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Students, today we are going to study a very basic thing found in chemistry
that is how to draw a Lewis dot structure
Lewis dot structure is needed because chemist want to show and a find
how to e-- moves between the molecules in a chemical reaction.
So to explain the mechanism of the reaction
One has to right molecules with e-- details electronic details means how the e-- are distributed between the atoms over among the atoms and how the charges are present on these atoms
so we will learn of very basic concept
, that is known as Lewis dot structure
. First thing, most of the times, we have come to know that we write hydrogen like this H2O
Remember, it is not Lewis dot structure, or if we write Hydrogen like this it is a word obviously it is not upto a standard of chemistry.
It is a simple word. It is not called a lewis dot structure
Remember, many times in book, we find that hydrogen is exhibited like this
, but again it is not Lewis dot structure
Lewis dot structure is going to be that structure where the valance e-- that means outer most of e-- are presented by dot
if the e- are represebted by dots,then the reason for the bonding is the presence of this 2e-- which reduces the repulsion between 2 nuclei is reason for the bonding, this is the (--)ve charge and the
the attraction force between the e-- and the (+)ve charge likewise this side the (+)ve charge & the electrons is the reason for bonding.
So this structure is called Lewis dot structure
. In order to draw Lewis dot structure you must have some basic idea about elements,
their position in periodic table and their capacity to hold the maximum no. of e-- three things I have you told,
1st thing the element or the atom which you are going to focus, where does it lie in the periodic table and what is the maximum capacity of holding the e--
So in a rough idea for the beginning
, I can take a sketch of periodic table. in this sketch, you find that the first row element lie here these are hydrogen and helium.
Remember, after watching this video, you can take the note, that this is called the first row, you should note it down in order to remember.
Then these elements just after hydrogen, we find lithium, sodium, potassium, rather potassium comes, here in this line, then rubidium, cesium, francium.
. In similar order, if I write the element, see here this is the second row in front of lithium, this section that is the d block, has no element.
So In place of hydrogen, this is the first row, the lithium going to be 2nd row. sodium going to be third row.
Potassium going to be 4th row. Like that so here is you find that berilium, magnesium, calcium, stransium.
These are the elements. If we look at the 3rd row, the corresponding elements are and the 4th group, the fifth group, the 6th group and 7th group, I shall draw here, and this is for the inert gases.
we know what are the enert gases? Now, what I mean to say that these elements of 2nd row, they have a limiting value of attaining maximum 8e--
they cannot have e-- more than 8. In their outer most shell. But those elements which are below to this, I mean all the section. Can have maximum 18 e-- in valence shell
All these elements, they can occupy, they can accommodate more than 8e-- in their valence shell.
So what is important, hydrogen has a limiting value of 2e--, the 2nd row has a limiting value of 8e-- and rest all below the 2nd row, I mean, 3rd, 4th, 5th. Like that :
All these rows can accommodate more that 8e-- . So in nut shell, I can say that hydrogen has a capacity for 2e-- at the maximum, Any element of 2nd row has a capacity of 8e-- at the max.
and any element below in 3rd, 4th, 5th, all these can have more than 8e-- in their valence shell.
So one can remember these values is very important Remember, I doesn't mean that hydrogen can have, suppose, one says that hydrogen has one e-- right now, so we will accepted it
. Now, if someone says that carbon has 2e-- , 4e-- , 5e-- , 6e-- , 7e-- , till this theoretically possibilities exist.
But after 8e-- . I can't put the 9th e-- , this doesn't exist. 9e-- cannot come these but you can have eight seven, six e-- in carbon, you can have 5e-- carbon, so carbon atom can put any of these electrons in their octet.
mathematically, or Theoretically, the limit exist here only, for these values. We do not posses or pass any comment on it,
but it cannot have 9e--, but if suppose you are having a simple atom like sulphur, then, I can have 2e-- , 4e-- , 8e-- like this I can go till 18e--.
So I can keep till 18e--.it can work, It can have 15e-- in the valent shell. It can have 10e-- in the valence shell. These is no (--)ve restriction all these values are possible,
If I want to put 10e-- is possible. If I want to put 12e-- is possible, till 18 I can put the e-- with the 2nd row elements
. So what we studied in the lower class is only a narrow, and restricted understanding of elements, that the only 2nd row elements, these can have maximum 8e--
Rest all these elements can posses more than 8 electron e-- well, Now,my goal is to explain you, how to draw the Lewis dot structure for different at molecules and ions.
What I did explain here is the background for that these rules this picturisation of periodic table is all needed, when you draw the Lewis dot structure of a molecule or ion
There are some rules, we need to follow the 1st thing is that is to identify the central atom in a molecule, what is mean by that ?
Suppose I have a formula CH4. In order to draw Lewis dot structure, you must have the ability to identify the central atom.
Now central atom is usually that atom, which is least electronegative or bigger in size
So with methane, C is seemed to be central atom. central atom hold all the peripheral atoms to it, creating a big molecule.
Another example,where you can understand. SO2, sulphur di-oxide. In SO2, sulphur is the biggest atom or least electro negative atom.
. Therefore we will call it to be central atom. If suppose I have SO3, then, sulphur is the central atom, because oxygen is more electronegative.
Otherwise in periodic table, sulphur happens to be bigger in size,i will call it to be central atom.
You can understand like this, to check central atom, you can go in this order, if you go downward, there are chances, you will get central atom,
if you go towards left, there are chances you will get the central atom for e.g., BCl3. In BCl3, Boron is the central atom.
the reason is simple. Boron lies far to left here as compare to chlorine. So from this diagonal, you should have maximum this placement to check the central atom, that is BCl3
I can take other e.g. also, COCl2 we have check the central atom so there is carbon, oxygen and chlorine. See chlorine is here, oxygen is here, they are not much displace from this point
. But when you look at the carbon, It is far away from this oxygen and chlorine as compared to this diagonal.
If you locate the position atom from the diagonal you can find that the central atom is farthest from the diagonal.
. It is very easy to understand. Let me take one more case. Suppose you have NH3.
Obviously hydrogen atoms are the monovalent atoms so in this case, you would not lie to take such a rule, because hydrogen can not be central atom.It has ability to bind with the only one bond
. Central atom have multiple valencies. So in that case you will say that nitrogen can have 3 valencies, therefore nitrogen is the central atom.
So nothing is taken to as a varbotom rule instead you should take them with some logic.
. Central atom must have multiple valencies, 1st thing& 2nd thing, central atom should lie as far as possible from this diagonal, either to down side or to left side.
Let us call one exercise we can understand by that a gas is there SOCl2, we have to identify central atom, sulphur is there, oxygen is there, chlorine is there,
if we find chlorine lies here and sulphur is just on the diagonal, oxygen is closer here on the diagonal obviously since it is lying on this diagonal, I will prefer sulphur to be central atom.
chlorine can not compete with that, Let us take one more case IF3, if you find fluorine is here, I is here,I is larger enough as compare to fluorine.
So I will take iodine as the central atom and fluorine to be the peripheral atom. Let us take one more e.g. PCl5
Now phosphorous, since it is lying on the diagonal, you can see it is phosphorous and it is chlorine, so that is one phosphorous and chlorine all these are the mono-valent in normal conditions, they form only one bond.
So P it is going to be tri-valent,this is di valent It is tetra valent. So chances for folding more and more atoms happens to be with phosphorous so I would like to take P is to be the central atom.
. Here iodine, Now here if I have IF7,I is the central atom, If I have ICl6--then I is the central atom. So first thing to identify the central atom. Let us take one more NO3
So obviously this is tri valent, this is die-valent so tri valent should be the central atom.
Let us take one more e.g. CO3--2, Now is C is tetra-valent and oxygen is die-valent. So I will take C to be central atom So this is the may you should locate the central atom.
Now, we shall go with some rules to draw the Lewis dot structure. First thing, I am going to erase this and remember it is the very important of this part.
First thing, decide how many valence e-- are possessed by each atom in a molecule let us talk with this, an e.g. of methane CH4, Here C has 4 valencies e--.
This idea can be taken from here this is 3rd group, this is 4th group, 5th, 6th, 7th & 8th. So whatever its group no. that many e-- it keeps in its valence shell. Then H let us talk about hydrogen.
H has only one e--, so 1st thing you should take out these atoms from the periodic table.. Periodic table is assumed is to be back,Its back.I am taking out carbon with 4e--
--. I am taking out H that comes with one e--. and I have such 4 hydrogen to draw the Lewis dot structure.
It is the 1st step. So 1st you should know how many valence e-- are Passed by each atom that idea is given by this periodic table.
. After that 2nd step, if that is more than one atom type in molecule put the most metallic or least electronegative atom in the center, Recall that electronegativity decreases as atom moves further away from fluorine on the periodic chart
. As I did explain in the previous discussion how do we get the central atom.
So if there are more than one atom, this is 1st type of atom, this is 2nd type of atom so, one which is less electro negative and away from the fluorine is going to be central atom.
In our discussion, C is central atom.
. For 2 reason, it is most away from fluorine & 2nd reason. It has 4 valencies it has ability to produce 4 valecies . It is tetravalent C is tetravalent as per the previous knowledge we had in lower classes, H is monovalent as we studied in lower classes
So we know about it then the third rule, you can note down this rules because there are some 7 rules you have to follow through then you can understand it.
. So you can pause it and you can note it down, third rule Arrange the e--so that each atom contributes one e-- to a single bond between each atom that means the third step,
this was your 1st step,I decided the e--, this is over, 2nd I decided there are 2 kinds and therefore the central atom, and the 2nd step is over, 3rd, Now we have arrange them C with 4 e--, & H also with one e--each.
and they all are going to form single bond by sharing of e--so I have shared these e--in order to produce single bond. Remember, after sharing this e-- which was the property of only this H is being allowed to be part of this C also
So we can have 2 e-- not only belonging to this H but also to this C.
We can put a box to their ownership. H has ownership of 2 e--. this H has also ownership of two e--.
This two has ownership of two e--,like this and C has an ownership of whole this e-.
this whole electrons come to a value of 8e-,and each of this H,have 2e-.
These values are as per the limiting values which we said earlier That means all these atoms have complete valance shell or I can say they have completed, the need of e--, by sharing.
. So this is called the Lewis dot structure, of C. Now I remove the bond it is perfectly Lewis dot structure, because lines need not be shown.
These are going to show the perfect lewis dot structure, more correct one,the but the most commonly used one is where you connect the lines with dots
Now, we are going for next rule that is rule no. 4. Count the e--around each atom :
are the octets complete? If so, your Lewis dot structure is complete. So, we have seen that C has 8 e--, each of the H has 2 e-- , So our Lewis dot structure is completed.
This is the end of the formation of L.W.S. of methane. This is how you draw the Lewis dot structure of CH4.
But thing do not end here, because it is very simple example of Lewis dot structure. We have to go with rather more complicated & tougher e.g. of Lewis dot structure for that, I will give you few more rules, you can write them.
The 5th step says that If the octets are incomplete, and more e-- remain to be shared, move one e-- per bond per atom to make another bond.
Now what is the meaning of 5th point, I will explain that, let me erase it.
I am giving an example to explain the 5th point say carbon di- oxide,as usual our idea for central atom comes from carbon because it is least electro negative second thing you should write each of this atom,with their valence e-.
as I told you earlier, this values are obtained from their position in periodic table, the group no. shows how many dots you have to put around that.
. C has group no. 4 means, I have to put 4 dots. oxygen has group no.6 means i have to put 6 dots.
like this so this was first formula that is condense formula, 2nd I have seen what set of atoms this is composed, It is composed of these set of atoms
3rd. thing I need to do is to identify the central atom and write it.
C is the central atoms and I have written like this Now, I should arrange both the oxygen of this 2 sides.
. Please always remember that atom with highest valencies used to be at center and atom with lower no. of valency should be the side. Oxygen is divalent.
It is also di. It is tetra,you can remember it,that's why I can put di on the sides peripherals and at the center highest one atom with highest valency, that should be placed.
Well, now this C has formed the bond why we are doing it because they have to reach up to the maximum value of 8,
, then C also has formed the bond, Remember after arranging the atoms,
you should first connect each of the atom to the central atom,otherwise they will not be the part of the molecule ,so to connect them at least the 1st bond should be secured, so I have secured one bond from each of this oxygen from two sides to the central atom C.
It is the condition of ownership of this C, there were 4 e-- already possessed by C. This is the 5th one, and this is the 6th one. Remember I am going to put the no. one, two three four this four e-- were originally belonging to the carbon
. This is the 5th. It got by the sharing. This is the 6th e-- got by the sharing. So if I draw at this stage this is the picture for carbon's octet.
Remember, I told you that carbon has the tendency, carbon can please come to this point, this is the carbon, it is belonging to the 2nd row, 2nd row have can maximum 8 e--.
So it will go for sharing one more e-- with this oxygen so it shares this e-- also with this oxygen. now e-- also come into the ownership of carbon.
This is 7th e--. If I draw one more e-- this 3rd e-- for sharing with oxygen, this is the 3rd and this is the 8th e--. So this is how, we got 8e--around carbon. Now let we see this is checked
. It octet has complete, because it said that if it is not share draw from more no.bonds you can draw multiple bonds, I have drawn the 2nd bond, here also I have drawn the 2nd bond.
. If I count these e--, this is one 2, 3, 4, 5, 6, 7, 8, e--, it is also completed and 1, 2, 3, 4, 5, 6, 7, 8, e--, It is also completed,
So In this manner, all the 3 atoms have achieved 8 e--in each octets. This is what the meaning of 5th point.