Page 101 - Physical chemistry understanding our chemical world

68 INTRODUCING INTERACTIONS AND BONDS

Table 2.6 Typical properties covalent compounds
Property Example

Low melting point Ice melts in the mouth
Low boiling point Molecular nitrogen is a gas at room
temperature
Physically soft We use petroleum jelly as a lubricant
Malleable, not brittle Butter is easily spread on a piece of bread
Low electrical conductivity We insulate electrical cables with plastic a
Dissolve in non-polar solvents We remove grease with methylated spirit b
Insoluble in polar solvents Polyurethane paint protects the window frame
from rain
a The polythene coating on an electrical wire comprises a long-chain alkane.
b ‘Methylated spirit’ is the industrial name for a mixture of ethanol and methanol.

Figure 2.13 Diamond has a giant macroscopic structure in which
each atom is held in a rigid three-dimensional array. Other covalent
solids include silica and other p-block oxides such as Al 2 O 3

to one or two others to form a linear chain; the hydrogen atoms are bound to this
backbone, again with covalent bonds. But the wax is a solid because dispersion
forces ‘glue’ together the molecules. Table 2.6 lists some of the common properties
of covalent compounds.
Finally, macromolecular covalent solids are unusual in comprising atoms held
together in a gigantic three-dimensional array of bonds. Diamond and silica are
the simplest examples; see Figure 2.13. Giant macroscopic structures are always
solid.

Aside

The word covalent was coined in 1919 when the great American Chemist Irving Lang-
muir said, ‘it is proposed to define valence as the number of pairs of electrons which
a given atom shares with others. In view of the fact ... that ‘valence’ is very often
used to express something quite different, it is recommended that the word covalence
be used to denote valence defined as above.’ He added, ‘In [ionic] sodium chloride, the
covalence of both sodium and chlorine is zero’.
The modern definition from IUPAC says, ‘A covalent bond is a region of rela-
tively high electron density between nuclei which arises (at least partially) from shar-
ing of electrons, and gives rise to an attractive force and characteristic inter-nuclear
distance’.