1. new unit 8
IMFAs

2. IMFA: intermolecular forces of attraction
“mortar”— holds the separate pieces together
(the IMFA)
“bricks”— individual atoms, ions, or molecules of a solid

3. IMFA: intermolecular forces of attraction

4. types of IMFA covalent network atoms such as C, Si, & Ge ionic bond
strongest
occurs between
covalent network
atoms such as C, Si, & Ge
(when in an extended grid or network)
ionic bond
cations and anions
(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)
van der Waals forces
dipole-dipole attraction
polar molecules
London forces
non-polar molecules
weakest

5. details about each IMFA
strongest
covalent network
ionic bond
metallic bond
hydrogen bond
dipole-dipole attraction
London forces
weakest

6. 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

7. London dispersion forces in action
1. temporary polarization due to any random little disturbance δ+ δ-
2. induced polarization caused by neighboring molecule
3. induced polarization spreads
4. induced polarization reverses
non-polar molecules, initially with uniform charge distribution

8. 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

9. dipole-dipole attractionsδ+ δ-

10. 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

11. hydrogen bonding (or ultra-dipole attractions)
ultra-polar molecule
(much higher boiling point)
hydrogen bonds
(between molecules,
not within them)
non-polar molecules
(lower boiling points)

12. hydrogen bonding (or ultra-dipole attractions)
Beware!! H O H O
These are not hydrogen bonds. They are normal covalent bonds between hydrogen and oxygen. H O H O
These are hydrogen bonds. They are between separate molecules (not within a molecule).

13. metallic bonding structure resulting properties
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
alloy = mixture of metals

14. 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

15. why are salts hard but brittle?
1. apply some force
2. layer breaks off and shifts
4. shifted layer shatters away from rest of crystal
3. + repels +
– repels –

16. 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:
SiC (silicon carbide), BN (boron nitride)

17. buckminsterfullerine
m.p. = 3550°C
C60
buckminsterfullerine
“bucky ball”
m.p. = ~1600°C

18. summary of properties network ionic metallic hydrogen dipole London
strongest
strength
m.p. & b.p.
conductive?
network
extremely hard
very high
usually not
(except graphite)
ionic
hard but brittle
medium to high
if melted or dissolved
(mobile ions)
strong, malleable, ductile
medium to high
metallic
very
(delocalized e–)
hydrogen
van der Waals forces
dipole
soft and brittle
low no
London
weakest

19. 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

20. 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 London or dipole forces, causing greater attraction
strategy to predict m.p. and b.p.
first sort atoms/molecules into the six IMFA categories
then sort those in each category from lightest to heaviest

21. same IMFA: sort by molar mass
melt
boil
ex: halogen family
all are non-polar (London force)
lowest to highest m.p. and b.p. matches lightest to heaviest I2
(257)
+184.4 °C
–250
–200
–150
–100
–50
+50
+100
+150 I2
(257)
+113.7
Br2
(160)
+58.8
thus at room temperature:
F2 (g)
Cℓ2 (g)
Br2 (ℓ)
I2 (s)
Br2
(160)
–7.2
Cℓ2
(71)
–34.04
Cℓ2
(71)
–101.5 F2
(38)
–182.95 F2
(38)
–219.62

22. same mass: sort by IMFA type°C
–50
+50
+100
+150
+198
ethylene glycol
(can form twice as many H-bonds)
ex: organic molecules
all are ~60 g/mol
different types of IMFA
+97.4
1-propanol (ultra-polar = H-bonds)
+56.2
acetone (more polar)
+10.8
methyl ethyl ether (slightly polar)
–0.5
butane (non-polar)
the stronger the IMFA, the higher the boiling point

23. 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

24. soap or emulsifier soaps and emulsifiers oil water
some molecules are not strictly polar or non-polar, but have both characteristics within the same molecule
oil
polar region
soap or emulsifier
non-polar region
water
this kind of molecule can function as a bridge between molecules that otherwise would repel each other

25. soaps and emulsifiers
with a soap or emulsifier present to surround it, a drop of non-polar oil can mix into polar water