Std 11 Chemistry Session on Electronic Configuration by Dr. D.P. Sankaran

[00:00:05]Good evening students.

[00:00:08]Welcome to Aaaguru classes.

[00:00:11]I am Professor Shankaran.

[00:00:13]On my behalf and on behalf of Aaaguru, I wish you all the best in all your examinations.

[00:00:22]And today my dear students I'll be doing electronic configuration at the fundamental level.

[00:00:31]I'm sure most of the school they would be just doing this particular chapter.

[00:00:35]That is why we have taken this topic.

[00:00:39]And today's class is from 6 to 7. The chat box is open. In case you have any doubts, you may post your doubts, forward your doubts to the mentors. I shall try to clarify them. So with that small introduction we shall proceed and my dear students I'm going to do electronic configuration of elements or atoms.

[00:01:08]I will not go into the atomic structure.

[00:01:10]It has to be done in two halves. One regarding the atomic structure, the other one is electronic configuration.

[00:01:17]second half of the structure of atom. So an atom all of you would have studied in the 10th standard has got the nucleus and electrons revolve around the nucleus and the nucleus contains a protons and neutrons they are packed in a very small space and electrons revolve around the nucleus like a it is compared to a solar system.

[00:01:43]In a solar system, sun is placed in the center and all the other planets revolve around the sun in various orbits.

[00:01:52]Similarly, planetary electron that's why they are called planetary electron because they are similar to the planets in a solar system and electrons and we have got various orbits.

[00:02:07]These orbits are known as shell.

[00:02:12]I shall write. They are also known as orbits and these orbits are identified as a main energy level. Main energy level of course energy levels are different. The energy decreases from the occasion and L shell etc. Each shell is named as the first shell immediately after the nucleus is called the K shell. The next one is higher energy level L shell. The next one is M shell, N shell etc. And there was there is no need for more than four shells in those days because the atoms were not discovered that much.

[00:03:01]And as the energy level increases from K to L to M to N energy level K shell less than L shell etc etc. So the stability stability goes by the energy of a shell.

[00:03:15]Lesser the energy greater the stability.

[00:03:19]So in that context K shell is the most stable shell. K shell is more stable than L than M than N etc. And my dear students each orbit or each shell or main energy level can accommodate only with maximum number of electrons as follows. The K shell is the smallest one. So it can accommodate only two electrons and L shell can accommodate maximum number of eight electrons. The third shell, this is the first shell, second shell, third shell and the fourth shell.

[00:03:56]The third shell can accommodate 18 electron. The fourth one can accommodate 32. And this is the number that can be obtained by using this formula 2 n² where n is = 1 whole number 1 2 3 4 up to infinity up to infinity when n= 1 the maximum number 2 n²= 2.

[00:04:21]When n= 2 2² is 4 4 into 8 like that they can ask you this particular question. Supposing I give you imaginary number n= 6. N is equal to 6.

[00:04:35]In case you have an atom containing the sixth shell also. So what should be the value of the maximum number of electrons in the sixth shell here. So it should be 2 into 6². So 2 into 36 and that should be it should contain maximum number of 72 electrons. 72 electrons. That is how one can find the maximum number of electrons in each orbit. It can contain not necessarily maximum, it can be less than the maximum also. The K shell need not contain always two electrons. It can contain one electron or two electron.

[00:05:12]Similarly, L shell need not always contain eight electron. It can contain electrons from 1 to 8. 1 to 8.

[00:05:21]So that is the the the macro number of electron that can be accommodated in a particular energy level. Now let us go into the the arrangement of electrons in these various shells and we need to know that total number of electrons in an atom and the total number of electrons is decided is obtained from the atomic number.

[00:05:45]Total number of electrons is obtained from the atomic number. You know you have studied this in the 10th standard.

[00:05:51]Atomic number is a total number of electrons or protons in an atom. And you know you have equal number of electrons and protons in an atom because atoms are always neutral. So protons are positively charged, electrons are negatively charged. So in order to have a neutral atom, you should have equal number of electrons and the protons in an atom. And that is normally represented by the symbol z mass number. Let us know the difference between the atomic number and the mass number. Mass number gives you the sum of the protons and neutrons inside the nucleus. For example, I have given you the various symbols of an atom and hydrogen, helium, florine, sodium, iod, bromine. Now this is what is known as atomic symbol.

[00:06:49]The atomic symbol you you are able to find the subscript atomic number and super superscript the mass number. Now in the case of the hydrogen atom it's a very unique element here. You look at this atomic number is one. Mass number is also one. Atomic number tells you the number of electron.

[00:07:12]So hydrogen has got one proton and one electron that's all and neutron is zero zero neutron no neutrons this is the only element which does not contain any neutron neutron now in this case two helium 4 it is four here therefore it has got two electrons two protons and two neutrons and the four is a mass number number of protons UTS plus the neutrons. Similarly, you can find the number of neutrons. Of course, we don't need the number of neutrons. What is important is we need to know the number of total number of electrons.

[00:07:55]Electrons are more important particle of an atom. From the number of electrons, we are able to write the electronic configuration. From the electronic configuration, you are able to have an idea about the valency. If you know the valency know you are able to compare the reactivity of that particular element and so on and so forth. Now my dear students we shall go to how these elements electrons arr the the hydrogen atom. Maybe we shall go to the next page and give you that page number two. Page number two, we have hydrogen atom.

[00:08:41]Atomic number is one. Therefore, we have one electron. And we shall write K, L, M, N. It's at a different energy levels.

[00:08:52]Since it has got only one electron, that electron should go to the or shell which has the least energy level. So, it should go to the K shell only. And that is going to be ground state electronic configuration. And of course if you supply energy you will find that the electron can be shifted to the second shell or third shell etc [laughter] etc. We shall study all those things a little later. Now we shall now try to understand the electronic configuration.

[00:09:21]The ground state lowest energy level ground state electronic configuration is the least energy level. Electrons with the lowest energy is called the ground state that is ker.

[00:09:39]The next one we'll go to the helium.

[00:09:44]Helium the atomic number is two.

[00:09:50]Atomic number is two. Now K shell can accommodate both the electrons because it it has it can accommodate two electron. Therefore both the electrons enter here.

[00:10:01]And the next element is going to be lithium.

[00:10:06]I shall write here lithium atomic number three. Now my dear students please note that it can accommodate only two electron the K shell and the remaining one electron goes to the L shell. So the electronic configuration is 21.

[00:10:23]Okay we'll go to nitrogen. Nitrogen atomic number seven.

[00:10:31]Now we shall go to two electrons the K shell the remaining five electrons in the L shell. The next one is florine. We shall take florine atomic number nine.

[00:10:44]So two electrons in the K shell remaining seven electrons in the L shell. 27 is electronic configuration.

[00:10:53]The next one neon neon I'm going to compare make use of the periodic table also simultaneously neon is present in the last group of the periodic table you just imagine it has got 18 groups and the 18th group element is reard gases. Now neon the electronic configuration 28. My dear students please note whenever an element has got eight electrons in the last shell last shell it is said to have posess it it is said to have got octed configuration octed configuration now we'll be making use of this particular information an element having octed configuration the last it is supposed to be a very stable There is no chemical reaction. It is inert and inactive.

[00:11:51]Inert and inactive.

[00:11:54]Inactive.

[00:11:56]And that is why those elements present in the 18th group they are called inert group elements. Helium is to start with. Then we have non, argon, krypton, zenon. I am I'm giving you additional information. These are called inert elements.

[00:12:18]Inert inert gas elements. They are in the gaseous. All of them are gaseous state.

[00:12:27]All of them are the gaseous state. Inner gas elements. And accepting helium.

[00:12:33]Excepting helium.

[00:12:35]You have accepting helium.

[00:12:42]Helium has got only two electrons in the last shell.

[00:12:47]Helium has got only two electrons in the last shell excepting helium has got two electron.

[00:13:02]Okay. Next we'll go to Yeah.

[00:13:31]Just a minute.

[00:13:52]Yeah right now helium only has got two electron that are shell all the other elements have got eight electron shell octed configuration we are going to see it now okay the next one we shall go to sodium sodium atomic number is 11 it has got 11 electron electrons. So you put K two electron the K shell and can you put remaining n electrons in the next shell?

[00:14:21]No. Because L shell cannot accommodate more than eight. Therefore we put next maximum is eight and then remaining one electron in the M shell. K L M it has got one electron the last shell. One electron in the last shell. Similarly we have got chlorine. Let us take chlorine.

[00:14:44]Let us take chlorine.

[00:14:47]Atomic number 17. You don't need to know the mass number. Atomic number 17. So we have got two electron shell and eight electrons the next shell. So 8 + 2 10 remaining seven electron in this. Now my dear students, now we shall come to an exception here. 20 potassium. K is potassium is the symbol for calcium.

[00:15:11]Calium the first letter is K. Calium is potassium.

[00:15:19]It's an alkalic group metal. Potassium.

[00:15:22]Now you see here K, L and M. We have two electrons in the cial eight electrons.

[00:15:31]We have put the next maximum 8 + 2 10.

[00:15:34]So remaining 9 electrons. Can you put nine electrons here? Because because it can accommodate 18 electrons but you are going to find that is an incorrect answer here. Potassium does not contain eight nine electrons. Now what is the actual electronic configuration of potassium? Two K in the K shell eight in the L shell another eight in the M shell remaining one. Now now what's the reason? What is the explanation here?

[00:16:07]Now my dear students, how do I know that it has to be put only eight here not though it can accommodate more than 18 here? We have got there are fundamental rules for the electronic arrangement of electrons in various shells. Now there are couple of rules. One was rule says the last shell cannot exceed eight electrons exception is helium.

[00:16:45]Now I shall come back I shall explain give the example and the penultimate shell penultimate shell does not contain penultimate not to exceed need 18 electrons is the rule three. Now what is the penultimate last but one last but one I shall explain give an example then we have got prep penultimate shell prep penultimate shell not to exceed 32 electrons and please remember these rules and then follow the electronic configuration.

[00:17:50]Now we shall take same potassium 19 here. This is going to be 2 8 that is why we are not going to put nine here.

[00:18:02]If you put nine here the last shell exceeds eight electrons. So as per the rule it's a qualitative rule thumb rule qualitative rule.

[00:18:14]Of course we have got explanation also later on where I'm going to explain it.

[00:18:18]Now therefore this is not the electronic configuration. Therefore we try to put the next maximum eight here and one here one here. Okay. So this is going to be the electronic configuration penultimate shell. The last shell is this is the in potassium K L M N. What is the first shell? First shell is always K shell. And what is the last shell? In the case of potassium n shell, fourth shell. And what is the last shell? Penultimate shell. In the case of potassium, the third shell is a penultimate shell. I can ask you this.

[00:19:00]This could be one question here.

[00:19:02]Chlorine, bromine and iodine. For example, iodine has got how many shells?

[00:19:09]How many shells? Chlor. Please remember I'm going to connect this to the periodic table. Periodic table we have got certain elements called H hallogens. The H hallogens are florine, chlorine, bromine, iodine.

[00:19:30]Now there are only two shells in the case of florine. We have already seen atomic number. So there are going to be 27. Sorry it is not 22. It is 27 in the electronic configuration. Now you are going to appreciate one more thing here. Chlorine has got 17. So add eight more.

[00:19:54]The next one broine of course you have to after some time you know the electronic configuration. I shall explain why this is. So the next one is going to be I'm just for the sake of example I'm to answer my question here. So florine has got two shells, chlorine has got three, bromine four and iodine. What is the last shell? I can ask you the number of electron the last shell for iodine is seven. Number of electrons the penultimate shell of the iodine 18 electrons. The number of electrons in the prep penultimate shell.

[00:20:31]So what is last shell which has the highest energy level? The penultimate shell last but one prep penultimate shell last but two. Okay, this is how we try to write the electronic configuration. I'm sure my dear student you would have observed one interesting factor here. All these elements kept in the 17th group of the periodic table. We are going to combine periodic table also. You know today you take it for granted that these are elements are present in the 17th group of the periodic table and they are called halogens.

[00:21:08]All of them have got seven electrons out and they they should have the chemical property is based on the number of electron shell. If the number of electrons is going to be identical then all of them will have similar chemical properties. So all h hallogens have got similar chemical properties. That is a correct statement. Okay. Now we shall go to another interesting factor namely atomic number is 26. Atomic number is 26. Maybe you want to have some more examples for writing the electronic configuration. Yeah, I shall we we shall take up this a little later.

[00:21:55]Now first group of the periodic table as I told you lithium, sodium, potassium, rubidium and cesium. Of course you have got franchium is radioactive element.

[00:22:16]Francium is the radioactive element. The periodic table is gi is available in the textbook. The atomic number also. Now these elements are called alkaly metals.

[00:22:34]You know the difference between an alkaly and a base in the 11th standard. Alkaly is a soluble base.

[00:22:45]Any base which is soluble in water is called an alkaline.

[00:22:50]What is a base? A base is the one which react with an acid to form salt.

[00:22:59]It's a neutralization reaction.

[00:23:03]Neutralization reaction. Neutralization reaction is reaction between an acid and a base. The base can be soluble and insoluble. If it is a soluble base, it's called an alkaly. Alkaly. Now sodium hydroxide is an alkaly. Sodium hydroxide is an alkaly. It's soluble. Calcium carbonate is not a alkaly. It's only a base. It's only a base. This is both a base and an alkaline. Both a base and an alkaline.

[00:23:39]All the bases need not be soluble.

[00:23:43]All the soluble bases are called alkaline. Alkalies. Okay. Now the I I'll tell you this is the second period please if you if you have an idea about the periodic table the first period there are only two elements hydrogen in the first group and helium in the 18th group 18th group then you have second group third group second group so we should know the first group first period we should let let me write the number of elements in the period also.

[00:24:23]I shall write period and elements in the periodic table. First period has got two elements. Second period has got eight maximum eight elements.

[00:24:39]The third period also has got eight elements. The fourth period 18 elements.

[00:24:45]Fifth period also has got 18 elements.

[00:24:48]Sixth period has got 32 elements.

[00:24:52]Seventh period is incomplete or it could be 32 also. 32 also. Now we shall see.

[00:25:00]These are called the magic numbers. 2 88 18 18 32 you need it's enough if you know up to 32. Okay. Now this you have to remember 288.

[00:25:14]Now look at this. In what way it is going to be useful to you these numbers.

[00:25:19]Now you know lithium is the third element in the periodic table. Lithium is the third element in the periodic table. Therefore the atomic number is three.

[00:25:31]Then if you look at the lithium periodic table there are eight elements in the second period. Eight elements in the second period starting from lithium. Now you have to cross all the eight elements then come back here. Don't you think you have to add eight here and that is the atomic number of this. You don't need to know the periodic table at all. You yourself can guess the periodic table.

[00:25:57]My dear students there are eight elements here. Third period also it has got eight elements maximum here. So you know what should be your potassium atomic number. So 11 + 8 19.

[00:26:11]Now in the fourth period there are 18 elements. There are 18 elements in the fourth period. Now you have to add 18 to the 19 that because this is the 18th element from this 18th element from this. This is the third element from hydrogen. This is 11th element from hydrogen. Eighth element from lithium. 18th element from sodium.

[00:26:41]19th element from hydrogen.

[00:26:43]Okay. Now you add another 18. 19 29 37 is the atomic number of rubidium. My dear students, you know, you know how to guess the atomic number of cium. This is the third period, fourth period, fifth, sixth, seventh period and seventh period. I told you it has got 32 elements. 32 fifth period sixth period onwards. So 1 2 3 4 1 2 This is the second period.

[00:27:15]Look at that.

[00:27:17]This is the second period.

[00:27:20]Third period, fourth period, fifth period, sixth period. There are 18 elements. So he have added it. And now from here it is going to be sixth period. Add 32. 32 is going to be 18 potassium. Huh? From here they have to add another 18, isn't it? From rubidium sium is 18th element. Add 18. So 37 47 55.

[00:27:59]Now there are 32 elements in the next period. You have to cross 32 element before you come to franchium. Add 32 here. And that should be 87. 87 is the atomic number. Anyhow, I shall take up one more example also before I I shall take take up that example little later.

[00:28:20]Now my dear students, having got the atomic number, having got the atomic number, you know the number of electrons and we know if you know the number of electrons, you can you know the electronic configuration.

[00:28:44]Now we shall write the electronic configuration. Now it's easy to write the electronic configuration. Okay. We shall go to Yeah.

[00:29:01]Now you see here we don't need all this informations.

[00:29:08]We shall straight away write the electronic configuration of these elements. Okay. So it is three here lithium.

[00:29:18]So it should be 21 and in this case I have already told you it is 281 potassium add eight. What is the difference between these two? Eight and put that eight in the penultimate shell.

[00:29:33]28 81.

[00:29:36]Now what is the difference between these two here? 18. Put that 18.

[00:29:42]Where do you want to put that 18? Here in an orbit where it can accommodate 18 electron then you have two in between.

[00:29:52]So the third shell can accommodate 18.

[00:29:54]So put that you don't blindly put it in the penultimate shell here. The third shell can accommodate 18. The next one is eight here. And so I'm putting that eight here between these two. Again we have got 18 here. So put one more 18 between the third shell and the fourth shell because it can accommodate 181.

[00:30:18]Now my dear students you see here this is going to be 32. And where do you put that 32 here? Between the third shell and the fourth shell because the it becomes the fourth shell. Four cell car 32. Therefore 28 18 32 18 81. So I have written the electronic configuration even without knowing looking at the textbook at all. Maybe I shall give you one more example my dear students in the case of H hallogens. Now from that how do I guess the atomic number and then write the electronic configuration. How do I guess the electronic number?

[00:31:00]We'll go to page number four.

[00:31:03]I shall take up H hallogens.

[00:31:09]Let me write the electronic. What are H hallogens here? Florine, chlorine, bromine.

[00:31:18]You have to go home and practice it.

[00:31:20]This you should know at least it's the first period and second. First period only there are two elements hydrogen and helium. Second period starts from lithium and the last one is neon. So you should know lithium burillium you should remember all the things berilium the examination they may not give you all this information berillium boron carbon nitrogen oxygen florine. Now it's if if you just write couple of times you should be able to write remember it. Now this is going to be nine here chlorine broine. The next one I told you this is the second period of the periodic table.

[00:32:04]Second period has got how many electrons? Second period. Third period.

[00:32:08]This is in the fourth period. This is the fifth period. This is the fifth period.

[00:32:18]The sixth period we have u aetin radioactive element iodine florine chlorine bromine iodine. Now I told you in the second period let me write the elements in the second period. Maximum number of elements is going to be number of maximum number of elements is going to be 7 8 here 8 here 2 8 8 18 18 32 you remember that. So it is going to be chlorine happens to be the eighth element from florine because so you add eight here and that becomes the electronic configuration.

[00:33:09]So 9 + 8 17.

[00:33:13]Now this is the 18th element. This is the 18th element for chlorine from chlorine. Therefore 35 17 + 18 35.

[00:33:25]This is again 18th element. Iodine from iodine is 18th element from bromine. So 35 + 18 45 53 and this is the 32nd element. There are 32 elements here.

[00:33:39]Aetine is a 32nd element from iodine at 32 85. And that's how we remember. They don't need to give you you don't expect them to give you the atomic number. You are an expert. You know how to calculate the atomic number. From the atomic number, we calculate the find the number of electrons and we write the electronic configuration. Look at that. What should be the electronic configuration? I told you it's easy. Now you don't need to follow those rules. 27. This of course very simple. What is the difference here? Eight. Where do you put that eight here between this first and the second?

[00:34:18]Because second shell can accommodate eight. Now what is the difference here?

[00:34:23]18 here and third shell can accommodate 18. So 18 and seven here I again what is the difference here? 13 18.

[00:34:36]So you can put that one more 80 in the penultimate shell.

[00:34:42]Now what is the difference here? 32 where which orbit which shell can accommodate the fourth shell can accommodate 32.

[00:34:55]So if you put 2 8 18 32 18 7.

[00:35:06]Now as I told you all of them got seven electron in the outermost shell. So they have similar chemical properties like alkaly metals all alkaly metals contain one electron lash shell lash shell. So it is going to be uh there is a close relationship between close relationship between the electronic configuration and the chemical reactivity periodic table also.

[00:35:33]Okay. Right. My dear students now it's time for us to see the exceptional properties. Now all of them follow those two rule. The last shell does not exceed eight electron. It is true. The penultimate shell the penultimate shell should not to exceed 18 electrons. It is true. The prep penultimate shell not to exceed 32 electrons. It is also true.

[00:35:59]Now we'll come to the an exception. Now let me take the electronic configuration of I add I is 26.

[00:36:17]Now the electronic configuration based on the following two rules I can put 28 88 8 24 + 2 last sh not to exceed eight penultimate shell. It is right. In fact, you can go ahead 28 97 also because the third shell can account in maximum 8. So it is also right or it could be 10 6 or 11 5 or 12 4 13 2 13 3 14 2 15 1 Of course you cannot have 16 zero 8 9 10 11 12 13 14 15. Now all of them follow those two rules. The last shell has not exceeded.

[00:37:23]Last shell has not exceeded eight electrons. And the penultimate shell does not exceeded.

[00:37:45]The penultimate shell also has not exceeded 18.

[00:38:03]Greek now which is going to be the elect correct electronic configuration.

[00:38:09]Now I'll tell you now I can explain to you also the correct electronic configuration of is 28142 I you should remember electronic configuration 28 142 14 + 216 24 26 electronic configuration and [clears throat] this is able to explain most of the chemical properties of we shall come back to the chemical properties Now why is it so why not why do you rule out the other one? Now in order to understand this we have to know one more we have to understand the presence of subshells.

[00:38:59]They are called as orbitals.

[00:39:04]Orbitals, subshells, orbitals.

[00:39:10]Okay.

[00:39:12]Or each shell. Shell is the main energy level.

[00:39:18]Subshell is the subsidiary energy level.

[00:39:23]Subsidiary energy level. Like this. K shell. The first shell has got only one orbitals. Now what are the various orbitals? The various orbitals are represented by the letters orbitals.

[00:39:45]These are the four orbitals we use now.

[00:39:48]S orbital, p orbital, d orbital, f orbital. We use only these four. There is no need for the other orbitals. In fact, they have named the fifth one also is the G orbital. But there is no need.

[00:40:03]No element has been discovered so far to accommodate to make use of the G orbital, S orbital, P orital like this.

[00:40:12]Now like your stability of our shells, the energy levels, we can compare the energy levels.

[00:40:23]S orbital is the least energy level than P orbital than D orbital than F orbital.

[00:40:32]Therefore I am able to say about the the stability the stability S orital is the most stable than P orital than D orital than F orital.

[00:40:53]Okay. And similar to we also you are able to predict the maximum number of electrons in each orbital.

[00:41:01]S orbital to contain maximum two electrons.

[00:41:06]P orbital to contain six electrons.

[00:41:10]D orbital max difference is four contain 10 electrons. F orbital to contain 14 electrons. And the remaining in case you have got a g orbital of course it should contain of course there is no need for this 18 electrons 18 electrons so each orbital can contain now let's see whether it matches with the shell orbital k l m and fourth shell now what is going to be the orbital s orbital first shell has got only one or second shell has got two orbital S orbital and P orbital.

[00:41:53]Third shell has got three orbital S orbital, P orbital and D orbital. The fourth shell has got four orbital S orbital P orbital and D orbital F orbital. Similarly, the fifth one should contain five orital. But there is no need to use the the fifth orital right now. Now it's it's to be compared like this. You have your uh flat. It's a threetory or fourstory building. Here you have got the ground floor is a car park. Okay. Okay. Ground floor is a living apartment. The first floor, second floor, third floor and the fourth floor. Third first floor has got only one hall. Big hall to accommodate the the car car park. The second floor has got the two rooms. two rooms only. Okay.

[00:42:46]Living room and the kitchen. Living room and the kitchen. The third floor has got three rooms. We got one living room and one bedroom and one study room. The fourth room, fourth floor has got four rooms like that. Like that. Okay. So, we can of course no one is going to stay in the car park. Just for the sake of example, I gave you that the electron we have got SROL in all the floors. ground floor, first floor, second floor, third floor, etc. We got the kitchen etc. in all the floors, we can have the kitchen.

[00:43:21]Similarly, D orital is present from third orital onward. Now, how do we identify each orital? The S orbital present in the first shell is called 1 S orbital.

[00:43:34]The S orbital present in the second shell is called 3S orital.

[00:43:40]3s or 2s orbital 3s orbital 4s orbital 5s orbital and 6s orbital and so on and so forth 7s orital similarly we got the p orbitals right from the second onwards it's a 2 p here 3 p here 4 p here 5 p here 6 p here 7 p here 7 p this is going to be 3d starts from third level 4 D 5D 6 D 7 D and F orital start from fourth level 4 F 5 F 6F 7F. Now we are able to identify each orital in each main energy level. Now please you have to remember S orbital can accommodate only two electron and p orbital can accommodate six electrons onwards. Now we'll go to the next page and we shall explain the electronic configuration of these elements in an orbital orbital configuration of an atom.

[00:44:56]Take hydrogen. Hydrogen has got one electron in the Keller and that two it should be present in the 1s orital. 1s orital and this one represent the main energy level K shell. The small one represent the number of electrons. And helium the atomic number is two.

[00:45:17]Therefore it has got two electron the K shell and both the electrons are present in the 1s orital 1s2. 1 s2 and lithium. Lithium has got three electrons. The electronic configuration is 2 one. 1 s2 it is full. Now we have to go to the next energy level. Higher energy level is 2s. The second level has got 2s and 2p. 2s is more stable less energy. But it can accommodate only one electron. Only one electron is available. But s electron s orbital can accommodate two electrons. Now we'll go to berilium atomic number four. We write a 22 configuration.

[00:46:04]Now it should be 1 s2 and 2 s2. Now we shall go to chlorine straight away.

[00:46:12]Atomic number 17. Atomic number 17. Now we shall take up florine first. We shall take up florine first. 9 atomic number 9 1 S2 2 S2 you should remember the increasing order of energy level 2 S2 2 P5 so 5 + 2 7 + 2 9 so it is going to be in the two in the K shell and seven in the L shell is it okay shall we write the chlorine now the electronic configuration is 2 H7 we have already proved it how they arranged in terms of orital So 1 S2 2 S2 2 P6 3 S2 3 P5 this increasing energy level of orbital you should be able to remember my dear students this is going to be 1 S 2 P 3S 3 P 4S after 4s you don't get 4 P it is 3D P we have exception here then 4 P etc. Now how do we remember this particular order increasing uh orbitals with increasing energy level. This is called offbar principle.

[00:47:41]Electrons are arranged in the increasing order of energy levels. Now earlier we have seen the arrangement of these orbital. You try to put the parallel lines here. I shall tell you the easy method to remember the orbitals in increasing order of energy levels. Now let me use the parallel lines parallel.

[00:48:21]Similarly, now you see here.

[00:48:38]See here.

[00:48:40]See here.

[00:48:42]Easy. Now you have to remember this is the starting point.

[00:48:49]This is the starting point is 1 s. After completing this, it comes a second line.

[00:48:56]It goes to the 2s orbital. From 2s orbital, it goes to the 2p orital. From 2p, it with six electrons, it goes to 3s orbital. After 3ps, next line starts enters with the 3p orbital. Then 4s orbital. Now you see here the exception is understood 4s 3D 4p 5s. Now this kind of ladder level energy level you if you are able to draw every examination there is one specific definite question on this. So in the margin we can draw this and have this increasing order of energy level. Now look at that. Now you write the electronic configuration of any element. For example, my dear students, looking at that, shall we write the electronic configuration of iron and that we had some uh exceptions there.

[00:49:57]26 26 1 S2 2 A S2 2 P6 3 S2 3 P6 up to 4 S2 you always remember don't be wasting your time it is 20 electrons then it is going to be 3d level now we let let Let us write the electronic configuration of ion 1 s2 up to 20. I told you up to 4 s2 20.

[00:50:34]Now we have to account for how many more? Six more electrons. Those six electrons go to this orbital. So this is electronic configuration. Having this is the halfpa principle. Having written the electronic configuration. What is the first shell? Second shell. Third shell.

[00:50:52]The fourth shell. Now first shell has got two electrons. Second shell has got eight electrons. Add all the electrons.

[00:51:02]Second shell. Add electrons in the third shell.

[00:51:07]Third electron. Leave the 4s orbital. So 6 + 2 8 + 6 14.

[00:51:15]And the fourth orbital two electrons. My dear students, are you not getting the electronic configuration of this also is 28142 and we were not other electronic configuration. So what is more important is the orbital electronic configuration.

[00:51:37]Now I shall write one more example.

[00:51:40]Let's see whether there are any questions over there.

[00:51:44]So what is the stability of a shell and what is the atomic number? Good. I shall explain. The stability of a shell is based on the energy level. All the shells do not have the same energy level.

[00:52:00]Though the first shell has got the least energy level. The next L shell, M shell, etc. As you go from the K to L to M to N, the energy increases.

[00:52:11]The stability is inversely proportional to energy. The shell which is which has the least energy level is the most stable and any electron would prefer to go to the most stable shell. So we have to complete the K shell. After completing the K shell then we have to go to the L shell etc. Now in terms of offba principle as I told you we have got as I told you we we are able to remember this the electronic arrangement of orbitals the first orbital is 1 s orbital then you have 2s orbital the higher level increasing order of energy level 3s level 3p level then we have 4s level and 3d level and we were able to write this increasing order of energy level looking at that the arrangement of electrons in form of ladder ladder. Now it is two electron whenever we are using s orbital two electrons. Whenever you are using p orbital six electrons a d orbital can accommodate this. Now my dear students we shall see whether what is the electronic configuration of bromine [clears throat] 35 atomic number. You go home surely. Please remember broine normally they will ask you give you examples in alkaline metals and hogen metals generally. So florine, chlorine, bromine and iodine. So this is going to be 1 s2 2 s2 2 p 6 3 s2 3 p 6 4 s2 up to 4 s2 I told you 20 electrons then we have 3d can it accommodate 10 electron do we have 10 electron yeah we have 15 electron we have 10 electrons available 4 S2 then remaining 4 P look at that the electronic configuration increasing order of energy level after 3d 4 P after 3D you have got 4 P please remember this particular chart 4 P now it is 20 electrons plus 10 30 electrons now we have to accommodate five electron can I put all the five electrons here here in the 4 P yes yes why because 4 P orital can accommodate maximum number of six electrons. Therefore, this is the electronic configuration. See whether it tallies with the shell configuration. K shell, L shell, M shell, N shell, etc. Now, K shell I shall put them in different colors. K shell is going to be red in color. How many electrons? Two electrons. And then we shall put green in the second shell. All the electrons the green shell. How many electrons?

[00:55:13]Eight electrons here. Eight electrons.

[00:55:15]The next one black in the third shell.

[00:55:19]Black. It's a double light. Third. These are all third shell. So this is going to be 6 + 2 8 8 + 10 10 18.

[00:55:31]Now this one is the uh green. I shall put green here. The fourth shell. Fourth shell green. How many electron? Two plus [snorts] five. Seven electrons in the outer position. So 2 8 18 7 that is what we wrote earlier. And this is a more accurate electronic configuration my dear students than writing only the ships writing only. Now I have to give you one more additional information.

[00:56:02]Where does the last electron enter into this orbital? Which orbital? Look at the bromine. The 35th electron, the 35th electron enters into the as per the halfbar principle. As per the offbar principle, therefore it belongs to a p block element. Please remember my dear students the elements in the periodic table can be classified into S block element, P block element, D block element, F F block element, F block element. [clears throat] I can ask you this is going to be this. Now for example potassium what is the which block element it belongs to? Now you write the electronic configuration.

[00:56:54]1 s2 atomic number 19 1 s2 2 s2 2 p 6 3 s2 3 p 6 4 s1 because up to 4 s2 20 electron but we have only 19 electron. So where does your 19th electron go to s orbital?

[00:57:21]Therefore this belongs to s block element. Not only this all the alkaly metals belong to s block elements.

[00:57:29]Lithium, sodium, potassium, rubidium, cium all these elements belong to s block element. Is that clear to you?

[00:57:42]So in fact it's a ocean I have just given you just the roodium and basic concepts of electronic configuration and to sum up quickly the electrons are obtained from the atomic number as I one asked at how is atomic number defined atomic number gives you it tells you about the total number of protons inside the nucleus or the total number of electrons you should have the same number of protons and electrons because all the atoms or neutral. So that is atomic number. From the atomic number, you know the electrons and if you know the number of electron, you know how these electrons arrange in various main energy levels. They are called shells. K shell, L shell, M shell etc. Each shell can accommodate maximum number of electrons only. And then you have orbital subshells. Each shell has got just as one main building has got various floors and each floor has got various rooms. Similarly, the main shells also have got the sub energy level subshells. They are called subshells which are known as orbitals.

[00:58:50]And each there are four different orbitals right now in use. S orbital, P orbital, D orital and F orbital. And each can accommodate maximum number also as as in the case of the shells.

[00:59:04]Then we have the Hoff principle increasing order of energy levels. If you remember that you are able to write the electronic configuration any element for that matter any element for that matter that based on that only an element is classified as S S block element, P block element, D block element, F block element in periodic table. Now this is going to be an interesting feature that you are going to study later on in the 11th standard and this is what this is how you are going to learn chemistry my dear students I suppose you have understood the thing what is atomic I think I have explained to you are there any doubts right now I prepare to explain to Okay.

[01:00:04]Is that okay? Is that okay? Please go home.

[01:00:08]Um maybe you have heard a faster version of electronic configuration but please go home and revise it. Revise it. It should become easier. And this is an important chapter wherein quite a good number of questions are asked in the any competitive examinations. My dear students local and mentor madam abia madam are there any questions from the students?

[01:00:40]>> No sir no more questions [clears throat] sir.

[01:00:41]>> Okay since there are no questions from the students I shall wind up now and this is what you are going to learn in fact more interesting factor about the electronic configuration. So once again I wish you all all the best my dear students. Thank you. Good day. Good night.