Covalent crystals are an extreme example of a large molecule--they are highly-ordered 3-dimensional arrangements of trillions of trillions of atoms connected by covalent bonds. These materials have high melting points and good mechanical strength. The most familiar examples are diamond and graphite, both composed of purely carbon atoms (just connected in slightly different geometries). Other examples include silicon, quartz (silicon dioxide), and silicon carbide; however, because silicon is actually a metalloidrather than a nonmetal, these latter materials are entering the "grey area" between the different material and bond types.
Metallic: Metal + Metal
The valence electrons are easily dislodged from all of the atoms in the sample, and behave as a "sea of fluid electrons" surrounding positively charged metal cores. Most metals are solids at room temperature, and they exist as highly-ordered 3-dimensional arrangements of vast numbers of atoms (i.e., as metallic crystals). The strength of metallic bonding varies significantly between different metal elements,
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and therefore melting points and mechanical strength also vary. Since the valence electrons are held loosely in all metallic solids, they are good conductors of electricity and heat, and they can be bent or shaped without breaking.
