Chemistry - A Molecular Science

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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