Solids and liquids: Bonding, Structure, Properties.
Reading: Chapter 16: (note, we’ll skip the section on x-ray analysis & sections 16.9, 16.10, 16.11).
Solid State Chemistry webpage: seas.upenn.edu/~chem101/sschem/solidstatechem.html
• Major Bond Types:
For an A-B bond the bond type depends on XA-XB = ∆X
X = electronegativity = “the ability of an atom in a bond to attract electrons to itself”.
Represent using “Bond Triangle”
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XA
4.0
1.0
Ionic
one wants e -’s the other
happy to give them away
Metallic
ts )
en m
m ato
ele e
am
(s
both happy to
give away e -s
XB
Type of bond
depends on
(XA-X B)
Covalent
both want e -’s;
“compromise” by sharing
4.0
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Intra-molecular versus inter-molecular forces
Lewis Structures:
description of strong covalent bonds formed between atoms within a
molecule:
INTRA-MOLECULAR bonds
(e.g. each O-H bond within an H2O molecule ~934kJ/mol)
• Intra-molecular bonds are a measure of the stability of a molecule to
decomposition/dissociation.
• For solids and liquids focus on the forces between different molecules/particles:
INTER-MOLECULAR FORCES.
• Stability of “intermolecular forces”: reflected by boiling point (and to some degree
melting point).
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• Strongest Bond Types in Solid/Liquid State:
• Metallic: see later
• Ionic: see later
• Extended covalent (Zumdahl uses the term “Network Atomic Solids”)
We should be clear about the difference between inter-molecular extended covalent
bonds and intra-molecular covalent bonds.
e.g. methane (CH4) and Diamond (C)
CH4 : C(sp3) & H(1s) strong intra- molecular covalent bonds.
Bonding within molecule “terminated” by H atoms
No covalent bonding “left” to interact with other CH4 groups
Diamond: replace all H’s by C’s
Covalent sp3 bonds extend throughout solid.
“Extended covalent”: very high m.pt./b.pt.
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• 1st look at weaker types of inter-molecular forces.
If there are no strong metallic, ionic or extended covalent forces, these are all we have:
then we have a molecular solid.
Three types: (1) dipole-dipole; (2) H-bonding; (3) induced dipole-induced dipole
(London Dispersion Forces)
• (1) Dipole-dipole (5-25 kJ/mol; ~1% strength ionic/covalent)
Present in polar molecules; strength varies with: magnitude of dipole
dipole-dipole separation
Factors affecting magnitude: ∆X of bonds
shape (VSEPR)
e.g.’s compare isoelectronic molecules: CO & N2 (10 VE’s)
N2
CO
non-polar
polar (∆X)
no dipole
dipole
b.pt. –196°C
b.pt. –192°C
SiH4 & PH3 (8 VE’s)
SiH4
PH3
sp3
sp3
tetrahedral
trigonal pyramidal
non-polar
polar
b.pt. –112°C
b.pt. -88°C
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(2) Hydrogen – Bonding (strength 15-40 kJ/mol)
Special case of dipole-dipole bonding: molecules with H bonded to high X atoms:
especially effective for H-N, H-O, H-F bonds
also present in other bonds but much weaker.
H-bonds considerably stronger than other dipole-dipole interactions.
Why so strong ?
(a) bonds highly polar
(b) small size of H
(i) concentrate δ+ charge into small volume
(ii) permit very close approach of oppositely charged end of
dipole on other molecules
(c) unscreened nucleus of H: when draw e-‘s away, only have bare
nucleus (proton)
• H-bonding leads to increases in boiling points:
e.g. consider two C2H6O molecules:
(a) ethanol: CH3CH2OH:
(b) dimethyl ether: CH3OCH3:
Oδ--Hδ+ effective for H-bonding
C-H bonds, no H-bonding
b.pt. 78.3°C
b.pt. –24.8°C
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(3) Instantaneous dipole-instantaneous dipole forces: London Dispersion Forces.
(weak, 2-20kJ/mol)
•
•
•
•
weak interaction present in all molecules.
Instantaneous displacement of electron cloud by neighboring molecule or atom.
Non-permanent dipole; weak and short-lived.
Magnitude depends on ease of distortion of electron orbitals (polarizability)
- typically increases with atomic number and size
- shape also important for close approach (closer = stronger)
These are the only forces present in many non-polar molecules: e.g. Noble gases
B.Pt. (°C)
He
-268.9
Ne
-245.9
Ar
-185.7 Atomic # increases
Kr
-152.3
Xe
-107.1
Ra
-61.8
• By considering what forces are present, you should now be able to rationalize trends
in boiling points.
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