 So we have seen how electrons revolve around the nucleus in fixed circular orbits. Each orbit has a fixed energy level and these are called, these are also called electron orbits. Now in this video, we will look at how are electrons distributed in these orbits or shells? Are there specific rules that electrons follow when they are getting distributed across the shells? That is what we will explore in this video. So to begin with let's say we have we have a sodium atom, let's say we have a sodium atom and there are there are total 11 electrons that a sodium atom has. And if our task is to distribute these 11 electrons in these orbits, maybe maybe we can we can have let's say we have four electrons over here and then let's say the remaining are seven and let's say we take or we have seven electrons in the next shell. This will be this will be the L shell right? K, L, M. That's how the shells go. But turns out this is not how electrons are distributed in a sodium atom. In fact the first shell it has two electrons then the next shell has eight, eight electrons and the outermost shell has one electron. The electrons are distributed in this manner in the shells for a sodium atom, not like how we did in the beginning. Why does the first shell has only two electrons? Why not more than that? Is there is there a limit to the number of electrons that a shell can have? And turns out there is the first rule is that the maximum number of electrons, the maximum number of electrons in any shell, maximum number of electrons in any shell is given by the relation of two n square. This is two n square where n is the orbit. So for the first shell that is K shell when n is one, the maximum number of electrons would be two into one square which is two for the next shell that is L. This is n equals to two. This is a second shell for this one. And the maximum number of electrons that this shell can have is eight. For the next one that is m, the maximum number that it can have is three square is nine, eight to two, that is eighty. So on and so forth. So now when we have a look at this setup, this makes sense right? Because the first shell can have maximum two electrons, so there are two electrons. The second shell can have a maximum of eight and we have eight electrons in L shell. And the last one only has one because there is only one electron remaining. But now let's take a different example. Let's take an example of calcium. So calcium atom has 20 protons and 20 electrons. Every atom is neutral right? So 20 protons, 20 electrons. And now if we distribute the electrons in the orbits, we can say that there will be two electrons. I'm showing electrons by crosses now. Two electrons in the first shell, it can have two and eight in the next shell that is L. So this is four, five, six, seven, eight. Now the number of electrons that remain are 10 and the third shell, m shell, n equals to three, it can have a maximum of 18, maximum of 18 electrons. So the number of electrons that remain are 10. Now let's say we have 10, 10 electrons in this one. So three, four, five, six, seven, eight, nine, 10. This is how the electrons are distributed in the shells. This is K, L, M. But turns out this is not how the electrons are distributed in the calcium atom. In fact you have one more shell. You have one more, one more shell. And then you have eight electrons in the third shell that is M. So again you have eight electrons here. You have eight electrons here. That's three, four, five, six, seven, eight. And then remaining, remaining you have two electrons. Two electrons come in the outermost shell. This is how the electrons are distributed for a calcium atom. But the third shell can have a maximum of 18 electrons, right? Why does it have eight? This brings us to the second rule. The second rule is outermost orbit. Outermost orbit can have a maximum of eight electrons. And this rule is also, it also has a name. It's really, it's called an octet rule. There are some exceptions to this rule. For example, all the transition metals like zinc, copper, they don't really follow this rule. For them the third shell can have a maximum of 18 electrons and they do have 18 electrons in the third shell. So the first rule is more of a general rule. The second rule is the third shell. So the first rule is more of a general rule. The second rule is kind of specific to some elements, not all the elements. There are exceptions like transition metals. But still for most of them, the outermost orbit can have a maximum of eight electrons. And for our relevance at this point, we will be following the octet rule. That is the outermost orbit should have a maximum of eight electrons. So following this rule, the calcium atom should look like this. This is how the electrons are distributed. K, L, M and N. The outermost orbit should have a maximum of eight, not more than that. And therefore they remain into electrons. They occupy the shell next to it. That is the N shell. There is one more question here. Why do we need to fill two electrons in the first shell? Why can't we just have one electron? Why can't we just have one electron? So let's say we only have one electron here and in the next one it can have a maximum eight. So let's say we have only seven in the second one. Let me write that. Let's say we have one in the first, seven in the second. The total is right now eight. The remaining electrons are twelve. Let's say we keep the electrons in the third shell to be as eight. So one comma seven comma eight. And the remaining electrons are now four. So let's say we have four electrons. We have four electrons in the outermost shell. Why can't the configuration look like this? Why can't the configuration look like this? This really brings us to the last rule. The last rule which says, the last rule says, electrons are filled in a stepwise manner. Or let's say electrons are not accommodated in a shell. Unless the inner shells are completely filled. Or we can say that electrons do not take up a new orbit unless the orbits before them are completely filled. So we cannot have a configuration like this. The first shell should be completely filled. That is two electrons. The second shell should be completely filled. That is eight electrons. And the third shell should also be filled. But we need to also keep in mind the octet rule. That is, if we have ten electrons in the third orbit, that will be the last orbit. But the outermost orbit or the last orbit can have a maximum of eight. So the remaining two, they occupy the shell next to it. Let's have a look at some more examples now. Let's say we take three more atoms. Carbon. Carbon has six electrons. Then let's say we take any atom. Let's say we have aluminium. Aluminium has 13 electrons. And the last one, let's say we have argon, which is an inert gas and it has 18 electrons. Now I'm going to fill for the first one carbon. So we have six electrons. The first shell, the first shell can have a maximum of two. The remaining of four. And the shell next to it can have four electrons. So if we draw it, here is a nucleus. This is the first orbit. And this is the second orbit. The first orbit has a maximum of two. So you have two electrons here. The orbit next to it has four. So you have four electrons like this. This is for carbon. Why don't you pause the video and try for these two cases. Aluminium and argon. This one has 13 electrons. How should the electrons be distributed? And this one has 18 electrons. How should the electrons be distributed for this one? Pause the video and give this one a try. All right. So aluminium has 13 electrons. The first shell can have a maximum of two. The second shell can have a maximum of eight electrons. The remaining electrons are three. Two plus eight ten. Total is 13. 13 minus 10 would be three. And three, they occupy the last orbit. So in this one, if this is a nucleus, you have the first shell. You have the second shell. And you have the third shell. The first shell, it has two electrons. The shell next to it, N equals to two or L. It has eight, three, four, five, six, seven, eight. And the last shell has three. One, two, three. All right. For argon, there are total 18 electrons. So the first shell can have a maximum of two. The second shell can have a maximum of eight. The remaining are eight more electrons. And so third shell also has a maximum of eight electrons. And for this one, if you have the nucleus over here, this is the first shell, this is the second shell, and this is the third shell. So the first one has two. The shell next to it has eight. This is seven, eight. And the last shell also has eight.