Preparing for Medicine Concours (FMBS) - Chemistry

Added: 2026-05-13

[00:00:06]Hello.

[00:00:07]Greetings to everyone.

[00:00:10]Welcome to today's class.

[00:00:15]Preparing for medicine concours chemistry, atomic structure and bonding.

[00:00:24]So, that is the program for today's class.

[00:00:28]And um I've been giving lessons.

[00:00:31]But today's class is going to be one of its kind because we'll be answering 30 questions on this very topic.

[00:00:43]So, atomic structure and bonding.

[00:00:47]I'll be answering with you 30 questions.

[00:00:52]I mean 30 past questions for the FMBS entrance examination.

[00:01:00]So, this is for you who is going in for the medicine concours.

[00:01:08]This is for you who is writing FMBS, FMPS.

[00:01:14]This is for you who is writing FHS, Faculty of Health Sciences, uh this is for you who is writing means santé concours.

[00:01:25]This is also for you who is writing any concours in which chemistry is involved. It could be midwifery, pharmacy, state nursing, and so on. Medical lab science, just to name a few.

[00:01:41]So, let's get started with our question one. I'll read the question.

[00:01:49]I may not read exactly as this because we have to answer. That's the objective.

[00:01:54]We'll answer in 30 questions in one session is not is not just so easy. So if I go on reading all the questions we may spend more time than I planned. So I'll read the question, I'll solve it, I'll answer in chemistry right. Some need answering, some need to be solved and I will explain. This is the most important part.

[00:02:23]In today's lesson of course, which is a special session.

[00:02:28]I'll explain everything clearly to your understanding.

[00:02:32]So now if you're following me from anywhere, let me know in the comment section your name and where you're watching me from and the concur writing.

[00:02:43]Then that is one.

[00:02:45]Make sure you exhaust this in a very calm environment.

[00:02:49]This is very important for study. When you're preparing for a concur, this is one thing I will tell you. You study in a very calm environment. Your mind relax. If you feel stressed, take a glass of water, cool down and then you pick up your lesson.

[00:03:04]So number two, make sure you have something, a notepad and a pen or your book. You have books.

[00:03:13]Don't just watch as if you were watching a theme, a movie.

[00:03:19]Also I'll become a very good actor. At the end of the day you understand nothing.

[00:03:24]But if you actually struggling stressing up your mind to also answer in the course of that. When I read the question, try to see if you can answer the question.

[00:03:37]Yes, try to see if you can answer the question. When I'm doing the solution, follow up follow up step by step.

[00:03:45]Yes, this will help you a lot. So I'll go on with question one. Let Let all my materials.

[00:04:07]All right. So, uh remember to give me a like.

[00:04:12]Yes, do so.

[00:04:13]And then >> [clears throat] >> uh feel free if there's anything you do not understand, you have the comment You can drop whatever you don't understand in the comment section. I'll be there with you in the comment section. So, I will do the first question.

[00:04:31]At Okay, we are on atomic structure and and bonding. Question one.

[00:04:37]The number of protons in lead two plus lead two.

[00:04:43]So, this is lead.

[00:04:49]The How many protons are here? That is the question.

[00:04:53]How many protons in lead two plus? To answer this question, >> [clears throat] >> we first of all have to discover or to ask your ask ourselves a question of how many What is the atomic number of lead?

[00:05:09]Yes, a lot of people uh may not know. The atomic number of lead is 82.

[00:05:16]82.

[00:05:18]So, now Why do Why did I have to ask the question of the atomic number of lead?

[00:05:25]Because the proton number, the number of proton is the same as the atomic number.

[00:05:31]So, once you get the atomic number of an element, you have gotten the number of protons in the nucleus of that of the atom of that element.

[00:05:42]Now, um someone will now think that here is not lead as an element. Here is an ion of lead because you can see it's lead 2 plus.

[00:05:54]So, it means this is lead that has given out two electrons.

[00:05:59]Uh, let me repeat that. Let me repeat that for you. This is lead that has given out two electrons. So, what it has given out is electron, not proton. But, what we want is the number of protons.

[00:06:11]So, if the question was how many electrons, then we will have 80 electrons. Why? There were 82 electrons when two electrons go off, we are left with with 80 electrons. But, the number of proton remain 82.

[00:06:27]Yes. So, I hope that is well explained clear to your understanding. We then go to question two.

[00:06:37]A minute. Remember, any question you don't understand anything at any level, use the comment section. Ask your worry in the comment section.

[00:06:55]Question two.

[00:06:58]Question two.

[00:06:59]In which of the following compound does nitrogen have an oxidation state of plus five?

[00:07:07]So, in which compound does um, nitrogen have an oxidation state of plus five?

[00:07:16]Now, we have We have H N O 3 We have N O 2 We have um, N O We have N2 and and finally, we have NH2OH.

[00:07:46]So, in all these in which of them does, you know, nitrogen all of them are compounds of nitrogen.

[00:07:54]In which of them is a molecule of nitrogen here? The rest they are compound of nitrogen. In which does nitrogen have an oxidation state of +5?

[00:08:06]So, this helps um This helps you to This question will help you to understand the the how to calculate the oxidation state of an element in a compound.

[00:08:20]Yes. So, um Simple and for sure, let us start with the first one.

[00:08:26]And by by by looking by looking at the elements we have, that's already the compound we have, I can already tell you boldly I can tell you boldly already that nitrogen in nitric acid HNO3 has an oxidation state of +5. Okay. What did I write here? +5, not two. The question says, in which of the following does nitrogen have an oxidation state of +5? The answer is nitric acid. I will solve it to show you.

[00:09:04]Now, um this is what we do.

[00:09:07]Uh the oxidation state for hydrogen is well defined as +1. For oxygen is well defined as -2. And then you let that of nitrogen to be X. And that is what you are calculating, right?

[00:09:21]So, now, in this compound, hydrogen, that is one + X. X for nitrogen. Then + oxygen -2. And there are three of them.

[00:09:37]So, we say + 3 * -2 = 0. I hope this equation comes clear to everyone.

[00:09:45]So, what we do at this level What we do at this level is easy.

[00:09:51]Um -2 * 3 is -6.

[00:09:55]-6 + 1 is -5. So, we are saying that x - 5 gives us 0. x = 5. When we arrive at this level to say x = 5, we are saying that the oxidation state of nitrogen in nitric acid is 5. Not just 5, this 5 here is plus. Why? Because um when I hope it's okay. Most often I I prefer saying that we add 5 on both sides, not just we send 5 here. But, I know this not an an arithmetic class. This kind of a little problem you are very okay with.

[00:10:32]If you have problems with it, let me know in comment section. I'll come back to explain this.

[00:10:37]So, however, uh this is not just 5, it is it is +5. So, the answer is nitric acid.

[00:10:44]It means that uh nitrogen in nitric acid has an oxidation state of +5. Question three.

[00:10:55]Given the following the given the following So, we have Planck constant.

[00:11:01]They have given us Planck constant, the speed of light as 3 * 10 to the power of 8 m/s. And we are told that one electron volt is equal to 1.6 * 10 to the -19 J.

[00:11:16]Now, um We are also told that the energy of um of hydrogen atom is -13.6 / n².

[00:11:29]Okay.

[00:11:31]Uh Now, here's the question.

[00:11:34]When an electron of the hydrogen atom passes from n equal to 2 to n equal to 4, the hydrogen atom Okay, that's the question. So, we have A, absorb a photon of energy 2.55 J. We have B, um Okay, the the the rest of the options are on the next paper. So, we we will first of all answer the question, then I will I take the options, and then we pick up the option that has the correct answer. So, now, what does it mean from the question? I have to clean the board here first.

[00:12:08]So, you give me a second for that.

[00:12:19]Okay, so here we go.

[00:12:22]Here we Here we go.

[00:12:24]Um This is the question says that the hydrogen atom, right, is passing from n equal to 2 to n equal to 4, right? So, let let let's look at that. We use what they have given us. We use what they have given us.

[00:12:46]Now, when they say from n equal to 2 to n equal to 4, there is something here I will have to show you. n equal to 2 to n equal to 4, it means that these are the n here represent what the energy level.

[00:12:58]So, this means that the hydrogen atom is moving from a lower energy level to a higher energy level.

[00:13:07]That's from energy level n equal to 2 to energy level n equal to 4, right? So, when that happens, we are now going to calculate the energy at the initial energy of hydrogen and the final, and we're going to look at the difference.

[00:13:25]So, that is principally what we're going to be to be doing here right now. So, follow up keenly.

[00:13:32]Now, the initial energy level that's E2, right? Because the initial is at n equal to 2. So, at n equal to 2 we have E2.

[00:13:42]Now, go back to the question. We're given We're given uh En, right? We're given En. Let me put it uh here. Okay. We're We're told that En is equal to um -13.6 divided by n squared.

[00:14:01]So, what does it mean?

[00:14:03]It means E2 is -13.6 divided by 2 squared. I hope that that's just all. That's That's the initial.

[00:14:13]Now, it moves from here to n equal to 4.

[00:14:17]The energy final energy here, which is E4, is -13.6 divided by 4 squared, right? Yes, 2 squared is 4, 4 squared is 16. So, from there you can uh Let's Let's go gradually. It means E2 is what? -13.6 on 4.

[00:14:40]Sorry, 4. And E4 is -13.6 on on 16. So, you use your calculator to to to to punch that, and then you obtain values for E2 to be -3.4 electron volt. So, here you have -3.4 electron volt.

[00:15:06]And here you have -0.85 uh -0.85 85 electron volt. So, this is E4, and this one here is E2.

[00:15:22]This is E2. Now, does the question end there? The answer is no, the question doesn't end there.

[00:15:28]Um we are look we are now going to look at the energy different, that is delta E, right? Delta E. So, what has happened is hydrogen has moved from N equals to 2 to N equals to 4. So, the the energy delta E energy energy different delta E is E4 minus E2.

[00:15:51]E2, which you have to put on your calculator, this minus this is going to give you now a positive value, 2.55 electron electron volt. So, now uh what can you see? What can you see about this um this uh this this is the answer you have.

[00:16:11]What can you say about it?

[00:16:14]Uh it tells you that the the the option with the correct answer is A, the option that I read before, which says that the hydrogen atom absorbs a photon of energy 2.55 electron volt. Now, but why do we say absorb? Why? I I We have that delta E is 2.55 electron volt, and I'm interpreting I'm telling you that it means that the hydrogen atom uh absorb a photon of energy 2.55 volt. Now, this is because the electron moves to a higher energy level, and it is clear that when electrons move from lower energy level to higher energy level, they absorb energy. They absorb energy. But when they are falling, when they fall from high energy level to lower energy level, they now release energy. So, that's question three. We go to question four.

[00:17:16]All right, I'm here with question four, which reads I'm here with question four, which reads, "Which of the following compounds will form hydrogen bonds?" Now, when we talk of hydrogen bonds, remember this is a topic, atomic structure and bonding.

[00:17:32]Now, uh all together this this this is not one topic actually. As you can see, it is something like three topics put together. We have atomic structure, we have bonding, and then we have intermolecular forces.

[00:17:46]So, now this question here is is from a topic known as intermolecular forces.

[00:17:52]Well, uh based on the our curriculum, how we have scheduled our work uh for the purpose of what we are doing, it is one topic now.

[00:18:06]Yes, because we sometimes put topics together when they when they are all uniform, right? So, we are talking about the formation of hydrogen bond.

[00:18:17]Hydrogen bonds. And hydrogen bond, these are the strongest intermolecular forces.

[00:18:23]And that is clear already. You know that um now let's look at what they have given us. HF, that's option one. HF, we have HCCl3, we have PH3, we have CH3OCH3, and finally, we have CH4. Yes. So, this this this clearly this clearly is weak van der Waals forces, right? Yes, van der Waals van der Waals forces. But leave that aside. Leave leave that aside, right?

[00:19:00]You can see that most of them on the board there, they are van der Waals forces. Now, but the very first one, HF, let me tell let me start by telling you the condition for the formation of what? Hydrogen bond.

[00:19:16]There are two conditions under which hydrogen bonds are formed.

[00:19:19]Number one, hydrogen must be present and it must be covalently bonded to a highly electronegative atom.

[00:19:28]Now, this highly electronegative atom can be um fluorine, can be nitrogen or oxygen.

[00:19:40]So, now the first thing we want to check is is hydrogen present in this? Yes, hydrogen is present. The next thing is is it bonded? Is it bonded to a highly electronegative atom?

[00:19:57]Yes.

[00:19:58]Fluorine is the most electronegative atom.

[00:20:03]Yes, on the periodic table, fluorine is the highest electronegative atom.

[00:20:07]So, what does it mean? It means option A satisfies the condition.

[00:20:12]So, that is your answer.

[00:20:14]The The ionization energy of an atom is Now, before we we check the options, let us define ionization energy, okay?

[00:20:24]What is ionization energy?

[00:20:30]Yes, that's the energy of ionization.

[00:20:33]What is abbreviated IE? What is ionization energy? And then, mind you, sometimes the question refer to the first ionization energy, IE1. Yes. So, but now let's let's focus on what we have.

[00:20:49]What do we mean by ionization energy?

[00:20:50]Short and simple, if you want to write you can put it down this way. Ionization energy is the minimum amount of energy required to remove one mole of electrons from one mole of gaseous atom or ion to form one mole of gaseous positive ion. So, that is it. That is the definition of ionization energy.

[00:21:12]So, ionization is ionization energy is the minimum amount of energy required to remove one mole of electrons from one mole of gaseous atom or ion to form one mole of gaseous positive ion.

[00:21:30]Now, options. Well, let me get it from here.

[00:21:33]That's question I think four now.

[00:21:36]Mhm.

[00:21:39]Now, question five. That's question five.

[00:21:42]Uh option A says the number of electrons on the outermost shell. That's already wrong.

[00:21:48]B, the ease with which an atom can lose an electron.

[00:21:52]From the definition I gave you, this is clear, right? Because ionization energy defines the ease with which an electron can be removed from an from a from an atom, right? Yes. The When the ionization energy is greater, it means that um electrons cannot easily be removed. But now, when ionization ionization energy is little, then it is easy now to remove an electron.

[00:22:21]Uh so, we don't have to read the rest.

[00:22:23]Anyway, let me read.

[00:22:24]Uh the ease with which an atom with which atoms can combine to form molecules. That is wrong.

[00:22:31]Um the charge on the nucleus. No. And finally, none of the above, which is wrong. So, we we have had the answer. Uh I think that is clear to everyone.

[00:22:44]Question six. Question six.

[00:22:47]Question six.

[00:22:51]Question six says, given that H they've given you first of all Planck constant H given that H is this.

[00:23:02]Uh you know Planck's constant is 6.62 now * 10 ^ -34.

[00:23:07]Then uh the speed of light in air, c, 3.0 * 10 ^ 8 m/s.

[00:23:17]Now, they have given you E0 to be 13.6 and they have they have told you that one electron volt is equivalent to 1.6 * 10 ^ -19.

[00:23:30]Now the wavelength of a photon emitted by a transition from n = 3 to n = 1 1 in hydrogen atom is All right. So, now uh you will notice this kind of question we have seen it We have answered this kind of question in this very session. And this this very topic in this very uh class but not exactly the same question.

[00:24:02]Yes, it looks alike.

[00:24:04]It would that by that time we were looking at um when an electron an electron passes from a a lower energy level to a higher energy level. That was from n = 2 to n = 4. But now, we are witnessing a situation where an electron falls from n = 3 to n = 1. So, we are still going to start by calculating the energy difference. So, that means we're going to find E3 and then we're going to find E1. This case now, E3 will be the Mm.

[00:24:43]All right, E3 will be the initial initial energy and E1 will be the final.

[00:24:51]Why? Because it is from E3 to E1. So, this is the initial and this is the final. Remember if you you have any question I'm in the comment section, all right?

[00:25:02]So, let's let's do that. E3 the the question gave you the the the question gave you already how to find E, right? So, how do you find it?

[00:25:15]We just go directly to delta E so that I'll just substitute. Yes, delta E is what? E final minus initial that's E1.

[00:25:30]That's So, that's E1 minus E3, right?

[00:25:35]So, we have 13.6 uh 13.6 times Okay.

[00:25:46]So, now for uh E3, let me E3 is supposed to be Please follow keenly. E3 is supposed to be um May it will just be 13.6 over 3 squared. Remember, we have done this. Now, um E1 is 13.6 over 1 squared.

[00:26:12]So, now it means this is 13.6 over uh times Let's take times 1 over 1 squared minus Now, we have 1 over 3 squared. So, I hope at this level people people are understanding. So, when you do this, it gives you that delta E is equal to 12.09 electron volts.

[00:26:39]Now, unlike the previous question, we're not going to we're not going to end here. We're going to We're looking for the energy now.

[00:26:48]So, E is what? We're going to multiply 0.09 * 1.6 * 10 ^ 19, which was given, right?

[00:27:01]Yes. So, when you do this, you're going to have the energy to be 1.93 * 10 ^ 18 J.

[00:27:10]Yes.

[00:27:11]Um Now, this is the energy. The question is asking for the wavelength. And I've given you Planck's constant. Wavelength is lambda. So, you're going to use the equation lambda equal to HC over over E, right? Yes. So, now, this is the E you're using.

[00:27:28]H here is Planck's constant. Uh Planck's constant was given to you. And uh if I should remind you, Planck's constant is 6.62 * 10 ^ 34.

[00:27:38]Then, C here represent the speed of light. Speed of light in air is well known.

[00:27:44]Yeah, that's 3.0 * 10 ^ 8 m/s. Then, um now, the energy is this value that you have. So, you do that, and your lambda lambda represent the wavelength. Lambda will be equal to 1.03 * 10 ^ 7.

[00:28:02]Since the wavelength, so it's in meters.

[00:28:04]So, this is the solution to question number six.

[00:28:16]All right.

[00:28:25]Our request on seven.

[00:28:28]In general, in general, in general, the ionization potential of elements decreases as one proceeds in the periodic table, we have bottom to top, right to left, we have top to bottom and right to left, we have bottom to top and left to right, we have top to bottom, left to right, we have none of the above. So, let's evaluate. We're talking about the ionization potential. So, what does it mean? We're looking at the trends on the periodic table, hm?

[00:29:09]And this takes us to periodicity, right?

[00:29:18]Like the period the study of the periodic table and it's in actually under the our topic today.

[00:29:25]So, now, the trends on the periodic table for, you know, ionization potential, ionization potential, ionization potential. So, uh first of all, let me state it clearly.

[00:29:53]Ionization potential decreases on the periodic table, decreases as you move down.

[00:30:05]Please wait.

[00:30:09]Decreases as you move You you get? It increases as you move down. So, let me see how I'm going to place the arrow.

[00:30:24]And give me a minute for that.

[00:30:38]All right. So, we are back on that question 13, what?

[00:30:42]Question seven.

[00:30:53]Okay.

[00:30:55]Question seven, the ionization potential on the periodic table.

[00:31:00]Uh let us look at something.

[00:31:02]When you move down down a group on a periodic table, what happens is that you have the formation of new shells and the ionization potential increases.

[00:31:18]In fact, or let Whatever about decrease, so it would decrease up.

[00:31:24]You know that's how I will put it now.

[00:31:26]It can It would decrease as you move up.

[00:31:30]Then now, what about uh um along or across a group from left to right and from right to left?

[00:31:39]As you move to the right across a period on the periodic table, ionization potential increases.

[00:31:49]It increases. So, it means that it would decrease backward this way. It means it would decrease this direction from right to left. So, now let's go back to the options.

[00:32:06]All right.

[00:32:07]Mhm.

[00:32:08]The ionization potential decreases from bottom to top and right to left decreases.

[00:32:22]From bottom to top and right to left. That's the answer, A.

[00:32:31]Now, we continue.

[00:32:34]In which compound does manganese have an oxidation state of negative four? So, I'm going to put the compounds here and then we find out which of them manganese has a the oxidation state of plus four. We have manganese chloride, that's MnCl2.

[00:32:55]We have MnO2.

[00:32:59]We have MnSO4.

[00:33:03]We have K, that's potassium permanganate.

[00:33:06]O4. Then uh we have none.

[00:33:10]So, in which of them does manganese have an oxidation state of of of minus minus four? Yes, with this question, just by looking, you can already uh know the answer.

[00:33:30]Yes, um let's see.

[00:33:34]This manganese peroxide. Okay, it's simple.

[00:33:38]From here, normally, you know already that this is called manganese peroxide as well as manganese four oxide.

[00:33:49]So, even without solving, the answer is without working it out, right? The answer is clear already, this is it.

[00:33:56]Here, manganese has the oxidation oxidation state of plus No, that's the oxidation state of minus four.

[00:34:04]Yes. So, uh we're going to we're going to look into that now. Let's see.

[00:34:12]Um question Yes, question eight. Let's solve Let's Let's solve to prove to prove this answer.

[00:34:24]Manganese here has a an oxidation state of minus four.

[00:34:28]So, this manganese peroxide, it has it it's here with oxygen and oxygen next to, right? So, we have manganese, you set it to be X. We have done something like this sometime ago, like today.

[00:34:41]Then, um plus two times oxygen is -2 equal to zero.

[00:34:48]What does it mean?

[00:34:49]X will be X - 4 is equal to zero.

[00:34:56]X - 4 is equal to zero. X is equal to four.

[00:35:01]X equal to four.

[00:35:03]So, let uh it means the answer is manganese peroxide because here, manganese has the uh the oxidation state of four.

[00:35:13]But, there's a problem.

[00:35:15]Here, from the equation we are seeing -4, not +4. Yes. So, it means here in here, manganese has an oxidation state of here of +4.

[00:35:30]of +4.

[00:35:31]So, we now we have to also do this do this and we confirm. So, give me the because we let's do that together.

[00:35:41]So, here manganese chloride. Manganese set to X, chlorine is -1.

[00:35:49]So, it can never give you that answer, right? It's not possible. Potassium permanganate.

[00:35:55]Uh that's still not possible. We go with that. One from potassium. X for manganese.

[00:36:04]Four times -2. It is not going to give you um It is not going to give you -4. Let me verify.

[00:36:25]All right. So, the question the question is plus four and not minus four. So, the answer is manganese peroxide.

[00:36:35]Yes, the answer is manganese peroxide.

[00:36:37]The question is plus four, not minus four.

[00:36:40]Now, the property of liquid that does not have intermolecular forces. Sorry.

[00:36:49]The property of a liquid that does not have intermolecular forces is The property of a liquid that does not have intermolecular forces is We have surface tension, boiling point, vapor pressure, heat heat of vaporization, none of these. The answer is vapor pressure, C.

[00:37:32]The answer is vapor pressure.

[00:37:35]If you need an explanation, let let me know.

[00:37:37]Which elementary particles are the main contributors to the mass of an atom? The mass of an atom We have the proton and the neutrons. Why? The proton The proton and the neutron Their masses The mass of the proton is one. The mass of the neutron is one. The electron is negligible or one on 1,000 or 1,840, which is nearly zero. So, uh now, in an atom, the in in the nucleus, you have the proton and the neu- neutron.

[00:38:10]And here you have the electron. So, when we talk of the mass of an atom, it is the mass of the nucleus.

[00:38:18]So, what are we saying? Proton and neutron.

[00:38:23]What is the time necessary for 99.9% of the atoms of a radioactive compound to be disintegrated?

[00:38:34]Half-life equal to 5 minutes. So, let's let's get on that.

[00:38:41]Now, this question um Okay, please guys, please follow me.

[00:38:48]I just get some future.

[00:38:50]All right, um Have any worry, let me know in the comment section. I will be there.

[00:38:56]I'll check the comment.

[00:38:57]And my collaborators also, that means other teachers in my team also check the comments to respond. So, you can feel free to comment.

[00:39:12]We proceed.

[00:39:14]Okay.

[00:39:16]I read the question again.

[00:39:20]What is the time necessary for 99.9 Sorry, for 99.9% of an atom of a radioactive compound to be disintegrated and the half-life is 5 minutes. So, uh that is already equation 10.

[00:39:38]No, no, that's question 11. Question 11.

[00:39:41]So, um For 99 per- 99% 99.9% already disintegrated, this means that this means that we we are left with 0.

[00:40:03]1 percent left.

[00:40:08]Right? See, what do you have percentage?

[00:40:11]So, it's on on a hundred. That is why when we say 99.9% have disintegrated, It means what is left now is 100 - 99.9, which is 0.1%.

[00:40:25]Now, we can use this formula.

[00:40:27]Um half Oh, is that clear? Okay, so half to the power of n to the power of n * 1 1 100 100% is 1. So, I was supposed to put 1 * 100% but 100% is 100 / 100, which is 1. So, the effect does not apply, right?

[00:40:51]So, we have half to the power of n is equal to 0.1%.

[00:40:58]Now, 0.1% means what? 0.1 over 100.

[00:41:05]So, we are now looking for n.

[00:41:08]We're looking for n first.

[00:41:10]Uh n n means what? N is going to give you the number of half-lives.

[00:41:16]The number of half-lives. So, uh now from here you're calculating n.

[00:41:25]You're calculating n and um you're going to have that n is equal to 10.

[00:41:32]N is equal to 10.

[00:41:35]So, it means that we have 10 half-lives.

[00:41:41]And then, one half-life T is equal to 10 T half.

[00:41:48]That's how you express it. The time is going to be 10 half-lives. Now, one half-life is 5 minutes. So, that's 10 * 5, right? That is 50 minutes.

[00:42:02]And the question is answered. 50 minutes.

[00:42:07]50 minutes.

[00:42:11]Yeah, and I want to believe that everyone is understanding and uh I'm in the comment.

[00:42:20]All I'm I'm I'm in the comment all the time. So, you don't have any worry. That is where you ask.

[00:42:25]Now, question 12.

[00:42:29]Thorium.

[00:42:50]The uranium 238 isotope has an atomic number of 92. What does it mean? Uranium 92 238.

[00:43:04]Okay.

[00:43:06]And then, through the emission of an of alpha rays, the uranium is changed into another element, thorium.

[00:43:17]Thorium.

[00:43:19]Oh, now, that means it gives out thorium and uh an alpha alpha ray.

[00:43:29]So, alpha, you know, is 4 2. You know that.

[00:43:37]With atomic number of 90, thorium is 90 normally.

[00:43:41]Normally, 90. So, what does the question want?

[00:43:45]Wait, it's relative atomic mass of thorium formed from uranium. So, it's just going to be fi um 238 minus 4, right? So, that's 234.

[00:44:00]The answer is 234.

[00:44:03]13.

[00:44:05]Let us assume that there are 20,000 atoms in a relates in in a radioactive element. Let us assume that there are Let us assume that there are 20,000 atoms in a radioactive element.

[00:44:23]Its half-life is 5 days. How many atoms remain or change after another 15 days?

[00:44:32]After another 15 days. Now, um this question uranium Okay.

[00:44:43]Yes, this question is coming So, some people will make error here thinking that it is a continuation of the question we solved uh on radioactivity some few minutes back. But, this is an independent question its own. So, you should study from fresh. Let's go.

[00:45:04]And it's a it's an easy one at that.

[00:45:10]There are two formulas you can use.

[00:45:13]Let's go.

[00:45:15]Um Let there there are 20,000 atoms in an radioactive element. After the time of five After the half-life is 5 days, and then after 15 days, what will be left?

[00:45:30]So, the formulas you can use you can use the exponential formula.

[00:45:38]N equal to N0 e to the negative lambda t. You remember that lambda lambda here represents what?

[00:45:48]The radioactive constant, right? Which is equal to in two over the half-life.

[00:45:57]Yes, these are some things that you shouldn't you you should already you already know.

[00:46:03]Yeah. So but we're going to use an easier formula to resolve this.

[00:46:08]I went to use an easier method to resolve. And the method we used before that's the easiest method. So that means half to the power n times times the n initial n which is n0 will give you your answer which is n after a certain time.

[00:46:28]Let's say n c. Right. So this is it. Now n0 here is the 20,000 they gave you.

[00:46:35]20,000. Then n here represent the number of t half. Half-life is 5 days. Now 15 days means what? Three half-lives. So it means that we're talking about three t halves. So what does it mean? n is equal to three. So your answer becomes half to the power of three then times 20,000.

[00:47:01]You compute this and you're going to have um 2,500 atoms.

[00:47:21]2,500 atoms.

[00:47:24]So use a calculator to compute this.

[00:47:30]Okay.

[00:47:32]Next question.

[00:47:35]When a neutron is transformed into a proton accompanied by a beta or gamma radiation, what what is the change in the relative atomic mass?

[00:47:46]Yes, a lot of people will say there is no change and it's logical. It's logical because the mass of a proton is equivalent to the mass of what?

[00:47:58]Of a neutron.

[00:47:59]Proton and neutron, they are almost the same in mass.

[00:48:06]The mass of a proton is about 0.08 0.

[00:48:11]The mass of a neutron is about 0.073.

[00:48:16]So, Sorry. Sorry. Not zero, it's one.

[00:48:20]Excuse me.

[00:48:22]One.

[00:48:24]So, we round We always round down.

[00:48:26]That's why we say that the mass of a proton is one. The mass of a neutron is one because this one is negligible. We just round it down.

[00:48:34]But, when you look critically, you will see that there's a thin difference.

[00:48:39]There's a thin difference in masses. So, uh some There are sometimes that when questions come critically, you can consider this thin difference. You will notice that you have the correct answer.

[00:48:51]So, uh Um now, what has happened here is a proton A proton is being transformed.

[00:49:04]A neutron is being transformed into a proton.

[00:49:10]Um The mass The option says Option A says is going to be smaller by one. It's going to be smaller, slightly smaller, yes, but not by one. So, that makes it wrong. So, the answer now becomes E, does not change.

[00:49:31]If you change a proton to a neutron, the mass does not change.

[00:49:37]How many electrons are found in the electron cloud of an element whose K, L, and M shells are fully are full?

[00:49:48]Okay, so the K, L, and M shells.

[00:49:53]The K shell L and M.

[00:49:56]The K shell takes two electrons. L shells take eight electrons and M shells take 18 electrons. So when all these shells are full, when they are full, it means we have 28 electrons. So you just add right. You just add you have 28 electrons.

[00:50:22]Next question.

[00:50:23]That's 16.

[00:50:25]The element chlorine is a mixture of two isotopes.

[00:50:30]Now 75% chlorine 35 and 25% chlorine 37.

[00:50:40]In this case they have said chlorine 36.

[00:50:42]Using the information calculate the relative atomic mass of chlorine.

[00:50:47]So that will be simple. We are given the percentage composition already.

[00:50:55]So you use the formula.

[00:50:58]The relative atomic mass is the sum of relative abundances.

[00:51:07]Relative isotopic mass times relative abundance.

[00:51:11]Yes, over 100% or the sum of relative abundances.

[00:51:19]I know some Maybe you may not really understand this formula but when I will solve you understand it. This I know some abbreviation. Some of the relative isotopic masses times relative atomic abundance divided by the sum of relative abundance. And when you have percentage composition given, you divide by 100.

[00:51:39]Yes, because when you sum of percentages, you will always have 100.

[00:51:45]Where is that?

[00:51:47]Okay. So what does it mean? The relative atomic mass of chlorine will be Okay.

[00:52:04]Um Chlorine, we are talking about chlorine 35 and chlorine 36.

[00:52:18]Chlorine 35, chlorine 36. So, chlorine 35 is 75% plus chlorine 36 uh 25% divided by 75 + 25 is 100.

[00:52:34]So, uh when they give you percentages, you just do that divide by 100. Yes. So, that is going to give you uh 35.5.

[00:52:47]35.5 as the relative atomic mass of chlorine.

[00:53:02]Now, 17. Let us assume that there are 20,000 atoms in a radioactive element. Its half-life is 5 days.

[00:53:13]How many atoms remain or change after another 15 days? This question has been answered already. But, it it it appears here again because the question repeated itself the following year. Yes. So, it came in the exam like this year and then the next year I will not want to call the year. The next year the same question repeated again. And that's what happens when a question come and then a lot of people feel, you need to in the next session the question will surely come back. Yes. So, as you would this that's the importance of treating the past questions. Past questions.

[00:53:51]Okay, um next so we'll go to question 18.

[00:53:55]How many shared electron Okay, how many shared electron pairs are there in we have oxygen, nitrogen, and chlorine?

[00:54:13]Oxygen oxygen nitrogen chlorine But we are talking about the molecules of these elements.

[00:54:24]So, we have oxygen nitrogen and chlorine. So, chlorine is like this.

[00:54:31]Cl Cl Right? One single a a single single bond.

[00:54:39]Uh oxygen is like this.

[00:54:42]Nitrogen is like this.

[00:54:45]Good.

[00:54:46]So, what does it mean?

[00:54:48]Here one shared electron pair.

[00:54:52]One for chlorine two uh two for oxygen and three for nitrogen. So, you can see that in option option C.

[00:55:04]Two for oxygen, three for nitrogen, and one for chlorine.

[00:55:09]Next question.

[00:55:10]Given this give the valencies. Give the valencies of the following ions respectively. So, we have sodium ion sodium. We'll write it for short like this. Sodium ion we have manganese ion. Manganese ion.

[00:55:31]Yes, they didn't want to write it because after writing they have already given you the valency. So, they prefer to say manganese ion. So, you should be the one to know to know that this is how manganese is actually written. And then we have the chloride ion, which is like this.

[00:55:47]So, now we want to come up with their valencies. And how be it, it is easy.

[00:55:54]You get that just by looking at the charge. The valency of an ion come from the charge of that ion. So, the valency of sodium ion is one. The valency of manganese is two. And the valency of chloride ion is what? One.

[00:56:12]But now, mhm, sometimes oh you can want to look at the sign, right?

[00:56:20]Yes, you can put plus one, plus two, negative one. It's all the same. Yes.

[00:56:27]Putting the plus here indicate that this is an ion and it it is a cation, meaning a positively charged ion. And putting a minus here indicate that this is an anion, right? An anion. A negatively charged ion. But all the same, this is this are the valencies, right?

[00:56:46]To question 20.

[00:56:52]Question 20 says, group five element consists of Please listen. Group five element, nitrogen, phosphorus, arsenic, and and antimony, and bismuth.

[00:57:09]Bis- bismuth, sorry. Bismuth.

[00:57:12]Which element is the in this group do you expect to be the most metallic?

[00:57:20]Now, think. Think. Think. Think. Think.

[00:57:23]Metallic character. Now, please.

[00:57:26]If you If you don't focus here, you may miss what I'm saying, what I'm going to be telling you.

[00:57:32]And you may you don't even know if we'll see this kind of question again before you write your exam. So, what does it mean?

[00:57:40]Pay attention.

[00:57:41]Pay attention.

[00:57:43]When you go to school and you already pay your school fees, you also pay attention. If you don't pay attention, you pay school fees again a second time. So, pay attention here.

[00:57:52]Um we have group five elements and we have those group five elements and listed are um nitrogen first, nitrogen, phosphorus, then we have arsenic, we have and antimony and bismuth.

[00:58:21]Now, it means this is in group five, right? Group five, group five. Uh we want to look at metallic character.

[00:58:29]Which one more behave more like a metal?

[00:58:32]Do does metallic character increase down the group or decrease down the group? So, try to think about it. Try to Try to think about it. And I would like you to know that on the periodic table, metallic character metallic character increases as you move down the group.

[00:59:00]As you move down the group.

[00:59:04]So, what does it mean?

[00:59:06]The um the most metallic of all would be the last one down, which is bismuth.

[00:59:14]I don't know if you're getting. We're not choosing bismuth because it is having a beautiful name. We're not choosing bismuth because we like the name. Because I remember I asked a question few days ago. Someone said she's choosing that that answer because the name is beautiful.

[00:59:32]The name is beautiful.

[00:59:34]Please, in your exam hall when you're writing when you're writing in the hall a question that you don't know how to It's true, you will never know everything, right? It's always You always That's why you always seek knowledge because being knowledgeable of everything is an impossibility.

[00:59:53]It's never possible. Don't ever think that you know enough.

[00:59:58]The more you learn, the more you feel like you need You have a lot more to learn. So, that is it. The person who feels like I've already studied is the person who has not even studied at all.

[01:00:10]You who already went through this topic, you have answered questions on this topic. Through this course, through this lesson today, you have seen that there are a lot of things you did not know, right?

[01:00:18]So, if you don't expose yourself as such, you will not learn anything.

[01:00:24]So, metallic character um Come down. Metallic character increases down the group.

[01:00:32]Down the group.

[01:00:34]Next question.

[01:00:36]Consider Consider the reaction.

[01:00:45]We have Okay.

[01:00:47]I'll put the reaction on the board.

[01:00:51]I'll put the reaction on the board so that we can now consider.

[01:00:55]All right?

[01:00:57]Um aluminum Okay, gives All right.

[01:01:20]Uh That's the reaction. That's the equation.

[01:01:36]The valency changes from the uh manganese four oxide to manganese and aluminum to aluminum peroxide.

[01:01:55]Is Mhm. Uh So, I'm first of all going to look for manganese and then for aluminum. We're looking at what the valency changes. The valency changes.

[01:02:09]So, here this manganese four oxide, the valency of manganese here is four. But, here what is the valency here? One.

[01:02:19]So, it means for manganese is four to one, right? Now, let's go to oxygen.

[01:02:27]Oxygen Is it four to one?

[01:02:33]What is the valency of manganese here?

[01:02:35]It's zero, not one.

[01:02:37]Right? Because you can notice manganese here is not is not reacting and it is it does not have a charge, right? So, my valency is zero. Now, let's go to aluminum. Aluminum here what? Zero. To what? Aluminum there is Al three.

[01:02:59]Al is Al three plus, right? So, the valency is three.

[01:03:06]That's the change.

[01:03:08]For manganese and for aluminum and for aluminum. So, I hope that's clear.

[01:03:13]Compared to carbon carbon double bond. A simple carbon carbon bond is Now, we are comparing carbon to carbon single bond and carbon to carbon double bond.

[01:03:32]And the first thing you need to know is that the single bond is longer than the double bond and is weaker than the double bond. So, the double bond is stronger and shorter.

[01:03:44]Is that okay? Let's go to the option.

[01:03:47]Longer, that's correct. Shorter, that's wrong. Of the same length, that's wrong.

[01:03:52]More polarized, wrong. More ionic, wrong. So, the answer is what? The single bond is longer than the double bond. Also, you need to know that the single bond is weaker than the double bond.

[01:04:06]23 The The molecule of carbon dioxide has a total of A, we have four sigma bond. B, four pi bond. C, two sigma bond and two pi bond.

[01:04:21]D, three sigma bond and one pi bond. E, three pi bond and one sigma bond. So, in this kind of question, if you do not know the answer, you better leave it empty because any any any answer you try to guess will probably be wrong.

[01:04:40]Yes. So, let's let's do that.

[01:04:48]Carbon dioxide Carbon dioxide is like this.

[01:04:52]Double bond, double bond.

[01:04:56]But now, listen.

[01:04:58]Listen.

[01:04:59]Listen carefully.

[01:05:01]We are talking about pi bond and and of course sigma bond, right? We are talking about pi bond and sigma bond.

[01:05:11]So, each double bond has one pi bond, one sigma bond.

[01:05:21]Here is the same. So, what does it mean?

[01:05:23]Carbon dioxide has what? Two pi bond, two sigma bond.

[01:05:33]You understand, right? Two pi bond, two sigma bond.

[01:05:38]Bonds.

[01:05:40]Now, sigma bonds are formed by a head-on overlap of orbitals, right? Yes, if you don't know that, you are hearing about it now. You are knowing about it now.

[01:05:53]Yes.

[01:05:53]Meanwhile, the pi bond can be formed by what? A sideways overlap of orbitals.

[01:05:59]So, for example, um single single bond You have a single bond. Let's talk about our double bond and then the the triple bond.

[01:06:12]Yes.

[01:06:13]Um The triple bond has one pi bond and two sigma bonds.

[01:06:25]All right? Now, the double bond has one pi bond and one sigma bond.

[01:06:35]All right? Now, the single bond has one pi bond.

[01:06:40]That is how That's what happens. So, for And now, that means it depends on the type of bond. It depends on the whether you have a single bond, double bond, or what? A triple A triple covalent bond.

[01:06:55]Now, that takes us to question 24.

[01:07:04]The empirical formula is NO. Simple, right?

[01:07:08]Let me read the question.

[01:07:15]The The empirical formula A compound contains six 46.7 nitrogen and 53.3 oxygen. What is the empirical formula? So, you put the element, percentage composition, you divide by You do the mole ratio. So, that means for the elements, you have nitrogen and oxygen.

[01:07:39]Uh composition percentage composition, you have 46.7 and 53.3.

[01:07:48]Now, mole ratio, you divide there by nitrogen is 14, oxygen is 16. So, it gives you values. Let me Let me punch that.

[01:08:00]Mhm.

[01:08:14]All right. So, we have 3.34 for nitrogen, 3.33 for um for oxygen. All right. So, now the ratio, we divide by the smallest. So, we divide by 3.33 3.33. So, we have 1 1.

[01:08:36]What does it mean? The empirical formula is NO.

[01:08:44]If you do not understand this empirical formula and you have any worry, you can let me know and I'll explain how to do the empirical formula step step by step.

[01:08:53]Now, we will continue with question 25.

[01:09:01]How many electrons are found in the electron cloud of an atom whose K, L, and M shells are full? The same question we have seen already. And I said that the K shell has two electrons, L shell has eight, and the M shell has 18 electrons. So, when these shells are full, you just add all, and it means you're going to have 28 electrons.

[01:09:28]26. An element has an atomic number of 11, and its nucleus contains five protons. How many neutrons does the element have? Okay, let's look at that now.

[01:09:49]And the atomic mass Atomic mass is A. Atomic mass is 11.

[01:09:58]And then, the new the neutron number is five, right? Now, what is the proton number? Proton number is what? A the mass number minus the neutron number. 11 minus five is six. So, there are six protons.

[01:10:19]Good. There are six Oh, the question All right, that's okay. There are six.

[01:10:24]Um the bond formed by the combination of sodium and chlorine. So, sodium and chlorine.

[01:10:35]Sodium is Sodium is a metal.

[01:10:40]It's a metal. Chlorine is a nonmetal.

[01:10:44]So, what does it mean? The bond between a metal and a nonmetal is an ionic bond.

[01:10:50]So, the answer is ionic bond. Sometimes, we also call it the electrovalent the electrovalent bond.

[01:11:00]I hope it's clear.

[01:11:05]Okay, that takes us to question 28.

[01:11:10]We have this.

[01:11:19]In question 28 Okay, this is what we have.

[01:11:57]Now, the question says the arrangement of ions in a crystal is called lattice. This is normal. The arrangement of ions in a crystal is called lattice. Now, what is the number of chlorine ions having the same distance to one single ion in the following crystal lattice? So, uh we are going to choose um Okay, let the chlorine. Chlorine is red.

[01:12:27]So, chlorine is in red.

[01:12:31]Chlorine, and then in green we have sodium.

[01:12:37]Is it sodium?

[01:12:38]Yes, sodium ion.

[01:12:40]And then chlorine ion in in red.

[01:12:43]So, we want to know the number of chlorine ions that have the same distance to a single ion in the lattice. So, we choose any central um sodium and then from here now we want to find the the closest chlorine.

[01:13:02]So, from here, this is where we are. If you check, you see that the next chlorine is exactly above, below, to the left, and to the right. And you count that you have 1 2 3 4. So, your answer is what? Four.

[01:13:22]Question 29.

[01:13:26]Second to the last question.

[01:13:42]So, question 29.

[01:13:44]In the following molecules, we have water, ammonia, we have and water is H2O, right?

[01:13:51]And NH3.

[01:13:54]Yes, we have CH4.

[01:13:56]That's water, ammonia, methane.

[01:14:00]Now, identify the letter which gives the exact geometry of um each of their structure. In fact, what do you have? One is the geometry.

[01:14:15]So, we have letter A which says angular, pyramidal, tetrahedral. We have letter B and so on.

[01:14:20]So, what is the the geometry of water, H2O? We We can either say it has a bent shape, bent shape, or angular.

[01:14:33]Right? Because water is like this. Water is like this.

[01:14:39]This oxygen.

[01:14:40]Right?

[01:14:41]Are you even seeing here? This what I'm seeing.

[01:14:45]Water is like this.

[01:14:48]Yes. So, that And this is this from what? The valence electron pair repulsion theory.

[01:14:58]All right. Now, ammonia. Ammonia Ammonia is pyramidal, so you can see this angular or bent. This is pyramidal for ammonia. Then this is tetrahedral.

[01:15:11]Tetra- tetrahedral So, that These are the shapes of these molecules. The last question.

[01:15:21]Identify the particles X and Y in the following equation. So, we have this equation and the equation says um N 14 7 plus X 2 giving O 8 17 plus Y 1 Let us see what is happening here.

[01:15:58]All right. So, we want to identify X and Y. We want to identify X and Y. So, first things first, they you check You check here. You have 1 2 You add 7 2 You have what? N- 9. Here you have 8. It means something is lacking. And what is lacking is 1, which is plus 1, right?

[01:16:20]1 You go up. This is 14. Well, this 17 plus 1 18. This is 14. What is lacking here? 4 By doing this, you now know the You now know what is X and what is Y. I think this was the last question, right? So, X is Excuse me.

[01:16:41]All right.

[01:16:43]So, X 4 2 is alpha, right? X is alpha particle. Because the alpha particle is a helium nuclide, 4 2. And then, Y is a positron.

[01:16:58]Positron.

[01:17:01]Let me check the options and then we confirm that You are 30. X is an alpha particle.

[01:17:08]Y is a positron.

[01:17:10]If you don't call it pos- positron, you can call it a proton.

[01:17:15]Y is a proton.

[01:17:18]Y is a proton. Ladies and gentlemen, dear aspirants, dear friends, we have now come to the end of our today's session.

[01:17:30]I am Sir Godwill Yacum.

[01:17:33]I'm lecturing you from Yaoundé, Cameroon.

[01:17:37]And, like I said at the beginning of this session, we had to go through the through these 30 questions, and I affirm it's not an easy thing.

[01:17:48]Um I I believe you have learned a lot.

[01:17:53]And, I will encourage you to uh keep answering more questions. Yes, the secret is to answer as many questions as you can find.

[01:18:04]Any question that you are lucky to see, don't leave it without answering. That is it. So, I am wishing you the best.

[01:18:14]Yes. I am wishing you anything else that can be learned.

[01:18:19]So, um we shall see again in subsequent sessions.

[01:18:25]You have the encyclopedia with you, the book from Acadestical Music.

[01:18:30]So, what does it mean? These questions are coming from the encyclopedia, a book that is crafted with expertise.

[01:18:39]So, anytime such sessions are coming up, I will encourage you to do what? Answer the questions before the session. So, for those of you who answer this question before I come on screen, you notice it will be easy for you to understand. You already know your problem. So, you are following the session to solve your problem, not just watching as if you were watching a movie.

[01:19:02]So, thank you. We end here to see you again in the next class. Bye.