 Good morning guys, can you hear me? Yeah, so what have we done last class? Because that's a class management platform, class flow. Okay, so P pi P pi we have done. Rather, I think you can reset the password, is it? Okay, so last class we discussed about P pi P pi back bonding, okay. And I said there are two types of back bonding we have. One is P pi P pi. And other one is P pi D pi back bonding, okay. So we have seen the example of the Lewis acidity of BF3, Lewis acidity of BF3, BCL3, BBR3 and BI3, right? Now the next one write down the second type of back bonding. That is P pi D pi back bonding. There's two small things we need to discuss and then we'll finish it. P pi D pi back bonding. So here we have P orbital and D orbital involved in back bonding. Back bonding, the definition is exactly same. There's no difference here. But the orbital that is involved is P orbital and D orbital here, okay. So if you see here, the stability of, suppose we have CH3 negative charge carbon ion and CF3 negative charge carbon ion. Because of more electronegativity of fluorine, CF3 is more stable than CH3 minus. The stability order is this. CH3 and CF3. It's CH3 minus carbon ion, it is proven. CH3 does not exist. CH3 minus is an intermediate carbon ion possible, okay. If you compare the stability of CCl3 negative charge and CF3 negative charge, then CCl3 is more stable than CF3, this is because of the back bonding. Stability order is this. So what happens you see, it is a kind of resonance we have here, carbon with three chlorine atom and we know chlorine is belongs to the third group, right. So each chlorine atom has vacant D orbital. Vacant D orbital, that's why this lone pair of electron it donates into the vacant D orbital of chlorine. And hence we have P pi D pi back bonding. P pi D pi back bonding. Where P orbital of carbon and D of chlorine involves. And that is why CCl3 is more stable than CF3 minus. Okay, understood. If you see these two molecules, the other example here, suppose we have N CH3 whole thrice, lone pair or nitrogen atom. And another one is N SiH3 whole thrice and lone pair. Okay, so this N CS3 whole thrice, this one is tetrahedral geometry. Tetrahedral geometry. But this one is, this one is trigonal pyramidal or simply pyramidal or simply pyramidal, okay. Why this happens? Because you see in N SiH3 nitrogen has one lone pair on it, but it is attached to SiH3 and silicon has vacant D orbital. Because this also belongs to the third group. So this silicon has vacant D orbital, D orbital. Hence this lone pair it can donate into vacant D orbital. And hence here P pi D pi back bonding is possible. P pi D pi back bonding is possible. But in carbon, there is no vacant D orbital. Hence the back bonding is not possible. And hence all the four, three CS3 group and one lone pair present tetrahedral to each other. Hence the geometry is tetrahedral, but here it is pyramidal. Understood this? No, it's not trigonal planar. It's like the three SiH3 present at the vertices of the triangle and nitrogen is above it, right, like this. So this is the two types of bonding we have. P pi D pi back bonding, P pi D pi back bonding. You see both P pi P pi back bonding and P pi D pi back bonding. It is a kind of resonance. Why it is a resonance? Because you see A atom has lone pair, then a sigma bond, B atom either has vacant P orbital or vacant D orbital. This is what the situation we have. This is P pi D pi P pi P pi. So this is also a conjugated system. Lone pair, sigma vacant orbital, right, conjugated system. All these P pi P pi back bonding and P pi D pi back bonding. It is possible because of resonance only. Yeah. Now the last one thing here is odd electron bond. Odd electron bond. What is an odd electron bond? Odd electron bond is a bond of one electron or three electron. This we also call it as half bond, half bond. It's bond order, it's bond order is half, that is 0.5. Only two examples we have here, which you have to keep in mind. Look at this example. The molecule which has odd number of electrons, the molecule which has odd number of electrons, are said to have odd electron bond. Like for example, you see NO. The number of electrons NO has, number of electrons NO has, you can see, eight for oxygen, seven for nitrogen, that is 15. If you talk about the valence electron here, five for nitrogen, six for oxygen, seven for oxygen, seven for oxygen, five for nitrogen, six for oxygen, that is 11. 11 is the odd number. Hence the bond, the molecule contains odd electron bond. Okay. So NO, if you see the bond here, see what I'll do with the help of Molecular Orbital Theory, MOT, we'll find out the bond order of this molecule first. Let's find out the bond order first. So the orbital is sigma one S, then sigma star one S, sigma two S, sigma star two S, then pi two P, x pi two P, y sigma two P, see, that is enough, right? So we have two electrons here out of 11, two, then four, then six, then eight, nine, 10, 11. This is the distribution of electron, okay? So bond order, if you calculate, number of electrons in bonding molecule orbital, that is two, four plus three, seven, minus number of electrons in anti-bonding orbital, that is four divided by two, and we have 1.5, the bond order. Is it 1.5? See, what we have done here, for bond order, we need to have the valence electron, and valence electron is 15 here. Sorry, total number of electron, not valence electron. The number of electrons, or we can write here, the total electrons is equals to 15, and that's why the order of this orbital, anti-bonding and bonding molecule orbital, that is this. Here we have sigma of two Pz, pi two Px, pi two Py, pi star two Px, pi star two Py, sigma star two P, see. So here we have two, four, six, eight, 10, 11, 12, 13, 14, and 15. Based on this, if you count, because we take the number of total electrons, not the valence electron, if you count this two, four, six plus four, 10 minus two plus two, four plus one, five divided by two, that is 2.5 we are getting. This 0.5 means we have a half electron, sorry, half bond, that's why this 0.5 we have. So 2.5 means we have two complete bond and one half bond, that's why the bond order is 2.5, okay. Now, if you draw the Lewis dot structure of NO, and this you have to memorize, we have nitrogen and oxygen, one pair of electron will place here, and then we'll give six electron to this oxygen, six plus two eight, then two electron to this nitrogen, one electron we are left with. So one, you see it is an odd electron, single electron, so this is the odd electron. So next what happens, since the atom has, since the bond order is 2.5, it means we have two complete bond here, it means one pair of electron, you need to shift, lone pair you need to shift and forms a bond pair, okay. So how do we do that? We'll have an O here. So one electron will shift from here and we'll get a double bond here, okay. And then in the last, this NO has three electron bond, okay. To make the bond order 2.5, what we do, the final Lewis dot structure is nitrogen has one lone pair and two bond, two complete bond like this oxygen and one bond we have here, which has three electrons with one lone pair on this oxygen. This three electron bond that you have, we call it as odd electron bond. So you should memorize this, that in NO we have a three electron bond. How do you know that the bond, that the molecule has an odd electron bond? You count the number of electron, if it is an odd number, the molecule has an odd electron bond, okay, got it? Yes, the strength of odd electron bond, the strength of odd electron bond is lesser than the single bond. Write down the strength of odd electron bond is lesser than a single bond and hence it is called half bond. Yeah, if you compare the stability of this compound with the normal compounds, it is lesser stable, okay. That's why this bond generally won't form, but yes, for a few examples it is true, okay. One last point you write down. Compounds containing, compounds containing odd electron bond, compounds containing odd electron bond are paramagnetic in nature, are paramagnetic in nature, paramagnetic in nature, since it contains unpaired electron. See, if the molecule contains unpaired electron, it is said to be paramagnetic, okay. If all electrons are paired, then it is diamagnetic. Okay, one more example we have into this one, the last one, that is the example of NO2, okay. If you call the valence electron for NO2, it is five for nitrogen and six into two for oxygen. So we have 17 electrons here, valence electron, okay. In this also, the structure that we have is this. N double bond O, one unpaired electron in this nitrogen, one single bond O, and we have a three electron bond. Like this, three electron bond. This we also call it as odd electron. Understood? Just these two examples you have to keep in mind for odd electron bond, and that is it. Properties you must remember, odd electron bond, we call it as half bond, because it has half bond order. Paramagnetic in nature, because of unpaired electron, and the strength is lesser than to that of a single bond. Yeah, you can also consider CLO2. Both are same, we can also take CLO2. Okay, Ruchita? Yeah, see Meghana, both are correct only. To draw the Lewis dot structure, we are taking valence electron, okay. Now to draw this molecular orbital, this configuration, I have taken total electrons, okay. So for this, I have taken total electrons, but to draw the Lewis dot structure, I am taking only valence electron. So both examples, I'm doing the same thing. I have not drawn here the molecular orbital configuration, sigma one and sigma star one is, because that is not required. I have just done in the first one just to make you understand what is the meaning of half bond and how do we get it? How do we get this idea that the molecule has an odd electron bond? Yeah, since the bond order is coming as 0.5, 2.5, it means we must have a half bond and hence it is the odd electron bond. NO has 15 electrons, so first we have sigma 2pz, then we have pi star 2px, and then we have pi 2px, pi 2py. It has one unpaired electron, and this is the odd electron bond we have. If the bond order is an integer, then it means we don't have an odd electron bond, because odd electron bond has half bond order, okay? Understood? Odd electron bond always contributes half bond order, so we always have 0.5, when the molecule has odd electron bond. Understood, guys? Okay, fine, so this is it for chemical bonding, okay? We have done everything in this chapter. Next, we are going to start with gaseous state, okay? You have exams, probably this session will help you in your exam also, one second I'll just start it.