Intermolecular Bonds
Four Types
The Strongest – IONIC BONDS
The formation of a crystal lattice is due
to ionic bonds BETWEEN different
(already built) ionic molecules.
I.
Ionic bonds are BY FAR
the strongest
intermolecular bonds as
they carry the same
strength that they do as
intramolecular bonds.
II.
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--
--
The Weakest – LONDON FORCES
weakest type of intermolecular bonds
occur between ALL molecules, but only
important between molecules who
possess non-polar covalent
intramolecular bonds.
other intermolecular forces overshadow
London Forces in molecules made up of
ionic and polar covalent bonds.
electrons possessed by the atoms of one
molecule are attracted to the protons
in the nuclei of neighbouring
molecules.
Electrons from one noble gas
atom are attracted to the
protons in another…
 The
attraction, however, is short-lived
due to kinetic energy overcoming the
weak London force.
 The more protons/electrons that a
molecule/atom has, the stronger the
London Forces and henceforth, the higher
the melting and boiling points.
 Exist mainly in noble gases and non-polar
covalent compounds: F2, Cl2, Br2, I2, H2,
N2, O2, He, Ne, Ar, Kr, Xe, Rn.
The 3rd Strongest – DIPOLE-DIPOLE Forces
-- Dipole – a partial charge separation that exists
when one end of an intramolecular bond is
more electronegative than the other (polar
covalent bond) resulting in partial positive
and partial negative ends.
-- the partial negative end from one molecule is
attracted to the partial positive end from
another molecule, and vice versa, creating a
dipole-dipole force.
-- Dipole-dipole forces are stronger than London
Forces, so molecules exhibiting them tend to
have higher melting/boiling points.
III.
DOTTED LINES
ARE USED TO
INDICATE DIPOLEDIPOLE FORCES
(BONDS)
The 2nd Strongest – HYDROGEN
BONDS
-- a special type of dipole-dipole force
(stronger) that only occurs in
molecules containing H-N, H-O, and/or
H-F bonds.
-- hydrogen bonded compounds exhibit
higher melting/boiling points than
dipole-dipole compounds.
Eg. Melting point for water = 0 degrees C
Melting point for CO2 = -55.6 deg. C
IV.
Again, dotted lines are used to
indicate hydrogen bonds…
Deciphering Which Intermolecular
Bonds Exist in Certain Compounds
Plan of Action:
1.
Using the electronegativity table, decide
which intramolecular bond exists within
your molecule.
2.
If the intramolecular bond is:
a. Ionic…then the intermolecular bonds
are also ionic (crystal lattice).
b. Non-polar covalent…then the inter are
London Forces.
c. Polar covalent…then the inter are
Dipole-Dipole Forces, unless the
intramolecular bonds are between H-O H-F
and/or H-N, in which case the inter will be
Hydrogen Bonds.
Try these:
a.
b.
c.
d.
HCl
Intra:
Inter:
I2
Intra:
Inter:
NH3
Intra:
Inter:
NaBr
Intra:
Inter:
Polar Covalent
Dipole-Dipole
Non-polar Covalent
London Forces
Polar Covalent
Hydrogen
Ionic
Ionic