Concept of Valency - CBSE 9

Why does bond formation occur? Can you give me the answer? Yes, it happens because atoms are always in a constant struggle to reach the stable state. And what is this stable state? Well, if atoms have a single shell, then the stable state is achieved when two electrons are present in it. And if the atom has two or more shells, then the stable state is reached when the outermost shell contains eight electrons. It's called an octet. But the stable state is seen only in the case of a few elements. We know that elements such as helium, neon, argon, etc.

They have atoms with the desired electronic configuration. But what about the case of atoms that do not have this octet? Yes, they will participate in the formation of bonds. That means they will give, receive, or share electrons. But wait, who decides how many electrons will participate in the formation of the bond? Or who tells atoms whether they should give or receive electrons? Well, the answer to these questions is valence. Let us understand this

concept

in detail. In simple words, valence is defined as the combinatorial capacity of an element. What exactly do we mean by this? When we say combining capacity, that means how much an atom can bond with another atom.

Does that mean how many electrons can be given, taken, or even shared? Yeah! Thus, the ability of an atom to give up electrons, accept electrons or even share them to reach the octet state can be called valence. And what about the case of elements with a single shell? That's right! The exchange of electrons is carried out to reach two electrons in the outermost and only orbital of said atoms. Let's take simple examples to understand this. In the case of Hydrogen, we have only one electron in the outermost shell. That means it has to somehow add one more electron to the outermost shell.

Thus, we find that the valence of Hydrogen is unique, as it accepts or sometimes even shares electrons with atoms of other elements. Can you tell me the valence of oxygen now? The electronic configuration of Oxygen is 2.6. That means it has 6 electrons in the outermost shell. How many electrons are needed to complete the octet? Yes, 2 electrons. Therefore, the valence of oxygen is 2. It accepts or takes 2 electrons from other atoms to complete the octet. Now tell me, what will be the valence of magnesium? We know that magnesium has 12 electrons. That means the electron configuration of magnesium is 2, 8, 2. So how many electrons will be needed now? 6, that's right!

And that's what we can see. 6 electrons are needed to complete the octet. But what atom will give up 6 electrons so easily? Is it even possible? Not precisely! So how will magnesium complete its octet? This is where the

concept

of electron donation comes into the picture. If managing the gain of 6 electrons is difficult, can you then think about giving or donating the 2 electrons? Yes absolutely! So if magnesium gives up 2 electrons from its outermost shell, then we say the valence of magnesium is 2. That means it's not just about how many electrons it can take, but also how many electrons the atom can give up.

This is how we calculate the valences of various elements. Now, to understand the concept better, let's take an example of elements that form a compound. Maybe we can think about sodium chloride. To form a molecule we need a sodium atom and a chlorine atom. By the way, what are the valences of each one? Sodium has the electron configuration 2, 8, 1. While chlorine has the electron configuration 2, 8, 7. Before I tell you what happens, I want you to try it. What do you think will happen? Doesn't this indicate that sodium will donate one and chlorine will accept one electron to complete their respective octet state?

Yes, that is absolutely correct! And that is what helps in the formation of the sodium chloride molecule. Chlorine needed an electron that it obtained from sodium. And sodium gave one electron, since it was the easiest way to achieve stability. So the valence of both sodium and chlorine is 1. Does this mean that only elements that have the same valence always combine to form a bond? Not precisely! Let's take the simple example of a water molecule. We have seen that the valence of Hydrogen is 1 while that of Oxygen is 2. Here, Hydrogen needs 1 electron, while Oxygen needs two electrons.

They both need electrons. So how do the atoms combine in this case? How will the formation of the bond occur then? Well, they will both share electrons with each other. But how? Its not that easy. We will understand it in one of our future videos. For now, we should know that one oxygen atom combines with two hydrogen atoms to give us one water molecule. This is how atoms with different valences combine with each other to form compounds. Now that we know how the concept of valence works, let's look at how atoms join together to form independent units.

Let's discuss the concept of molecules in the following video.