The electrical conductivity of a substance is a measure of the ease with which the valence electrons move throughout its structure, and thus is dictated by its bonding. Metallic bonding produces the greatest conductivity, as it involves a lattice of positively charged nuclei, with electrons free to move throughout the lattice (Science Daily, 2010). Thus, when an electrical charge is applied to the metal, the electrons are able to easily move through it and therefore it can be said to be a good conductor. Substances bound by covalent bonding, on the other hand, are usually poor conductors (insulators) as the electrons are tightly held within the covalent bonds.
There are some exceptions. For example, the covalent molecular substance graphite. Graphite is a pure carbon substance, where three of its valence electrons are covalently bonded to three other carbon atoms, forming a layered structure. However, the fourth valence electron is left unbonded, and thus is able to move freely. These valence electrons allow the flow of electricity through the substance in certain directions when an electrical current is applied to graphite.
The “lead” in pencil is, contrary to its name, predominately made up of a combination of graphite and clay, with wax and other additives in small quantities. Clay, unlike graphite, is an insulator: that is it does not conduct electricity well, due to the covalent bonds holding valence electrons tightly in place. The shade of pencil is dependent on percentage of each component. Pencils range from 9H, with 41% graphite and 53% clay, to 9B, with 93% graphite and 1% clay (Everything2 Media, 2012).