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Ch 5.1 Valence Bond Theory | General Chemistry | YouTubeToText
YouTube Transcript: Ch 5.1 Valence Bond Theory
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everyone so up until now we've been
working with lewis structures
and the problem with lewis structures is
that we're not really told
what a bond actually is so we're going
to work today
on valence bond theory so valence bond
theory is built
as the first theory from quantum
mechanics that actually explains the
origins of where our bonds come from
so these are our two learning outcomes
you should be able to describe how
covalent bonds are formed in terms of
atomic orbital overlap and you should be
able to define
and draw examples of sigma and pi bonds
so if we start to take two hydrogen atoms
atoms
and we start them really far apart and
we move them closer together
so we start moving them together they
will actually
start to form an energy drop so if we measure
measure
energy as we go over distance
and then we start to look at this from
down here we bring them closer and
closer together we'll see that the energy
energy
actually starts to fall and when it
actually falls it actually hits a
well in here where we start to get some overlap
overlap
of our electron density from the two
hydrogen atoms
and if you push them closer together
they start to repel each other the
nuclei repel
each other so as you go here energy goes
in infinite
but we're interested in this energy well
right here
where we actually start to form an
overlap of
atomic orbitals so one of the orbitals
that we can kind of think about
in here in terms of the uh overlap are
sigma box
so this is when we have our two hydrogen atoms
atoms
and when we have these two hydrogen atoms
atoms
we get this overlap that is along
so we normally draw our bond as a line
in between the two nuclei but in the
sigma bond we end up getting lots of
overlap with this bond
so we end up getting something that
looks like this
where we get lots of this electron
density in the middle
and we will normally draw this as just a
line on here so that is an example of an
s to s kind of overlap you can also have
this with an
s and p orbital where we draw the line here
here
uh for the bond and we see overlap of
the p
lobe and the s orbital and we can also
have p
to p overlap in our orbitals
so if we draw our bond on here the two
lobes actually overlap with each other
to form lots of electron density along
the bond itself
then we come along to a pi bond so the pi
pi
bond the electron density is
so if we have our p orbitals we can
actually look at them
so if we have our two atoms on here we
have our two p
orbitals these dumbbell shapes
they can actually start to form overlaps
with each other so you end up getting
something that looks like this we draw our
our
our bond is typically drawn in here
you'll end up getting lots of electron densities
densities
above and below the bonds themselves so
you have this nice little bond
that forms and typically you'll see a
picture like
this in your in your books and you'll
see lots of overlap with these p
orbitals to form this cloud of electrons
above and below the bond itself
so the bond is in here so what can we do
with this we can actually start to count
sigma and pi bonds in our different
lewis structures
so if we start off with just a single
bond and this hydrogen chloride
hcl atom on here and just a single bond
that is always due to a sigma bond
if we look at oxygen we draw the lewis
structure we have a double bond in here
the double bond in here is actually a sigma
sigma
and a pi bond so you have one sigma
and one pi bond
on here and then if we look at nitrogen
which we draw the lewis structure we have
have
a triple bond one of these is a sigma
and then you have two pi bonds so each
of these lines that we're drawing in our
lewis structures
are a sigma and two pi bonds
so this theory actually tells us what
these these bonds are
actually about and we can start to
sketch them out
and what i also like about valence bond
theory is we can start to think about dipoles
dipoles
and you know that you know how we share
our electrons
so if we start off with something like
hcl we already know that the chlorine
side is going to be slightly negative
the proton side is going to be slightly
positive and valence bond theory will actually
actually
help us to draw that most of the
electron density
is going to be around the chlorine so
you have
lots of electrons around the chlorine
which gives it a slightly
negative charge mostly electron density moves
moves
away from the hydrogen so you get a
slightly positive
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