 # Concept of Valency - CBSE 9 Why does bond formation take place? Can you give me the answer? Yes, it takes place because atoms are always in a constant struggle to reach the stable state. And what is this stable state? Well, if the atoms have only one shell, then the stable state is achieved when two electrons are present in it. And if the atom has two or more than two shells then the stable state is achieved when the outermost shell contains eight electrons. It's called an octet. But the stable state is seen only in case of a few elements. We know that elements like Helium, Neon, Argon etc have atoms with such desired electronic configuration. But what about the case of atoms which do not have this octet? Yes, they will participate in bond formation. That means they will either give or take or share electrons. But wait, who decides how many electrons will participate in the bond formation? Or who tells the atoms if they've to give or take electrons? Well, the answer to these questions is

### valency

. Let's understand this

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in detail. In simple words,

### valency

is defined as the combining capacity of an element. Now what exactly do we mean by this? When we say combining capacity, that means how much can an atom bond with another atom. Does that mean how many electrons can be given or taken or even shared? Yes! Thus, the capacity of an atom to give electrons, accept electrons or even share them in order to achieve the octet state, can be termed as its

### valency

. And what about the case of... elements with just one shell? That's correct! Trading of electrons is carried out for attaining two electrons in the outermost and only orbital in such atoms. Let us take simple examples to understand this. In case of Hydrogen, we have just 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

### valency

of Hydrogen is one as it accepts or sometimes even shares electrons with atoms of other elements. So can you tell me the

### valency

of Oxygen now? The electronic configuration of Oxygen is 2,6. That means it has 6 electrons in the outermost shell. So how many electrons are needed to complete the octet? Yes, 2 electrons. Thus the

### valency

of Oxygen is 2. It accepts or takes 2 electrons from other atoms for completing the octet. Now tell me what will be the

### valency

of Magnesium? We know that Magnesium has 12 electrons. That means the electronic configuration of Magnesium is 2, 8, 2. So how many electrons will be needed now? 6, that's correct! And that's what we're able to see. 6 electrons are needed to complete the octet. But which atom will give away 6 electrons so easily? Is it even possible? Not really! Then how will Magnesium complete its octet? This is where the

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of donating electrons comes into the picture. If managing the gain of 6 electrons is difficult, then can it think of giving or donating the 2 electrons? Yes, absolutely! So if Magnesium gives away 2 electrons from its... outermost shell, then we say that the

### valency

of Magnesium is 2. That means it's not only about how many electrons can be taken, but also about how many electrons can be given by the atom. That is how we calculate the valencies of various elements. Now to understand the

## concept

better, let's take an example of elements forming one compound. May be we can think of Sodium chloride. For the formation of one molecule, we need one atom of Sodium and one atom of Chlorine. What are the valencies of each by the way? Sodium has the electronic configuration 2, 8, 1. While Chlorine as an electronic configuration 2, 8, 7. Before I tell you what happens, I want you to give it a shot. 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's absolutely correct! And that is what helps in the formation of Sodium chloride molecule. Chlorine needed one electron which it got from Sodium. And Sodium gave one electron, as it was the easiest way to achieve stability. So the

### valency

of both Sodium and Chlorine is 1. Does this mean that only elements having the same

### valency

always combined together to form a bond? Not really! Just take the simple example of a water molecule. We have seen that the

### valency

of Hydrogen is 1 while that of Oxygen is 2. Here, Hydrogen needs 1 electron, while Oxygen needs two electrons. Both need electrons. So how do the atoms combine in this... case? How will the bond formation take place then? Well, they will both share electrons with each other. But how? It's not that straight forward. We will understand it in one of our future videos. For now, we have to know that one atom of Oxygen combines with two Hydrogen atoms to give us one water molecule. This is how atoms with different valencies combine with each other to form compounds. Now that we know how the

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of

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works, let's have a look at how the atoms come together to form independent units. Let's discuss the

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of molecules in the next video.