unshared electrons with examples you will get it Okay, one more point you write down if a molecule is hypervalent if the molecule is hypervalent Then the charge on the central atom should be zero if the molecule is hypervalent Then the charge on the central atom should be zero or The charge on the ion I repeat this point again if the molecule is hypervalent Then the charge on the central atom should be zero or the charge on the ion Tell me all of you have done. We'll start the examples then Yes, with examples only you will understand the rules that you have written. Suppose the first molecule I am taking that is carbon tetrachloride, CCl4. We need to draw the Lewis dot structure of this, okay? Lewis dot structure of this. The first step is to calculate the number of valence electrons. So we will calculate the number of valence electrons. Valence electron means in the valence shell how many electrons are there. For carbon the number of valence electron is 4 and for chlorine it is 7 but there are 4 chlorine atoms 7 into 4 so it is 32. 32 valence electrons we have. Next is we will find out the central atom. Identify the central atom. The least electronegative element is the central atom. Carbon should be the central atom. Around the central atom we will place the outer atom which is chlorine here in this case. Next is what we need to distribute this 32 electrons among these atoms. Carbon and chlorine. How we will distribute that? We will start from this. Means one electron, one pair of electron we insert between the central atom and the outer atom like this. 32-8 electrons we have used. Remaining is 24 electrons. That 24 electrons we start placing on the outer atoms in order to complete their octet. You see this chlorine if you ignore this one. This chlorine has 2 electrons. This chlorine requires 6 more to complete its octet. We have 24 electrons. Out of 24 electrons, 6 electrons I will place here. This chlorine octet is complete. For this also we will do the same thing. We are left with 12 electrons after this. Out of 12, 6 electron will place here. 6 electron will place here. We are left with 6 more electrons and the last 6 electrons will place on the chlorine and this is the distribution of electrons. This is a very basic example I have done. If you draw this, you see octet of all the atoms are complete. The last step when you do everything, the last step is to calculate the formal charge. Let's see how to calculate the formal charge. Formal charge actually we calculate on each atom including the center atom and the outer atom. The bonding of all chlorine are same. The formal charge on carbon is what? Formula we use, number of balanced electron 4 minus half of the shared electrons. 246, 8 electrons are being shared. 8 plus the number of unshared electron on this carbon. There is no unshared electron. Sorry, we have negative also in the last. There is no unshared electron. Unshared means any lone pair is present or not. That is 0. So this should be 0. The formal charge on carbon atom is 0. For chlorine if you find out the formal charge, 7 is the valence electron. And for each chlorine the bonding is same, anyone you can take. This is the unshared electron. The shared electron is 2. So half of 2 electrons are shared minus we have 6 unshared electrons, unpaired electrons. This is also 0. Hence the formal charge on chlorine as well as carbon is 0. So this is the Lewis dot structure of C-C-L. If you want you can place the circle in order to complete show the octet of the atom like this you can place. The carbon, the octet is complete you see this. For chlorine the octet is complete you see this. The chlorine also the octet is complete. The chlorine also the octet is complete and the chlorine also the octet is complete. So all the atom has complete octet with 0 formal charge. So this is the best possible arrangement of atoms. Remember if you have placed everything and formal charge if you do not calculate, if you do not put, if any, then the entire thing will be wrong. Without formal charge nothing is correct. Did you understand this? Let's take another example CO2. Tell me the valence electron for carbon dioxide. The number of valence electron and which one is the central atom here? How many valence electrons are there? If you look at these examples you will get it. What is the Lewis dot structure? Tell me the number of valence electron guys. What is the number of valence electron? 16 you are getting. Number of valence electron for carbon it is 4. Oxygen it is 6 into 2. So we have 16 electrons. Central atom. What is the central atom here? It's in other value. We have carbon. So carbon is the central atom and oxygen will place on the other side of carbon. Then this 16 electron we have to distribute. So I will place one electron here, one pair of electron here, another pair will place here. Now we will distribute the electron on the outer atom starting with the most electronegative one. But we have oxygen only so we will place anywhere. But suppose we have two different atoms here, oxygen and other atoms. Then the one which is more electronegative will give the electron to that particular atom first. So here I have given you the rules for this. Central atom is generally the least electronegative one. And if you have three to two oxygen and one carbon obviously it's common sense that oxygen cannot be the central atom. Symmetry if you see. So carbon is the central atom least electronegative one. So we are left with 12 electrons. So we will place 6 electron on this oxygen so that the octet of oxygen is complete. So that the octet of oxygen is complete. Then I want you to go through one point here. Maybe the point number 9 or 10 in which it is written that all of you listen. If central atom has less than 7 electron. Could you see that point? 10th, 1 or 9th or 11th you can check. If the central atom has less than 7 electron. Did you see that? What is that rule? If the central atom has less than 7 electrons then move an unshared pair of electron from outer atom to form double or triple bond. Correct? So what we will do? We will move one electron here. Right. You see this? Carbon has 4 electron in this bonding state. So one pair will move here in order to make a double bond with carbon atom like this. You see it is still the octet of oxygen is complete. Oxygen has no loss here. Whether you place this a 2 electron here. The octet of oxygen is complete. If you place the 2 electron here it is still the octet of oxygen is complete. Oxygen has no loss into this one. But the advantage is what carbon has had 4 electron. Now it has 6 electron. Again you shift this electron pair over here and we are doing it according to the rule. Then you see oxygen is still the octet of oxygen is complete but carbon also has complete octet. So this is the distribution of electron. The perfect distribution of electron. The last one is what? It is not the bonding. We are trying to understand the distribution of electron. How do we draw the Lewis dot structure? In Lewis dot structure we do not draw the line. Single bond or double bond we don't draw like this. We just put this dot. There is no point of talking about here the coordinate bond or covalent bond or something like that. Because we don't represent the bond. We just represent the electrons here. Now last step is what? To calculate the formal charge. The formal charge if you count on oxygen. Could you tell me the formal charge on oxygen? For oxygen it would be 6 minus 4 electrons are there. So half of 4 minus 4 electrons are there. So 0 formal charge on oxygen. For carbon also if you count it will get 0 formal charge on carbon. So this is the structure of CO2. See you can draw other structures also. You can draw other structures. Like you can ask me like I have done this. If you are asking first of all if you are asking why to count formal charge. You can also ask me why to distribute electron like this. We are calculating formal charge to write down the loose order structure. That's the answer your question. But the thing is there are many other distribution possible. It's not like only one kind of distribution is there. But with the distribution of electrons the formal charge changes. Without the formal charge the molecule is not stable and it is not correct structure. Not stable we cannot say. Like you see I will give you one example here only. I had done this. You did not ask me this question but you could have asked me this question also. This was the arrangement we were talking about. Because we have then I said what? That one electron one pair of electron I will remove from here and I will place it here. And then what I said one electron from this side will place it here. You can ask me so why not you place this electron here only. Can we do this? Can we do this? The point is we can do this. But this structure is not right now. It's not right now. When you place the formal charge we will have a formal charge on this oxygen. If you find out you will get a negative formal charge here. If you find a positive formal charge on this oxygen most probably you can count you will get that. So with this formal charge this structure is correct. If you do not draw the formal charge your structure is wrong. First thing is that second thing which one is the acceptable one. Acceptable one is the one which is in which the atoms has no formal charge. This and this if you compare this is negative and this is positive. The more stable structure that's why we draw this way. Did you get my point? Yes. Tell me. Yes. I can show you one more thing here. Let's discuss this way also. If you draw a bond here this structure is nothing but this one. O double bond C double bond O isn't it? Yes. And this structure is nothing but O single bond C triple bond O. With one lone pair with three lone pair one negative charge and one positive charge. Okay. Now if you see this. This is a conjugated system by Sigma lone pair. If you draw the resonating structure for this one. Then what do you get? You'll get this one. Yes. Could you connect this with resonance? Yes. So now you can answer your question itself. If you do not put the charge over here. Will this structure be correct? No, it's not. Is it right? So the thing is that when you distribute the electron, the distribution of electron leads to some charge on the atom. Correct. Without placing those charge that structure is not correct. It's a different matter that which structure is more stable one. This one or this one? The more stable one only will draw, right? But we can draw this structure also. Obviously this is the resonating structure for this one. So this is not wrong. But yes, it is not as stable as this one is. Is it clear? Tell me. Could you draw this one? We'll do many examples into this one because with example only you will understand this. XCF2 you try. Let me know once you're done. So in this one, how many valence electron? 8 for xenon and 7 for chlorine. 22, right? Obviously xenon is a central atom. The least electronegative one. Floating will replace this side. Then one pair in between the atoms. We are left with 18 electrons. So 1, 2, 3, 4, 5, 6 octet is complete. How many electrons? We are left with 6 plus 6, 12, 12 plus 4, 16. We are left with 3, 6 more electrons. So we have 3 lone pairs on xenon. This is the structure. If you calculate the formal charge, what is the formal charge on xenon? Xenon the formal charge 8 minus half of 4 minus 6. So 0. There's no charge on xenon. Floating also we have 0 formal charge. Hence, this is the structure we have. This one XEF4, yeah. So XEF4, how many valence electron? Pavati. Shatish, how many valence electron? Is it 36? Yeah. 8 plus 7 into 4, 36. So central atom is obviously XE. Then fluorine, fluorine, fluorine and fluorine. Okay. So we'll have 1, 2, 3, 4, 5, 6, 7, 8, then 6 electron on each fluorine atom. That plus this is 32. 32 electrons we have used. So xenon will have 2 lone pairs on it. Is it 2 lone pairs on xenon? Right. Formal charge on fluorine is 0. Xenon it's also 0. If there's any formal charge, you have to place it like minus 1 plus 1 on the atom. You have to place it. Right. You cannot count like this and leave. Okay. So this is just I'm doing it for you to understand how to calculate the formal charge. But if it is any formal charge there like minus 1, minus 2 plus 1, then you have to place here on the atom. Okay. Understood this one. We'll do some more examples into this. Okay. Next class and then we'll continue with the other theory of bonding. Okay. Some more examples required. Assignment I've given you already. Module questions you need to solve, redox and mole concept too. Okay. We'll see you in the next class. Take care.