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Chapter 4.5b Resonance

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welcome to the second video for chapter four

four

section five formal charges in resonance

in this video we'll be focusing on resonance

resonance

our learning objective is to explain the

concept of resonance and draw lewis

structures representing resonance forms

for a given molecule

so uh to begin we're going to think

about the the concept of resonance and

why we need to think about it

before we get to what exactly it is so essentially

essentially

the idea comes from a problem we have

when we run into uh

structures like nitrate and i will go

ahead and draw these for you if you

if you need the practice on lewis

structures go ahead and pause your video

now and give this a shot

and then come back to it and i will show

you what the structure for nitrite

so essentially we have a central

nitrogen and it is bonded to

two oxygens um and because of the number

of electrons

we need um to have one double bond and

one single bond

so a question becomes how do we choose

which oxygen

gets the double bond right because we

could also draw this exactly the same

with the other oxygen with the other

double uh with the double bond

to the central nitrogen and then um

a single bond to the uh the other oxygen

these are right there's no difference

when i when i make the choice to draw the

the

double bond on the right side versus the

left side

there's no difference so why do i choose

one or the other

and essentially this is where we get

into the concept of resonance

and the the concept of resonance really

is that if you can draw

two equivalent structures like this with

the same arrangement

and connectivity of atoms um that are equivalent

equivalent

but you can see that there's kind of a

difference the actual distribution

is an average of the two shown

so it's not that i've given the double

bond to this oxygen or this

oxygen it's that the double bond is

somewhere in between

um and that is really the concept of resonance

and we can call these each individual structure

structure

uh each one of these guys we can call

resonance forms

which are the individual resonance uh

lewis structures so resonance forms

um or sometimes resonance contributors

and then the actual electronic structure

of the molecule which

is not equivalent to either of the

resonance forms again it's an average

of of the two resonance forms is what we

so what this means is that when we're

looking at these two structures where

we have a double bond on this side or a

double bond on this side

the reality is it's somewhere in the

middle so i will go ahead and draw this

in blue so the reality is that we have

a nitrogen a central nitrogen bonded to

two oxygen atoms and they both have

two-ish lone pairs um but some of the

time they have

an extra lone pair and then we'll kind

of draw these sort of dashed lines to represent

represent

that extra um that extra sort of partial bond

bond

um and we know that this is the case we

know that this is an

average that that neither of these two

resonance forms really exists but this

is an average

because of certain properties that we

can measure and one of those is the bond length

length

so when we measure the bond length we

can tell the difference between single

bonds and double bonds

it turns out that double bonds are

shorter double bonds are actually also

stronger than single bonds

and if we had one of these two resonance structures

structures

in reality one of these two resonance

forms was in reality the actual

structure of nitrate

we would be able to see that one of the

oxygen bonds was shorter

than the other one um and what we

what we see is that that's not true we

see somewhere we see a bond

length that is somewhere between what we

would typically see for a double bond

in a single bond so it's slightly longer

than a single bond but it's somewhat shorter

shorter

than a double bond again it's somewhere

in the middle

another important thing that we use to

sort of

note resonance is we use double-headed

arrows to connect the resonance

contributors or resonance forms to

indicate that these are resonance

structures of each other

they're not individual or separate structures

so the question becomes well uh

if we say that we have a resonance

hybrid what does that actually mean

about where the electrons are spending

their time

and um here it's really important to

understand that we think about you know electron

electron

distributions as probabilities but uh

what it turns out is that

um the the structure is always

the resonance hybrid it is not ever

either of the two

resonance forms or contributors um so

this is kind of like

the two uh resonance forms or resonance

contributors are imaginary

they're like a unicorn and a dragon um

the actual molecule

has characteristics of both but it is never

never

actually either one so this is like if the

the

uh rhinoceros has right it has

characteristics of the dragon it has the

horn or the this you know the scaly

stuff and it's angry and

and it snorts perhaps um but it has the

horn like the unicorn and it can run

really fast

um so it has some characteristics of

both a dragon and a unicorn but it is

not equivalent to either one

the rhinoceros is always a rhinoceros

it's never a dragon and it's never a unicorn

unicorn

what this means is when we look at our resonance

resonance

hybrid versus our resonance contributors um

it's really common to sort of have a

misconception that the electrons are

moving around they're switching back and

forth really fast

and that's actually not the case they

are always somewhere in the middle

um and so so it's not that these bonds

are switching back and forth or that the

electrons are moving really fast it's that

that

this resonance can or this resonance

hybrid is

so i'm just going to show you one more

example before we leave this behind

we'll talk about carbonate the um

anion the polyatomic anion carbonate

so again if you need the practice

drawing the lewis structure of carbonate

go ahead and pause the video at this

point and come back and then i will show

when you straw the when you draw the

lewis structure of carbonate you will

find that

it is a carbon with three oxygen atoms

attached to it

and one of those oxygens is double

bonded and the other

two are single bonded

so again how did i choose which oxygen

to give

the double bond to well it's not really

any of them right it's all of them at

the same time so to draw this structure

properly i really should

represent all three resonance

contributors or resonance forms

because this is not actually true right

this oxygen is not different than the

other two oxygens

so i will go ahead and draw this i had

that charge wrong it's a two minus

so i'll go ahead and draw this

carbonate with the double bonds in all

of the places that it could be

because in reality it is in none of

those places

but somewhere in the middle and then i

have run out of room sideways so i'll go ahead

ahead

all right so again um none of these

structures are actually true

right they they are they are useful

images for us to have

just like a unicorn and a dragon are

useful imaginary images

um and they help us understand the they

can help us sort of understand

what this atom or what this molecule

this ion might look like

um but they don't perfectly represent

the actual truth

of the ion the truth is somewhere in the middle

middle

where where this double bond is

simultaneously everywhere

um so each of these bonds has partial

double bond character

and we can represent that with the

resonance hybrid which i will draw

in a different color so we'll have

the oxygen oxygen oxygen

and essentially each of these has

partial double bond character

um and we'll give it a two minus and

so um yeah

so uh our our resonance structure is

somewhere in the middle

each of these bonds is somewhat longer

than a single bond but somewhat shorter

than a double bond and strength wise also

also

each of these bonds is somewhat stronger

than a single bond but somewhat weaker

than a double bond and that's due to the

fact that

the electrons are sort of

simultaneously making partial double bonds

bonds

around around this carbon with all of

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YouTube Transcript:
Chapter 4.5b Resonance