Intermolecular Forces and
Polarity
Intra – versus Inter –
• Remember that
INTRAmolecular
forces are the
covalent bonds which
hold a molecule
together, and
INTERmolecular
forces are the
attractive forces that
hold different
molecules together.
Intermolecular Forces (IMF)
• The attractive force a molecule can exert is
important in understanding what that
molecule can mix with.
• As well, knowing the strength of the IMFs
acting on a molecule helps us understand
melting/boiling points
Polarity
• Polar Molecules = molecules
created due to an unequal
sharing of electrons
• This unequal sharing creates a
DIPOLE, a molecule with partial
positive and a partial negative
end
• Since positives and negatives
are naturally attracted to one
another, dipoles exert an
attractive force on each other.
Polarity and Lewis Structures
• From a Lewis Structure it is quite easy to
identify a polar molecule, it will either have
– Lone pairs on the central atom
– One of the atoms bonded to the central atom will
be different than the others
– Both
Polarity and Mixing
• The ability for one
compound to mix with
another is based on
whether the two
compounds can physically
connect
• Polar molecules can only
mix with other polar (or
ionic) molecules.
• Polar molecules CANNOT
mix with nonpolar
molecules
Polarity and Phase Changes
• Recall that states of matter are described by
the arrangement of particles
• Changing from one state to another involves
changing the particle arrangement
• The ease at which this occurs is based upon
the force of attraction between the individual
particles
Polarity and Phase Changes
• Polar molecules require more energy to
change state than nonpolar molecules
• But remember, ionic compounds will always
melt/boil at higher temperatures than
molecules
Types of Intermolecular Forces
• All molecules are first divided into Polar and
Nonpolar
• Nonpolar molecules only exhibit LONDON
DISPERSION forces (van der Waals)
• Polar molecules can either exhibit normal
DIPOLE-DIPOLE interactions or in some cases
the stronger force of HYDROGEN-BONDING
London Dispersion
• This is the weakest IMF
• It is only present in nonpolar molecules
• It’s created by the momentary grouping of
electrons on one side of the electron cloud
Dipole-Dipole
• All polar molecules are dipoles
• The partial positive end of one dipole is
attracted to the partial negative end of
another
Hydrogen Bonding
• This IMF occurs in the special case when
HYDROGEN is bonded to NITROGEN, OXYGEN,
or FLUORINE