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Dear Student
We are actually going to start chemical bonding
This is very important chapter
It is like blood to our body
like we have different organs,kidney lungs, heart, brain
, like wise chemistryas different chapters
but one important organ of our body in blood
like wise chemical bonding in the blood of chemistry
. If u understand it properly you can understand the language of chemistry which is written by different author and the way the question is being asked,
, so in that first part is actually determining charges which we usually with on the species so to understand the charges.
I will start with the term formal charges actually there are two terms one is formal charge
&other is oxidation state
These two term are little confusing and very closely related, so I would like to start with formal charge.
Lets see here is an example of sulphate ion is given
This is the structure of sulphate ion.
You will notice that sulphate ion has one sulphur & 4 oxygen atom,
but these oxygen atoms are not carrying similar situation i mean these are neutral and these are negatively charged
How does a chemist really come to know that which oxygen should be kept neutral and which oxygen should be kept charged and how many oxygen is neutral and how many are charged.
Well to understand this
. I am giving are more illustration these are different species related to nitrogen
This is ammonium ion
. This is ammonia, this is called amide ion, this is called imide ion and this is called nitride . I can write it.
This is called ammonium ion. This is called ammonia
its a gas,it is called amide ion it is called imide ion & it is called nitride ion
, you may notice that sometimes nitrogen acquires +ve charge
in this structure I didn't write any charge on nitrogen
here I put --ve charge this is -1 , -2 &-3
so how do we really come to know that whether we should put some charges on these atoms or not
again this is answered by the formal charge
so see sometimes we have to keep them neutral sometimes negatively charged, positive, neutral, negative 1, negative 2 & negative 3,
, so there must be some mathematics or calculation involved & we should be done at mental level
in order to write chemical species
well this is one more set of e.g.which is similar in different charges, see +ve charge, neutral negative 1, negative 2
This is called hydronium ion,
, this is called water,
and this is hydroxide ion
& this is called oxide ion
until you know
that why we placed this charges or when 2 place the charge with oxygen atom
till then we cannot write correct chemistry
well do understand it, I am just taking an example of sodium atom & sodium ion
one should remember that in chemistry we keep nucleus with fixed no. of protons
we are not given with the right to charge the no. of proton
or what we can do, we can remove the electrons
or well we can add the electrons
, so during chemicals reaction either the loss of electrons takes place or the addition of electrons takes place
so please look here this is a neutral sodium atom when we simply write sodium atom
what is the story inside
there are eleven protons which is the atomic number of sodium and about the proton which is there in the nucleus there are eleven electrons as so you studied in 9th or 10th class
so the positive and negative charge are equal is number
so what happened atom being an entity
finds that each proton is cancelled by an electron
therefore what happened the resultant charge of this atom is O
but just imagine if onechemicals reaction takes places where we have removed are electron
when remove an electron the system in left with only 10 electron and 11 protons
Remember I told you in chemical reactions we cannot touch the nucleusI mean we cannot disturb the nucleus and we cannot change the value of nucleus
therefore it is very important to remember what can I manipulate only the electron
their number can be increased or decreased what is sodium ? Sodium is a metal what is the tendency of metal? To lose the electron
so as it's tendency. I saw that an electron is lost so when electron is lost it is outside the box of sodium
so what happens that is an imbalance of charge
what is the imbalance? There are eleven protons and ten electrons each of the proton is cancelled by electron but 1 proton is left without cancelled
so we put its positive charges here so next time when you see a positive charge or sodium atom you have to visualize that the protons are in domination no. over the electrons that's why you got a positive charge
now this is an another pictorial explanation of the same description
see the positive and negative charges are equal in number. Therefore this box has neutral charge
but imagine if I remove an electron and electron is lost, loss of electron then positive charge dominates than I will say that if this entity is A
A so this should be than A+
just imagine if I remove one more electron in the next step what happens?
? It has this many +ve charges
but since I remove two electrons so for an electron here and an electron here is left with two unbalanced +ve charge so I will call it A2+
here we can notice from positive & negative charges are equal, A is neutral
since I added an electron since I added an electron positive negative cancel you find that an extra negative charge in left
so I will call it A--
and if I further add an electron the total balance will be there towards the electron such that all the protons of positive charges are cancelled and two electrons are left so it is 2-
so this is how the entities gain the charge on them
so next time when you visualize any thing like that,suppose i write cl,and then i write cl-then I write Cl+what is the meaning of writing Cl to Cl--
if there is no charge an it that means the no. of protons & electrons are equal
if there is extra electron an chlorine that mean its atomic number I mean that is 17 protons to that an extra electron is added so it becomes 17+ 1 = 18 electron so imbalance of 1 electron makes it negatively charged
Likewise here if it is positive that means there are 17 protonsand 16 electron
that's why the resultant is one positive charge actually 1 positive charge does not show that it has gained a proton
rather it has lost an electron because in chemicals reaction you cannot exchange the protons, you can only add electron or remove the electron
so when you see an entity neutral that means it has some no. of protons as its atomic number surrounded by equal no. of electrons so that they are cancelling each other and resultant charge is zero
Well now we will go little mathematically to understand the formal charge
here a formula is written formal charge is actually group no. if that atom minus half of shared electron and unshared electron,
now what is the meaning of that group number
You may notice here, I have written some commonly existing element which we come across initial stage of chemistry
these are in 1st group elements you cannot than because it is very important and if you remember them with their group numbers
this is these all are 1st group elements
these are 2nd group elements, 3rd group, 4th group, 5th group, 6th group, 7th group and these are the inert gases
there are different entities present usually in covalent bond formation these elements takes part. Therefore these are very important
well so group no. is actually the family in which it belongs so this is the group no. of hydrogen one is the also the group no. of sodium, 2 is the group no. of magnesium or if you are asked what is the group no. of strontium answer is 2
what is the group no. of silicon is 4. What is the group no. of sulphur is 6.What is the group no. of iodine answer is 7
7group no. of tin answer is 4 like that you have to remember them one another meaning also exists with the group number what is that?
If I write an atom herewith its group number means I have to show one dot in its Lewis dot structure,
, if this is an atom for group number 2 I mane I can show that here this is 1 electron, group number 2 means in its valence shall there are 2 electrons
group number 3 that means if this is the atom, in its valence shall there are 3 electrons, group no. 4 means valence shall has 4 electrons. Let's see group number 7 that means the valence shell has 7 dots around it,
these are 7 electrons I have drawn 1, 2,3,4,5, 6 & this is the 7th electrons
So group number indirectly indicates how many electrons are presents in its valence means outer most shell
one can understand valence shall is actually that periphery of atom from where if you imagine you are very small you can go and pick out an electron, It is a surface of an atom
although there is no such term which exists in standard chemistry surface of atom but just imagine like that on a tree mango tree there are some mangoes placed, you can see that in the periphery some mango do exist you throw stones and the mango falls out like that valence shell is actually that periphery where you can hit from where you can interact to the atom
you can remove the electrons from valance shall you can add the electron into valance shall whatever happens it happens at the outer most area &outer most place that is a boundary of the atom.
Well so let me tell you what was this formula, this is formal charge is equal to the group number minus half of shared electron minus unshared electrons
so group number I told you this we recall from the periodic table shared electrons & unshared electrons
these are the two terms which we should understand see in this formula this is ammonia
Let's see how the ammonia molecule is derived.
Ammoniais written like this nitrogen with 5 dots 1,2,3,4 & 5. 1,2,3,4 & 5 because nitrogen belongs to 5th group now you put the peripheral atom here
and put 3 hydrogen, these hydrogen have sharing of electrons
and after sharing you find that they are forming the bond
I will repeat at hydrogen has 5 dots because its group number these hydrogen has 1 dot because its group number. There, these are the valence electrons I put 3 hydrogen so that they can share and form covalent bonds
. So a covalent bond is formed here. Another is formed here. One more is formed here
and these two electrons are not used because nitrogen is already having 8 electrons now
2+2, 4+2, 6+2, 8, but this 8 electron are composed of 6 shared electrons why 6 shared ?
Because 2+2,4+2, 6 exist in the bond
well this pair of electron it is not sharing between any two atom its only belonging to nitrogen it is not shared by 2nd atom.
Therefore it is called; unshared electron or it is called the lone pair.
These are called shared electrons or bonded electrons
so there are 6 shared electrons, 1, 2, 3, 4, 5, & 6 actually it is two times the bond 3 × 2=6 and two electrons are unshared.
Two unshared electrons this is how we identify them
in order to get these values we should draw Lewis dot structure without drawing Lewis dot structure you cannot get related answer, and how to draw a Lewis dot structure?
? As I told you earlier first write the valence electron of control atom than put peripheral atoms
make possible sharing & after that you are left with structure which has both lines& dots
so you count them how many linesare there, how many dots are these
so in this formula we can put this value & can calculate the formal charge
in short shared electrons are actually the bonds so see in one line is equal to 2 shared electrons.
so,if i place this value here,shared electrons are 2,so 1/2 of two will given lines only lines so this is an easier way to evaluate the formula so let me calculate the formal charge of nitrogen in ammonia.I am doing that in here
.Formal charge is equal to group number see nitrogen is there in the 5th group,here.
This is nitrogen it is in this 5th group therefore this group no . value will be 5 minus lines surround 1,2, 3 three minus dot there are 2 dots well the results is O/zero that's why we do not place any charge on nitrogen.
You should be clear about one this. These formula are not Lewis dot structure these are not these are actually cooper structures
there you don't have the electrons, only the lives are given
In order to solve it you must have Lewis dot structure, this is not Lewis dot structure
so for every entity you have to draw a Lewis dot structure so in a simple word I can say if there is a formula and see if the dots do just as for completion octet
if they exist then you can calculate the formal charge, I will tell you 1 by 1.
These are examples let us see this example how to calculate the formal charge, please notice here,
this formula is appropriate this reason is that it has shared as well as unshared electrons
so here that question can be solved
let me solve for this atom let me number them 1, 2, 3, I have arbitrally numbered them
for the 1st nitrogen atom what is the group no. minus lines minus dots what is the group no. of this atom, this is nitrogen group no. is 5 minus lines there are 2 lines& any kind of dots or 1, 2, 3, & 4 dots are there
so what is the figure out this results that is minus 1, this is 5 minus 6 so minus 1 so this is nitrogen should carry one negative charge so in my correct formula there should be a negative charge
for this nitrogen let me calculate,
again group no. i.e., 5 minus lines surrounded 1, 2, 3, & 4 lines and is there only dot, no. there is no dot zero what is the result,
what is the result? Results is -1 so I will have to put here -1
so when one write the formula N3
this is in complete because it doesn't tell you about the balance of the charge in the 3 atoms,
so balance of the charge in resulted here and this is the correct formula for the azide ion.
in short if I write this as a condensed form N3, this minus plus is cancelled resulted value is only -1 so I have been N3--so it is called azide ion ,This question is been asked in JEE past time
where the azide ion has formal charges having minus plus minus.
Let us take this example what is the formal charge on these oxygen atoms
, I will do it mentally oxygen group no. is 6 because it is there in the 6
6 now line 1, 2, 3 lines and dots 2 dots.
The results is +1 so I will put here +1
let me calculate about this oxygen group no.is 6, lines 2, dots 1, 2, 3, & 4 result is O. so this is zero formal charge,
let me find this oxygen group no. is 6 lines these is only 1 line and any-any dots 1, 2, 3, 4, 5, &6 dots results is --1 so it should put here -1 formal charge
similarly here group no. is 6 minus lines, there is only one line, dot there are 6 dots. The results is -1, I have to put here -1
so see there are different oxygen atoms some oxygen has plus charge another one has no charge There are two oxygen with minus charge
so when you are writing chemical formula you should know that all atoms are not always balanced in terms of electron and protons
there might be some imbalances. This should be reflected in their formula otherwise we do not understand exactly how this molecule behave
to understand their behavior properly the charges must be mentioned and charges can be mentioned quickly by this formula if we know the position of atom in the periodic table that in which group it is there
let solve this 3rd question also,
first atom group no is 4 no. of lines surrounded 1, 2, 3 & 4 if there is any dot zero
so resultant in zero, I will put here zero charge that means this carbon is neutral
let me find here this is nitrogen group no. 5 lines 1, 2, 3, & 4 is there any dot no. results is 5-4-0 that is +1.
What is here group no. is 5 is there any line yes there are 2 lines
2 any dots, there are 4 dots what happen this result is -1 so I should put a minus charge here
so overall this entity is neutral because when I write its formula I write it CH2N2. It is called di azomethane; overall it is neutral
because the formal charges of nitrogen cancel out each other & carbon already is neutral
this is more structure here we have to calculate some formal charges on 3 different carbon atoms
This question is different than last because here all the 3 are the carbon
let us calculate group no. is 4, carbon below to 4th group
minus lives 3 any dots no, zero what Is the results +1
this carbon is positively charged, what about it? Group no. 4, minus any lines there are 4 lines
has only dots given to you no. zero
so 4-4 that is given zero no, charge on it,
here group no. 4 minus lines 3 minus dots 2, results is 4-3-2 is -1. This carbon is negatively charged
so you may notice that all are carbon but in different chemical circumstances.
Therefore according to that they possess the charges on them
with this carbon I can say that electrons are deficient around it
despite the bonding,this carbon is having a perfect bonding state, all the electrons and protons are equal in number in the surroundings of carbon with the nucleus
Here an excess electron exists
Now one can say that there are two dots
then why do we only put -1 charge?
. Remember dots are not actually producing the charge
dots are first being cancelled by the proton
& we write only the final balance of the charge here
It is something like that suppose I have a cheque of 50,000 rs.
With me and I have given one cheque from any account to somebody of 30,000 rs.
so though I have a cheque of 50,000 rs. but since a had given a cheque of 30,000 rs to Someone else so what is I am left with? Only 20,000
So if someone looks at this cheque and say that you have 50,000 rs. with you
them it would be wrong because at the sometime I have are debit not which say that 30,000 to be given
so the difference in only 20,000
. I am having actually 20,000 likewise if you have 2 electron it doesn't mean that the charge of 2 electron should appear here
actually the charge is the result of cancelation of protons
& electrons to the each other that what only we can write here
after explaining all these I would like to give you a word of caution that is very important, the word of caution like see here, please note that none of these formula is written with any dot
These are not the correct form where you can calculate formal charges.
If you have to calculate the formal charges you should satisfy their octet and you have let put appropriate no. of dots
and then you can apply that formula and then only you can get the correct charge over here
so if I want to calculate here my answer will be wrong because dots are not present in it
so if a question which is asking formal charges is given to you then it must carry both lines and dots.
Ok it may be case that there is no dots, dot is not mentioned. So how do I come to know that correct no. of dots are given or not
So the answer is simple. Just look at the valency, if an atom has normal valency
than it may have dots around it
And if an atom has more than expected valency, I call it hypervalent
then it may have lesser no. of dots
and if an atom has lesser no. of valency than its normal valency. Then it may have more no. of dots.
I will tell you how. See nitrogen is belonging to 5th group. That's fine.
But nitrogen is a trivalent atom. You can ask anywhere even in from the smaller from the lower classes from the simpler examples you know that nitrogen is trivalent.
What is about oxygen. That is divalent. What is about hydrogen.. It is monovalent
So please look here and understand it. Whenever an atom have its normal valency. It has no charge.
Like nitrogen shows 3 lines which is trivalent it has no charge.
Water -- oxygen has 2 valencies it is divalent so it has no charge. Likewise if I write hydrogen it is monovalent it has no charge But if an atom has more valency that as normal valency it will carry the positive charge.
So look here 1,2,3& I put the 4th line
so, it has an extra valency. It is a hypervalent by 1 unit so + charge.
This is its normal valency 3. if I cut off 1 valency it will be left with only 2 valency. So it is going hypovalent whenever anything is hypovalent equal number of -- ve charge comes on it.
This is its normal valency 3. if I cut off 1 valency it will be left with only 2 valency. So it is going hypovalent whenever anything is hypovalent equal number of -- ve charge comes on it.
So it has -1. ifI cut 2 hydrogens obviously 2- & if I cut all the hydrogens then 3 -- Look here If it get hypervalent it is going hyper
It is going to acquire more valencies so it will have a positive charge. It is going hypovalent. This oxygen has lost 1 bond
So hypovalent. So it has -- 1 charge Here it has lost both the bonds so it has 2 charges. Now you can notice here with the first question.
Why this oxygen was not having any charge. This two oxygen. But this two oxygen had the negative charge.
Let me explain you here. First thing you should know that you have to calculate the formal charge with the help of term Lewis dot structure.
How do we do it ? I will write the sulphur at the center and put around 6 electrons
These are the 6 electrons because sulphur belongs to 6th group Now take the oxygen, I told you earlier also that whatever group no. that many dots it has
Oxygen it has also 6 electrons
Let me put another oxygen that two has 6 electrons, 3rd oxygen 6 electrons 4th oxygen 6 electrons,
now let me make the bonds, remember one thing any atom can have no. of bonds varying form group no minus 8 to group no.
It is its maximum value of bond it can form and it can go from group no. minus 8 to group no.
. Sulphur has the maximum covalency of group no minus 8 to group no so group no. is its maximum covalency
How can I know that?Very simple. Group no. is 6 belonging to 6th group so its maximum convalency is 6
Let me see one bond is formed, 2nd is formed, 3rd is formed, 4th is formed 5th is formed Now I cannot form the 6th that means the way I was drawing the bonds was wrong. I should correct it how?
? I should correct it by drawing 1st one covalent bond to each of the atom and still this maximum no is not achieved
I will make the double bond to,Lets see 1st bond with this oxygen, it is held up now,
2nd is connected, 3rd is connected, 4th is not connected but now connected so I used out of 6,4 as sigma bonds to connect 4 0 atoms
Now till I have 2,2 more so this is 2nd bond, it is double bond now, till this I have 5 lines, 6th line is also there This is 6th line
No matter this 2 oxygen are not connected with double bond but at least they are bonded, they are with the sulphur.
So this in the correct way of drawing the Lewis dot structure. I will repeat. What are the steps we have to draw. Use to draw the Lewis dot structure.
First step draw the central atom put the valence electrons around it.
Draw the peripheral atoms put their valence electron around them now join each of the peripheral atom at least one covalent bond to the central atom.
Even then after that of the central atom is not reaching to its group no.then extend more bonds so that its group no.is achieved.so valency equal to group no .is the maximum valencey of the bond it can draw.
So it has we have drawn 4. Now, if I calculate the formal charge. This is group no. 6 minus lines 2, 4 dots The result is zero.
Here group no. is again oxygen 6 minus lines 2 dots 4. result is zero
Here group no. is 6, line 1, dots 1,2,3,4,5 so I will write here this is 5 let me check it again
1,2,3,4,5,6 dots are 6, oxygen must have 6 electrons around it.
So the result is -- 1. here to this is group no. 6 minus lines, 1line minus dots 1,2,3,4,5 so there are 5 electrons so there is no. charge on it But octet is not complete. 1,2,3,4,5,6,7 we have to add an electron to complete the octet
Here also I added an electron to complete the octate. After that it became -1.
. Now, why one question comes. Why did I add these two electrons extra? The answer is simple. From where do we get sulphate ion
. Some of these sources you know. This can be H2SO4, This can be NO2SO4, just imagine if it Na2SO4 how it has come Na has given Na+ by an electron Another Na have lost an electron in a similar way.
This two electrons are added to 2 oxygen atoms
How? I will draw that again you will get it sulphur having 6 electrons like this one oxygen another oxygen,one more oxygen 4 oxygen
Imagine each of these oxygen is having their valence electron Its valance electron is like this here 2,
valence electrons, Here 2 the valence electrons
now more make the bond. One bond, one bond, & one bond, and one bond
Now- 1 have to make a double bond. I can put this electron here
So with this electron I can make 2nd bond. With this electron I can make the 2nd bond
Now see the oxygen. The purpose of bonding is to make 8 electrons.
1,2,3,4,5,6,7 so the 8th electron , this electron I will add here This is an electron.
. I will make it a dot likewise this electron is added here. Why?
Because 1,2,3,4,5,6,7,8 see look again 1,2,3,4,5,6, I have to count this many electrons
1,2,3,4,5,6,7 & this is the 8th electron so there are complete eight electrons.
Now so I can calculate the formal charges.
. 6 minus lines 2 dots 4 result in zero. Here 6 minus lines 1 dots 6 , 1,2,3,4,5,6, result is -- 1 Here group no. is 6 lines again the some 2, dots, 4,1,2,3,4 result is zero. Here 6 minus lines 1,dots 6, result is -1.
Here group no. is 6 lines again the same way 2, dots, 4,1,2,3,4 result is zero
. Here 6 minus lines 1,dots 6, result is -1. This is how I put the negative charge.
The first part of formal charge is over here
Now if you find that how these dots are put, little difficult. You should watch the lecture for how to draw laws dot structures.
Once you I suppose that is the step before this. If you have not studied properly. In lower classes
how to draw Lewis dot structures, you may find a little difficulty here and if you have studied them,
, you can easily draw. But without Lewis dot structures we cannot calculate the formal charge.
Well this lecture ends here.
So please watch it for the next time how the formal charge is related to arrow
and how the formal charge is related to some other concepts of chemistry
. Well then after I will teach you how the formal charge &oxidation state are different.
. Well Thank You.