 All right, so mass of, you know, you can say roughly that mass of one proton is around one unit only and mass of one neutron is also around one unit. It will be slightly different than one unit, but more or less it will be close to one unit only, okay? Because it is understood that the C12 will be what? Around 12 times the mass of hydrogen atom, right? Because carbon, because the carbon will have, you know, nucleus which is 12 times heavier than the hydrogen nucleus, fine? So that is the reason why and we know that hydrogen nucleus has only one proton, okay? So when we say that one-twelfth of carbon-12, it will automatically mean that, you know, it is mass of just one proton, all right? Now inside any nucleus, suppose this is the nucleus, okay? Inside this nucleus, there will be integer times proton and integer times neutron, right? So when you talk about the total mass of the atom, it will be what? Z times mass of one proton which is around one unit, okay? Plus again one integer mass number minus atomic number multiplied by one neutrons mass which will be again around one units, okay? And also you can account for the mass of the electron also which is extremely less, okay? So it will be like 0.000, it will be like this, 0.00555 units, okay? This multiplied by the number of electrons which will be Z only which is the atomic number, okay? So this whole thing, can I say should be close to an integer? Yes or no? This should be close to integer only? When I measure the mass in terms of number of units? Yes sir. Okay? But then it is, okay, but then it is found out, okay, using mass spectrography that at times the mass of an atom is not close to an integer. It is like, you know, let's say for chlorine, chlorine the mass is found to be 35.47 units, okay? Which is nowhere close to an integer. It is actually in between 35 and 36, okay? So the reason for this mass is that the chlorine exists in two forms, okay? So the chlorine exists in the form of, you know, two isotopes. There will be two isotopes. The first isotope has mass of 34.98 units and the second isotope has the mass of 36.98 units, okay? So when you take naturally occurring chlorine sample, it will be a mixture of these two isotopes, fine? So how will you account for the, you know, the mass of the mixture of these two? Any guesses? Say it again. So you basically take the average of the mass? Yes sir. Yes sir. So how will you take care of that? So you take the sum of all the isotopes you have and then you divide it by the total number of isotopes you have taken. I have taken two isotopes suppose, these two. It would be the sum of the atomic masses of the isotopes divided by 2. Are you sure? Like for example, I am taking a mixture of these two. This out of 100 gram, it is found out that this particular isotope is present 75. sorry 75.4 gram is this and the remaining amount is that which is 24.6 gram. So can I take just the average of these two? So you multiply with the percentages and then you have it. Exactly. You have to take the weighted average, okay? What if the 99.99% is just this? So the chlorine mass will be closer to this, isn't it? So that is why you have to take the weighted average. So what is observed in the mass spectrograph is the weighted average. So 24.6% of 36.98 plus 75.4% of 34.98. So you can see that these two are close to integer. When you talk about the chlorine in a very pure form, like a single isotope, but then when they exist in a mixture, what you observe is the weighted average of their atomic masses that will come out to be a fraction which need not be close to an integer. So this comes out to be 35.47 units, okay? So I mean this chapter, it's straightforward in nature, but it has a lot of calculation involved. So you may at times feel that conceptually this is very easy. So let me just read it and just understand all the concept. But the 50% of it is related to solving problems. So make sure when I say solving problem, what I mean is, you know, calculation part of it. So make sure you are not ours to solving, sorry, the calculation part of it. So can you do this particular question? For example, neon exist in three isotopic form, neon 20, then 21. All of you write down this and neon 2210, okay? The natural abundance of this is 90.51% of neon 21, it is 0.27% and don't use calculator please. And that will be 9.22% unit to find the atomic mass of the neon. The neon 20, the mass is 19.99 units, neon 21 is 20.99 units and 22 is 21.99 units. Find out the mass of the next So is it 20.189? Others? So 20.177. 20.177 approximately. Okay. So this is how you do it, 19.99 multiplied by, you know, you can also, rather than using percentage, you can multiply it with fraction, then you don't need to divide it with 100. You can just multiply it with 0.9051 plus 20.99 multiplied by 0.0027 plus 21.99 multiplied by 0.0922. Okay. So you'll get around 20.17 units. Okay. And one unit is one 12th of the mass of one carbon atom. Okay. Fine. So this is how the simple questions can be asked from whatever we have learned till now. Okay.