and melting temperatures for several metals are listed in Table 2.3. Bonding may
be weak or strong; energies range from 68 kJ/mol (0.7 eV/atom) for mercury to
850 kJ/mol (8.8 eV/atom) for tungsten. Their respective melting temperatures are
and .
Metallic bonding is found in the periodic table for Group IA and IIA elements
and, in fact, for all elemental metals.
Some general behaviors of the various material types (i.e., metals, ceramics,
polymers) may be explained by bonding type. For example, metals are good conductors
of both electricity and heat, as a consequence of their free electrons (see
Sections 18.5, 18.6 and 19.4). By way of contrast, ionically and covalently bonded
materials are typically electrical and thermal insulators, due to the absence of large
numbers of free electrons.
Furthermore, in Section 7.4 we note that at room temperature, most metals and
their alloys fail in a ductile manner; that is, fracture occurs after the materials have
experienced significant degrees of permanent deformation. This behavior is explained
in terms of deformation mechanism (Section 7.2), which is implicitly related
to the characteristics of the metallic bond. Conversely, at room temperature ionically
bonded materials are intrinsically brittle as a consequence of the electrically
charged nature of their component ions (see Section 12.10).