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5.4a MO Theory Intro

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everyone now we're going to go on to our

next theory so valence bond theory gives

us an idea of what

our orbitals actually look like and

what the bonding actually could look

like however it does fail in kind of

describing some of the properties of the atoms

atoms

of the molecules so for example if we

look at something like magnetic

properties so this is a video

of of a strong magnet and

they're going to pour liquid nitrogen

through it and it passes

right through the magnets so it doesn't

have any type of magnetic

attraction to it however if we take that

same exact

magnet and we pour oxygen through it

we'll find out that oxygen actually sticks

sticks

to the magnet so it has some sort of

magnetic property

so we can think about what kind of model

we can use to actually explain what is

happening here

so that model is molecular orbital theory

theory

and here you're learning outcomes for

molecular orbital theory

you should be able to outline the basic

quantum mechanical approaches

to deriving the mos from the aos

you should be able to describe traits of

bonding and anti-bonding

mo's calculate the bond order based on

the mo electron configuration

and you should also be able to write

molecular electron configurations

for diatomic homonuclear molecules

and then relate these properties to

things like stability

and magnetic properties like in the

video we just saw so we can actually explain

explain

why oxygen is attracted to the magnetic field

field

so let's first think about um you know

our electrons again

and just remember that we can think

about our electrons

as waves so waves we have a couple of properties

properties

we can actually think about what happens

when you take two different waves

and add them together so we can have two

different things happen we can have some

sort of construct of interference uh

on here and then what happens in

constructive interference

your peaks and valleys are lined up so

you get an amplification

of the actual wave so it's additive

to each other another thing that could

happen is if you're

exactly off phase with each other you

can have destructive

interference and that's where your your

peaks and valleys are exactly

opposite of each other so we can think

about up here as positive wave function

this is negative wave function

this is positive wave function this is

negative wave function

if we take this positive and negative

region and we add them together

and then we take these negative and

positive regions and add them together

we end up getting a flattening or deconstructive

deconstructive

interference of those uh waves on there

so this is actually what we can think

about for our electrons so our electrons

and our bonds

can either have some sort of construct

of interference

where we have a bonding interaction

because the the

electron waves are matching up with each

other or we can have some sort of destructive

destructive

kind of interference where we get what

we call an

anti-bonding kind of situation so let's

see how this kind of plays

out let's first think about the 2s

orbitals so as a reminder the only

electrons that are going to

form some sort of bonding interactions

are our valence electrons

so let's think about our 2s electrons

in here so we can have our

our 2s electrons over here so we kind of

know what they are

that's that spherical with that nucleus

and then what we can do is take our wave

function so what happens if we have wave functions

functions

that are the same sign as each other so

if they are

the same sign of each other you can get

some sort of construct of interference

in here so both of these wave characters

are positive you end up getting some

sort of

constructive interference and we're

going to call this

a sigma bond in here

so if we were able to draw the electron

density it's exactly what we see in

valence bond theory

so thinking about our uh bonds and

and the electrons are along the bonds

themselves and we also know that if we

form a bond

we're lowering in energy so this x act

those y-axis

is energy on here and we can actually

match this

up what happens if this one over here

is positive and this one is negative

what happens when we think about that

when we try to bring those together

their wave functions are going to cancel

out from each other

so we're going to form something that's

higher in energy

so we're going to get this kind of

interaction here

where we have positive wave function and

negative wave function

and what happens in here is that we end

up getting

um destructive interference

in here so we end up getting something

the electron cloud that kind of looks

like this

there is no electron density between the

two nuclei

and they are actually a little bit

repulsive each other so they're actually

going to go

higher in energy and we're going to now call

call

this a sigma star and that star means that

that

molecular orbital

and down here we have a sigma that is now

now

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