new unit 8
IMFAs
IMFA: intermolecular forces of
attraction
“mortar”—
holds the
separate
pieces
together
(the IMFA)
“bricks”— individual atoms, ions, or molecules of a solid
IMFA: intermolecular forces of
attraction
types of IMFA
strongest
covalent network
occurs between
atoms such as C, Si, & Ge
(when in an extended grid or network
ionic bond
cations and anions
van der Waals forces
(metals with non-metals in a salt)
metallic bond
metal atoms
hydrogen bond
ultra-polar molecules
(those with H–F, H–O, or H–N bonds)
weakest
dipole-dipole attraction
polar molecules
London forces
non-polar molecules
details about each IMFA
strongest
covalent network
ionic bond
metallic bond
hydrogen bond
dipole-dipole attraction
London forces
weakest
London (or dispersion) forces
 non-polar molecules (or single atoms)
normally have no distinct + or – poles
 how can they attract each other enough to
condense or freeze?
 they form temporary dipoles
 electron clouds are slightly distorted by
neighboring molecules
sort of like water sloshing in a shallow pan
London dispersion forces in action
δ+
δ-
1. temporary
polarization
due to any
random little
disturbance
2. induced
polarization
caused by
neighboring
molecule
3. induced
polarization
spreads
4. induced
polarization
non-polar molecules, initially with uniform charge distribution
reverses
dipole-dipole attractions
 polar molecules have permanent dipoles
 the molecules’ partial charges (δ+, δ-) attract the
oppositely-charged parts of neighboring
molecules
 this produces stronger attraction than the
temporary polarization of London forces
therefore polar molecules are more likely to be liquid at
a temperature where similar non-polar molecules are
gases
dipole-dipole attractions
δ+
δ-
hydrogen bonding (or ultra-dipole attractions)
 H—F, H—O, and H—N bonds are more polar
than other similar bonds
these atoms are very small, particularly H
F, O, and N are the three most electronegative
elements
these bonds therefore are particularly polar
 molecules containing these bonds have much
higher m.p. and b.p than otherwise expected for
non-polar or polar molecules of similar mass
 the geological and biological systems of earth
would be completely different if water molecules
did not H-bond to each other
hydrogen bonding (or ultra-dipole attractions)
ultra-polar molecule
(much higher boiling
point)
non-polar molecules
(lower boiling points)
hydrogen bonds
(between molecules,
not within them)
hydrogen bonding (or ultra-dipole attractions)
Beware!!
H
H
H
O
H
H
O
H
O
H
H
O
These are not
hydrogen bonds. They
are normal covalent
bonds between
hydrogen and oxygen.
These are hydrogen
bonds. They are
between separate
molecules (not within a
molecule).
metallic bonding
 structure
nuclei arranged in a regular grid or
matrix
“sea of electrons”—delocalized valence
electrons free to move throughout grid
metallic “bond” is stronger than van der
Waals attractions but generally is
weaker than covalent bond since there
are not specific e– pairs forming bonds
 resulting properties
shiny surface
conductive (electrically and thermally)
strong, malleable, and ductile
ionic bonding (salts)
 structure: orderly 3-D array (crystal)
of alternating + and – charges
 made of
cations (metals from left side of periodic table)
anions (non-metals from right side of periodic
table)
 properties
hard but brittle (why?)
non-conductive when solid
conductive when melted or dissolved
why are salts hard but brittle?
1. apply some force
2. layer breaks off and shifts
3. + repels +
– repels –
4. shifted layer
shatters away
from rest of
crystal
covalent networks
 strong covalent bonds hold together millions of
atoms (or more) in a single strong particle
 properties
very hard, very strong
very high melting temperatures
usually non-conductive (except graphite)
 examples
carbon (two allotropes: diamond, graphite)
pure silicon or pure germanium
SiO2 (quartz or sand)
other synthetic combinations averaging 4 e– per
atom:
m.p. = 3550°C
m.p. = ~1600°C
C60
buckminsterfullerine
“bucky ball”
summary of properties
strongest
network
strength
extremely hard
m.p. & b.p.
very high
conductive?
usually not
(except graphite)
ionic
hard but brittle
medium to high if melted or dissolved
(mobile ions)
van der Waals forces
metallic
strong,
malleable,
ductile
medium to high very
soft and brittle
low
(delocalized e–)
hydrogen
dipole
London
weakest
no
consequences of IMFAs
 melting points and boiling points rise with
strength of IMFA
increasing molar mass
 substances generally mix best with other
substances having the same or similar IMFAs
”like dissolves like”
non-polar mixes well with non-polar
polar mixes well with polar
(polar also mixes well with ultra-polar and ionic)
 other physical properties such as strength,
conductivity, etc. are related to the type of IMFA
predicting melting points, boiling points
 stronger IMFAs cause higher m.p. and higher
b.p.
when atoms/ions/molecules are more strongly
attracted to each other, temperature must be raised
higher to overcome the greater attraction
 more polar molecules have higher m.p. and
b.p.
 atoms and molecules that are heavier and/or
larger generally have higher m.p. and higher
b.p.
larger/heavier atoms (higher molar mass) have more
e–
larger e– clouds can be distorted (polarized) more by
same IMFA: sort by molar mass
melt
boil
+184.4
(257)
+150
I2
+113.7
(257)
+100
+58.8
–7.2
(160)
Cℓ2
0
–50
–101.5
(71)
–150
(38)
–219.62
–34.04
Cℓ2
(71)
–100
–200
F2
Br2
(160)
+50
Br2
I2
–250
°
C
–182.95
F2
(38)
 ex: halogen family
 all are non-polar (London
force)
 lowest to highest m.p. and
lightest to
 b.p.
thus matches
at room temperature:
heaviest
 F2 (g)
 Cℓ2 (g)
 Br2 (ℓ)
 I2 (s)
same mass: sort by IMFA type
+97.4
 ex: organic
(can form twice as many H-bonds)
molecules
 all are ~60 g/mol
 different types of
1-propanol (ultra-polar = H-bonds) IMFA
+56.2
acetone (more polar)
+10.8
methyl ethyl ether (slightly polar)
butane (non-polar)
+198
+150
+100
+50
0
–50
°
C
–0.5
ethylene glycol
 the stronger the
IMFA, the higher the
boiling point
isomers (and an isobar)
butane and 2-methylpropane
glycerol and 1-propanol
n- and neo pentane
1-propanol and 2-propanol
1-propanol and methyl ethyl ketone
soaps and emulsifiers
some molecules are not strictly polar or non-polar, but
have both characteristics within the same molecule
oil
polar
region
water
this kind of molecule can function
as a bridge between molecules
that otherwise would repel each
soaps and emulsifiers
with a soap or emulsifier present to surround it, a drop of
non-polar oil can mix into polar water