 If you know the atomic number of an element, can you pinpoint its position on the periodic table? And by position, basically, I mean the period number and the group number. See, if you know the period number to which an element belongs to and the group number, then you can exactly pinpoint the position of the element. And even vice versa. If you know the position of an element, basically its period number and group number, can you find out its atomic number? So in this video, we are exactly going to see how this happens with the help of some examples. So let's begin. We have been given with an element whose atomic number is 19. And we need to find out its position on the periodic table. Basically, which period number and which group number it belongs to. Now you would recall from the video of modern periodic table that as we go to the next period, the electrons get added to the next shell. Basically, for example, if we look at the elements of the first period, hydrogen and helium, they have electrons only in the first shell, the K shell. If you look at the elements of the second period, lithium, beryllium, boron and so on, they have electrons in two shells, K and L. And if you look at elements of the third period, sodium, magnesium, aluminium, then they have electrons in three shells, K, L and M. So this kind of keeps on continuing. With every new period, electrons get added to the next shell. The fifth period will have electrons in five shells, okay? So by that logic, if we figure out how many shells are these electrons occupying, then we can predict which period this element belongs to, right? So let's try that. Let's write its electronic configuration and see how many shells are being occupied with electrons. So there are 19 electrons in total. First shell is K. Its maximum occupancy is 2. So the first two electrons will go to the K shell. Now, L shell has a maximum occupancy of 8. So the next 8 electrons will go over here. So in total, so far 8 plus 2, 10 electrons, 10 out of 19 electrons have been distributed. So remaining 9 electrons, where should they go? See, M shell, it has a maximum occupancy of 18. So can the remaining 9 electrons go over here? Well, no. See, we know that we cannot have more than 8 electrons in the outer most shell. If we put the last 9 electrons in the M shell, that makes the M shell as the outer most shell. And now it has more than 8 electrons. So that breaks the rule, right? So therefore we will have 8 electrons in the M shell. And the last electron will go to the N shell. So over here, you can notice that electrons are occupying 4 shells. That means this element should belong to the fourth period. So let me write it down over here. This element belongs to the fourth period. Now let's see which group it belongs to. So from the video of modern periodic table, you would recall that all the elements in the same group have the same valence electrons. For example, if we look at the elements of the first group, they all have one valence electrons. If you look at the valence electrons of group number 2, they all have two valence electrons. Similarly, if we look at the elements of group number 3, they all have three valence electrons. But by the way, we don't have to talk about group elements in the group number 3 to 12, because in your syllabus, we are only concerned with elements till calcium. After that, we don't care in your syllabus. So let me go ahead and remove that. We will never be facing them. So yes, let's go to the group number 13. If we look at them, boron and aluminium, all of them have three valence electrons. So group number 13 has three valence electrons. If we look at group number 14, all the elements over here have four valence electrons. Group number 15, they have five valence electrons. 16 has six, seven, and 17 has seven. And 18 has eight valence electrons. Except helium, of course, because helium poor guy has only two electrons. Okay, so based on this, we can say that if the given element has one or two valence electrons, then it will belong to group number one or group number two. If it has three valence electrons, then it could have belonged to group number three. But since group number three to 12 is not there in our syllabus, then, you know, definitely it's not going to belong there. It will belong to 10 plus 3, 13th group number. If the given element has four valence electrons, then it will belong to 10 plus 4, 14th group number, and so on and so forth. So this is going to be our rule. If valence electrons is less than two, then group number is equal to valence electron. And if valence electron number is greater than two, then group number is equal to 10 plus the valence electron number. Okay. Now just for caution, this is only true for your case when we are not talking about the elements in group number three to 12. Okay. Otherwise, if we talk about them, then this rule will not hold true. Now let's come back to our own element. Our element has one valence electron. Now according to the rule, it should belong to the first group. So this element belongs to the first group. So let's find out its position. It belongs to the first group and fourth period. So this should be its position. And as you can see, potassium occupies this position. So we are talking about the atom, the atom of potassium. So, okay, if we are given with the atomic number of the element and we are asked to find out its position, the first step is to write down its electronic configuration. Now, depending on the number of shells that have electrons, we can find out the period number. And depending on the number of valence electrons, we can find out the group number. So okay, after this, let's try to solve one similar problem. So this time we are given the element has the atomic number six. We have to find out its period number and group number. Now, can you pause the video and try this by yourself first? Now, if you have tried it, let's see. So the first step is to write down its electronic configuration. So KL, M, N. Now, K will have maximum two electrons. So out of six, two are gone, remaining are four. Now, all of those four can go to the L shell. L has a maximum occupancy of eight, right? So over here, we can see that there are only two shells that have electron in them. That means this element belongs to the second period. The second period. Well, what about the group number? Well, for that, we look at the valence electrons. So over here, we see four valence electrons. And according to our rule, if the valence electrons are more than two, then the element belongs to 10 plus the valence electron, 10 plus four, 14th group. 14th group. So our element belongs to the second period and the 14th group. Now, let's do the opposite problem. If we are given with the position of the element, basically it's a period number and group number. Then how do we find out its atomic number? Okay. Well, whatever you need to solve this problem is already given to you. We have already spoken about that. So why don't you pause the video and try to answer this by yourself first? Now, if you have tried it, let's see. So this given element, this belongs to the fourth period. Now, we have seen that the number of period that tells us in how many shells electrons are present. So fourth period means there are four shells in which electrons are present. Let me write it down, the K, L, M and N shell. Now, currently we don't know how many electrons are there in each of the shell. If we can find out the total number of electrons in each of the shells, then we can count the total number of electrons and get to our atomic number. So let's try that. See, we know that the K shell, right? It will, first of all, electrons will go to the K shell and its maximum occupancy is two. Then the electrons will start filling in the L shell and its maximum occupancy is eight. There could be eight electrons here. Now, next we will start filling the M shell. Now, M shell can hold up to 18 electrons. But you know what? First we fill eight electrons in the M shell and then we move on to the next shell, the N shell, okay? And then only come back. And I know that this could be slightly confusing for you. Well, you know what? This is an old method of writing electronic configuration. When we go in higher classes, we will look at a better method, a more awesome method of writing electronic configuration. For now, just remember that in your course, we are only talking about elements till calcium. And till calcium, M shell only has eight electrons in it. So you can even remember that. Okay, now we have seen the number of electrons that K, L and M shell has. What about the number of electrons in the outermost shell, in the valence cell? So basically we mean how many valence electrons are there? And that could be given by the number of group. So group number can tell you the number of valence electrons and we have seen that if the group number is one or two, the valence electrons are one or two. If the group number is 13, 14 or 15, then the valence electrons are three, four, five and so on. Okay, so here the group number is two. That means we will have two valence electrons. Okay, now let's count the total number of electrons to get the atomic number. Here we have two plus eight is 10, 10 plus eight 18, 18 plus two 20. So there are 20 electrons. And if this is a neutral atom, 20 electrons will have 20 protons also in the atom. And 20 protons, meaning the atomic number is 20. Now let's look at one more similar problem. So this time our element belongs to the third period and the 14th group. Now can you pause the video and find out its atomic number? Now if you have tried it, let's see. So the period number, that hints at the number of shells that have electrons in them. Okay, so since over here there are, the element belongs to the third period, that means its three shells are having electrons. So namely the K, L and M shell. Let's see how many electrons does each shell contain. So K is the, in a more shell, its maximum valence is, maximum occupancy is two. L can have a maximum of eight electrons. M shell. Now see this is the valence shell. So we need to actually find out the number of valence electrons. And for that we will use the group number. So our element belongs to the 14th group. Now 14th group means it'll have four valence electrons. Four valence electrons. So this is going to be the electronic configuration of our element. Now let's count the number of electrons. So two plus eight is 10, 10 plus four, 14. So 14 electrons, a neutral atom of this element will have 14 protons. That means the atomic number will be 14. Now let's summarize the video. In this video we saw how to find out the position of the element given its atomic number and even vice versa. For this you need to know how the number of period is related to the number of shells and how the number of groups is related to the number of valence electrons. I hope you remember the relationships. If you don't then please go back and watch the video again.