 Hello guys. How are you? Okay. Please type in your name. Okay. So today again in this session, we are going to discuss some board level questions on chemistry okay, so Till now, I think we have already finished four to five years question Four to five years previous year questions of chemistry which has been asked in class 12 board exam Okay, so today we are going to discuss one more Set of questions. Okay, which is Rome has been asked in Board exam. Okay, so All of you who are there, please type in your name in the comment box and We'll start the class in few minutes. Okay You see this question question number 30. I'm giving you. We are starting from the back Think about it few minutes Then we'll start discussing it. Yeah. Hi, resist justice again, let me you know Justice justice again, so this question number 30. Yeah, tell me what is the answer of the first one? Why do transition elements show variable oxidation is just one key what you write down you don't need to write down all the answers Electrons in N-1 DNS can participate in metallic bonding due to a similar energy in So it is basically because of uncured electron in the orbitals. Okay. Yeah, it's correct So that's how it shows variable oxygen state depending on the compound. Okay What is the answer for the second one name the elements showing maximum number of oxidation is state Among the first series of transition metals among the first series of transition metals. What is the answer for this one? Right, it's magnies Next question name the element which shows plus three oxidation state Only plus three not plus three only plus three. What is the answer? Elements which shows only plus three oxidation state Scandium right Scandium is the only elements in 3d series which shows plus three oxidation is state You can draw the electronic configuration and you'll understand. Okay, and there are a few more things into that the size of the elements and Intered electronic repulsion also. So basically because of all these factors is caddy Scandium can only, you know a Release three electrons and so that it can show plus three oxidation is state. What is lanthanide contraction? Tell me, okay So what is lanthanide contraction? It's more than the expected decrease in size of atoms across lanthanide series due to poor shielding effect of 3d and 4f orbital 3d also you mentioned Okay So lanthanide contraction is basically is the reduction in size in the elements of lanthanide series Okay, reduction in size in the elements of lanthanide series due to poor shielding of d and f orbital completely filled d and f orbital, it's correct Yeah, and what is this mesh metal? It is MIS CH I think This is check this spelling is it MIS CH mixed metal Yeah, so the example is Mish the spelling I'll write down here the important alloy which contains Some of the lanthanide metals. The name is M or just a second. I'll just reduce this MIS CH This is the spelling. It is the alloy of I'll write down here alloy of cerium Lanthanum lanthanum there. These are the male two elements present here and apart from this there are other elements also present Okay, okay, so this is the example of Alloy which contains lanthanide metals cerium lanthanum mainly it contains and apart from these two They also contain some rare earth metals. Okay and This this kind of you know alloy it is used for the you know Production of the flints of lighters, okay flints of lighter used to products used to produce with the help of this, okay? So that's the thing. Okay. We use to prepare. So that's what use we have here flints of lighter views Okay, next question you see we have to see no Just a second we have done this now this one Yes Complete the following equation CR the four two minus in acidic medium What is the answer in the second one CR two or seven two minus process two is correct? Second one. What is the answer K2 MnO four plus MnO two is the reaction is balanced You just check you write down KMnO four on heating gives K2 MnO four plus MnO two is it balanced? The reaction is balanced or not So with O2 it is balanced. So the product in the second reaction will be K2 MnO four Plus Will get here MnO two and O two now you see the reaction is balanced here We have to KMnO four then the reaction will be minus this forms CR two or seven two minus with H2O Now all these you see charge Atom everything is balanced here next question Give reasons for all these Mn three plus is the changes to Mn plus two The orbital really accepts another electron as T5 for configuration. Yes Correct the reason is correct, but you see one thing. I will tell you here I mean three plus where it get oxidized When it gets reduced and oxidize others, which is nothing but the oxidizing agent behavior So when it is it behaves as an oxidizing agent Mn three plus will have tendency to convert into Mn plus two right like all of you are saying This configuration is extremely stable. Okay, so that is also right. It's not wrong But it's better to write Mn plus two has exactly half fit the configuration Which is highly stable because of symmetry, okay All these keywords are important when you write down the answer in your answer sheet for board exams. Okay, so you must use this keyword that it has half fit configuration, which is extremely stable because of Symmetry, okay, so that's the keyword you should write here. So take that in keep that in mind Okay, so it's correct. Your answer is correct, but all those keywords that I told you keep that in mind now Second one E naught Mn two plus two M values are not regular or first drop term is an element. Tell me Yes, good Yes, variation in ionization in thalpe's are irregular and why is that so because of the variation in size? Okay, so size also you can write down here left to right if you go in a periodic table the size What happens size size decreases because of increase in effective nuclear charge, right? So when you go left to right in a critical the size decreases because of increase in effective nuclear charge But usually in case of transition element, what happens when you go from left to right in 3d series The electrons are continuously going into the orbital Okay, and as the electrons are increasing in number into the orbital there is more inter-electronic repulsion Right and because of inter-electronic repulsion and the effective nuclear charge both are Both are contradicting to each other, right? Both are contradicting each other because of inter-electronic repulsion the side the elements has Like because of that repulsion the size should be more but but because of effective nuclear charge the size should be less Okay, so both are contradictory to each other. So when you keep on going from Left to right direction in a period Number of electrons in the orbital is increasing which increases the inter-electronic repulsion Which gradually overcome the effective nuclear charge and hence the size becomes similar After a few elements five or six elements and then for the last two elements copper and zinc the size increases because the inter-electronic repulsion then dominates the Effective nuclear charge the point I'm trying to make is what as we go From left to right in your periodic table in transition series 3d series the size first decreases First the size decreases then becomes constant and then increases for the last two element Okay, so you see there is a irregularity in size of the elements Okay, and because of this irregularities also Ionization enthalpy is also not regular ionization enthalpy is also Irregular because of this only this value which is nothing but the ionization energy this enthalpy is Not regular. That's what the question. Okay, so you have to mention the size thing also here Right now the next question is what although Floating is more electronegative than oxygen the highest magnees fluoride is Mn F4 Whereas the oxygen the highest oxidized oxide is Mn 207. Okay, and why is it so?