let's classify the types of intermolecular forces so we're going to define intermolecular forces
here as non covalent and non ionic interactions that add up to govern many properties of matter
including boiling point and vapor pressure melting point surface tension adhesion and cohesion so if
you had something where the adhesive forces to a graduated cylinder or stronger than the cohesive
forces then what you're going to have is a meniscus that sticks to the wall so adhesion
winds and in that same thought if you had cohesive forces that are stronger than adhesion you're
going to have a meniscus that does like this so like mercury versus water mercury is going to bend
up its cohesion stronger than adhesion water would have a dip down because it has adhesion stronger
than cohesion we always measure from the top or bottom of the meniscus on a graduated cylinder
on viscosity the resistive flow all these properties are governed by the intermolecular
forces and non covalent interactions and so let's talk about the types dipole-dipole is
our first type here it requires two permanent dipoles so we need differing electronegativity
between two atoms two polar molecules so for example if we have a Cl in the gas phase not
to be confused with HCl aqueous which is an acid hydrochloric acid is HCl aqueous we're
talking about hydrogen chloride gas so this is a molecular compound well if we look at the bond
chlorine is more electronegative than hydrogen so it's going to hog more than its fair share of
electrons okay and so it's more exaggerated than I so let's try there more than its fair share of
electrons on one side of the molecule so what that does is it forms a partial negative charge
to one side and a partial positive charge to the other this is lower case Delta and it just means
partial okay so a partial positive partial negative that's a dipole whenever you have
two atoms of differing electronegativity electronegativity increases as you move up
here's my crude periodic table here up and to the right of the periodic table towards
fluorine we increase in electronegativity ok so chlorine is more electronegative
than hydrogen and so that dipole that's a covalent bond that's a polar covalent bond
okay but there's an interaction that occurs between two HCl's where two dipoles come
into play here the interaction that we get between positive and negatively charged ends
of a polar molecule that's the dipole dipole IMF molecular force represented here it's weaker than
a covalent bond much weaker than a covalent bond but it's still powerful when it adds up
hydrogen bonding is a special case of polar covalent bonds where we get a
greater electrostatic attraction than even you would expect from polarity alone and so what
we have here if we look at water oxygen is more electronegative than hydrogen so we're going to
develop polarity here and the OH NH and FH bonds exhibit this behavior so both of these bonds would
be a hydrogen bond potential candidate and so what happens is you arrange these atoms
and molecules where their minus charges near up to their positive counterparts and the polar
molecules and you get these interactions so I can even draw one more here where we have a hydrogen
Delta plus talking to the Delta minus of
our oxygen atom and so those non covalent interaction a full bond I'm just showing
them as partial attractions here okay they're really electrostatic attraction they add up to
something very powerful so I've shown three different hydrogen bond intermolecular force
interactions here between hydrogen and oxygen of an OH bond you've got to be an OH bond or
an NH bond and then the actual intermolecular force that occurs is between molecules it's not
within the same molecule the covalent bond OH is not the intermolecular force it's the
partial positive and partial negative charges attracting each other on between molecules
a dipole induced dipole interaction is when you have a permanent dipole that you induce
a charge in a non-polar molecule or atom so nonpolar um textbook definition is the
electronegativity difference so what's the electronegativity difference is less than
0.4 less than or equal to 0.4 or less than 0.5 you'll see from some places CH falls into
that category and anything bonded to itself has an electronegativity difference of 0 so
these are considered to be nonpolar ok and anything greater than that point 5 or point
4 um threshold is considered polar so if we take our HCl gas molecule and bring it near a
non-polar oxygen molecule not showing the lone pairs here but there there don't forget about
the lone cares this is before before hand we're going to have a delta plus a permanent dipole
on the polar molecule and we're going to have no interaction the electron cloud is
perfectly symmetrical on that two oxygens bonded to themselves that O2 molecule okay
and so no charge we can then induce this is not a reactionary I'm just saying we and this
is like time let's go over time bring these together bring the HCl closer to the oxygen
molecule and over time you induce a dipole to occur because of the dipole that you brought
nearby so what's happened here is you started with an even distribution of the electrons but because
it's negative charge is here you kind of push away the electron density from one side of the oxygen
molecule to the other the nonpolar molecule and create a delta plus Delta minus that is temporary
or induced cancer we can see the whole thing here temporary or induced dipole and so that
attraction then occurs and that is be induced dipole intermolecular force is the permanent
dipole to the induced dipole a little bit weaker than a dipole-dipole interaction
in most cases okay and then finally we have induced dipole induced dipole where you have
two nonpolar molecules they interact to produce temporary dipoles when they're near each other
these are called dispersion forces and greater surface area means greater dispersion forces
are induced dipole induced dipole so the one gives the other so if we look at the halogens
fluorine is a gas nonpolar not very strong IMS chlorine we would consider to be a gas bromine
we consider to be a liquid but then iodine we consider to be a solid okay so what makes
iodine a solid bromine liquid they're stronger intermolecular forces because what you're doing
if you're looking at the fluorine to fluorine molecule every time you go down the periodic
table you're getting bigger and bigger and bigger so your surface area is increasing now all of
these are supposed to be nonpolar okay so iodine through iodine is coming together now go back to
a smaller scale for room purposes here before they induce dipoles they're both nonpolar no
charge this is before they come together but after two molecules that are nonpolar come
together temporarily they can induce a dipole in one another where this sides now a little more
electrons than it should have has a little more electron density this side has a little bit less
this side a little bit more and so then you get that electrostatic attraction between the two
molecules that constitutes the temporary induced dipole induced dipole interaction
and when your lobes of your molecule are big enough you're said to be highly polarizable
highly polarizable okay so big molecules are highly rode and big atoms are highly
polarizable and they're very um they create very strong induced dipole induced dipole interactions