8.10
COMPARISON OF SOLID TYPES
Table 8.4
Covalent and van der Waals radii in Å of
selected nonmetals
H
r
cov
0.32
r
vdw
1.2
C N O F r
cov
0.77 0.75 0.73 0.72
r
vdw
1.70 1.5 1.40 1.35
Si P S Cl
r
cov
1.11 1.06 1.02 0.99
r
vdw
2.10 1.9 1.85 1.80
Ge
As
Se
Br
r
cov
1.22 1.20 1.17 1.14
r
vdw
2.0
2.00
1.95
Table 8.5
A summary of the key properties of the different types of solids.
Type Particles Forces Strength* Examples
Metallic
Atoms
Metallic bonds
Variable
Na, Cu, Ag, Fe
Ionic
Ions
Ionic bonds
Strong
NaCl, NH
Cl 4
Network covalent Atoms
Covalent bonds
Strong
C, ZnS, SiO
2
Molecular
Molecules
Intermolecular
Weak to moderate
H^2
O, sugar, SO
(^2)
- The strength of interaction is indicative of the melting point of the solid and how hard it is because these forces must be overcome to melt the solid or to break it. Example 8.10
Which compound in each pair h
as the higher melting point?
a) MgO or NaF
Both contain a metal and a nonmetal and ar
e therefore ionic. The strength of the
interaction in ionic solids increases with the charge on the ions, so we predict that the interaction of the Mg
2+ ion with the O
2- should be much greater than that between Na
1+^
and F
1-. The melting points are 2800
oC for MgO and 988
oC for NaF.
b) NaCl or Cl
(^2)
NaCl is ionic, but Cl
is molecular. Ionic substances 2
usually have higher melting points
than molecular substances, so NaCl is expected to have the higher melting point. Indeed, NaCl is a solid at room conditions, while Cl
is a gas. 2
c) SiO
or SO 2
(^2)
Neither contains a metal, so it might appear t
hat they are both molecular solids, however,
SiO
(quartz) is a covalent solid with no identifiable SiO 2
molecules. Consequently, 2
covalent bonds must be broken to melt SiO
. SO 2
, on the other hand, is molecular and 2
only relatively weak intermolecular forces must be broken to melt it. SiO
melts at 1700 2
oC, while SO
melts at -73 2
oC.
Chapter 8 Solid Materials
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