Network covalent solids vary from insulating to semiconducting in their behavior, depending on the band gap of the material.
Among the ionic solids are compounds formed by alkali and alkaline earth metals in combination with halogens; a classic example is table salt, sodium chloride.
Metals can be strengthened by introducing crystal defects (for example, by alloying) that interfere with the motion of dislocations that mediate plastic deformation.
[4] The Si–O bonds in quartz, for example, are polar yet largely covalent, and are considered to be of mixed character.
The example given, paraffin wax, is a member of a family of hydrocarbon molecules of differing chain lengths, with high-density polyethylene at the long-chain end of the series.
High-density polyethylene can be a strong material: when the hydrocarbon chains are well aligned, the resulting fibers rival the strength of steel.
With cross-linking, however, polymer networks can become continuous, and a series of materials spans the range from Cross-linked polyethylene, to rigid thermosetting resins, to hydrogen-rich amorphous solids, to vitreous carbon, diamond-like carbons, and ultimately to diamond itself.
For example, sodium stearate (the main constituent of traditional soaps) consists entirely of ions, yet it is a soft material quite unlike a typical ionic solid.
The charged components that make up ionic solids cannot exist in the high-density sea of delocalized electrons characteristic of strong metallic bonding.