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5.4a MO Theory Intro | General Chemistry | YouTubeToText
YouTube Transcript: 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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