 Valency of an element tells us its combining capacity. Basically, how is it going to combine with other elements? If you know the valency of two different elements like A and B, you can predict what kind of molecules are they going to form. Will they react and form AB molecule or A to B molecule or AB2 or some other kind of molecule. So isn't knowing the valency pretty useful? But imagine if I tell you to find the valency of, let's say nickel, well then you will have to write its electronic configuration and then calculate its valency. That's going to be long and tedious process, right? Wouldn't be nice if we remembered the valency of nickel. Well, in that case, you would say that there are 118 elements in the periodic table. How are we going to remember all the valencies? Well, that's where in this video we are going to explore if there is a trend of valency across the periodic table, meaning a trend, meaning that, you know, wouldn't be nice if all the elements in this period had the same valency, let's say in that case, we will not have to remember the valency of each and every element or calculate it for that matter, right? So that's where in this video we are going to find out if there is any periodic trend of valency. So let's begin. Well, first let's recall how to calculate valency. So one way to calculate valency is by counting the number of electrons an atom needs to gain or lose or share to have a complete octet or attain the nearest noble gas configuration. So we saw that atoms react in a chemical reaction to attain the nearest noble gas configuration and become more stable. So the number of electrons an atom will need to gain, lose or share in order to do this can be thought of as its valency. For example, let's see over here, oxygen has eight electrons. So its electronic configuration is going to be 2,6. It has six electrons in its outer most shell. And that's important because, you know, in a chemical reaction, only the outer most electrons participate. So we give them a special name. We call them valence electrons. So oxygen over here has six valence electrons. Now, if oxygen gains two electrons, its configuration will be 2,8, just like the noble gas neon, 2,8. Or if it loses six electrons completely, then it will have only two electrons, just like the noble gas helium. So either by gaining two electrons or by completely losing these six electrons, oxygen can have a noble gas configuration. It can be stable. Well, you know, in a chemical reaction, it is easier for oxygen to find two electrons and gain them rather than losing completely six electrons. Okay. So that's why its valency is going to be 2. Well, there are more reasons which we will talk about later. Now, let's talk about magnesium. Magnesium has 12 electrons and its electronic configuration will be 2,8,2. So it has two valence electrons. Okay. Now, if magnesium loses two electrons, its configuration will be 2,8, just like neon. Or if it gains six electrons, its configuration will be 2,8,8, just like a noble gas argon. So either by losing two electrons or by gaining six more electrons, magnesium can become stable. Well, it's always easier to gain or lose less number of electrons. So over here, it'll be easier for magnesium to lose two electrons rather than gaining six. So its valency will be 2. Now, let's see how the valency varies as we move across a period. So let me focus on the period number two. Okay. Let me bring these elements close together. Now, let's try to find out their valencies. For example, lithium, it has three electrons. Its electronic configuration is going to be 2,1. So it will have one valence electrons. And it'll be easier for lithium to lose that valence electron and attain the noble gas configuration of helium. So its valency is going to be one. Now, similarly, can you pause the video and find out the valency of all the other elements? Pause and try. Now, if you have tried it, let's see. I've already calculated the valency of every element and written it down. You can pause the video and compare your answers. Okay. Now, let's see. Beryllium and boron, they respectively have two and three valence electrons. And it'll be easier for them to lose these electrons and attain the noble gas configuration like helium. So their valency is going to be 2,3 respectively. Now, if you focus on carbon, it has four valence electrons. It will share these electrons and get a complete octade, just like the configuration of neon. So its valency is going to be four. Now, if you come to nitrogen, it has five valence electrons. It'll be easier for nitrogen to gain three electrons and get a complete octade, just like the noble gas neon, rather than losing five electrons, right? So its valency is going to be three. Now, if you focus on oxygen and fluorine, they have six and seven valence electron. And similarly, it'll be easier for them to gain two or one electron to have a noble gas configuration. So their valency is going to be two or one, respectively. Now, if you come to neon, see neon itself is a noble gas, okay? It has a complete octade already. In a chemical reaction, it does not need to gain or lose or share electrons. So therefore, its valency is going to be zero. Now, if you see the pattern of valency, as we move across the period, you can see that valency is first increasing, one to two to three to four, and then it's decreasing four to three to two to one and zero. So we can say that as we move across a period, the valency first increases and then it decreases. That's going to be our pattern. Across the period, valency increases to four and then decreases. Well, that's how generally valency changes in a period. There are some exceptions to it, but we'll talk about them later. Now, let's see how the valency changes as we go down a group. Let's focus on the elements of group number one. Well, hydrogen is not technically in group one, but I'm still including it. You'll see why. Now, can you pause the video and write down the valency of at least few of the elements in group number one? If you have tried it, let's see. I've already written it down for you. Hydrogen has only one electron. So if it gains one more electron, it will have the electronic configuration of the noble gas helium. So its valency is going to be one. Lithium has the electronic configuration of two comma one. So if it loses one electron, it will have the noble gas configuration. So its valency is going to be one. Sodium, its electronic configuration is two comma eight comma one. So if it loses one electron, it will have a complete octet, just like the noble gas neon. So its valency is going to be one. Similarly potassium, its valency is also going to be one. So you can see that all the elements over here have the valency one. In fact, rubidium, cesium, and franzium, all of them will have the same valency. So here as we move down a group, you can notice that the valency remains same. The valency is one for this group. Yes, and that's the pattern. As we move down a group, the valency remains the same. And that's why we put hydrogen in group number one. Even though it does not have similar chemical properties, it does have the same valency as the group one elements. Let me actually show you one more group, the elements of group number two. If you write down their electronic configuration, you will see that all of them have the valency two. So now this is very convenient. Now I don't have to write down the electronic configuration of barium and then find out its valency. I can say that barium is in group number two, just like beryllium and magnesium. So just like beryllium and magnesium have the valency two, beryllium will also have the valency two, okay? So these are the trends that we see of valency. As we go across a period, the valency increases to four and then decreases. And as we go down a group, the valency remains the same. Now you might be wondering, why do the elements of the same group have the same valency? Because when we were making the periodic table, we did not group the elements based on their valency, right? So is this a coincidence? Well, no. So back then when we were making the periodic table, we arranged the elements showing similar chemical reactions into one single group. Meaning elements in one group will show similar chemical reactions. But we did not know why elements undergo chemical reactions. Now we do. Now we know that elements undergo chemical reactions to attain the noble gas configuration and become more and more stable. So if the elements in the same group undergo similar chemical reactions, that means these elements will have to lose same number of electrons or gain same number of electrons or share same number of electrons to attain the noble gas configuration, right? Now this means that the elements of the same group will have the same valency. So I hope now you understand that the elements of the same group have similar chemical properties undergo similar chemical reactions because they have the same valency. Now one last thing. Let me show you how knowing this trend of valency is going to be super helpful. Now imagine you know that how lithium reacts with oxygen. Two atoms of lithium react with one atom of oxygen to form lithium oxide, Li2O. Now just because of the fact that lithium and rubidium belong in the same group, rubidium will have the exact same valency. It will have the exact same combining capacity, okay? So we can say that rubidium is also going to combine in a similar fashion with oxygen. Even two atoms of rubidium will react with one atom of oxygen to form rubidium oxide, RB2O. So just because you know how lithium reacts with oxygen, you can predict how rubidium is going to react with oxygen. Let's take one more example. For example, if we know how magnesium reacts with chlorine, it forms magnesium chloride. One atom of magnesium reacts with two atoms of chlorine, two atoms of chlorine, MDCl2. So just because of the fact that barium belong to the same group as magnesium, it will have the same valency, it will have the same combining capacity. So therefore, one atom of barium will react with two atoms of chlorine to form barium chloride, BACl2. Isn't this pretty amazing? Now let's summarize the video. In this video, we saw how the valency changes as we go across a period and as we go down the group. We also saw the reasons for that. I hope you remember what the patterns were. If not, please go back and watch the video again.