Electrons are shared between pairs of nonmetal atoms, each of which has a relatively strong attraction for those electrons.
Covalently bonded materials come in 2 major types--molecular substances and covalent network solids (or covalent crystals).
Molecular substances: Most covalent substances exist as molecules, which are small to medium-sized groups of atoms connected by covalent bonds. Because a molecule is attracted to other molecules in the sample by weaker secondary bonds, most molecular substances exist as gases or liquids at room temperature. The nature of the attraction between molecules depends a great deal on the way that the atoms in the molecule are arranged (i.e., on the molecular shape).
Since the strength of secondary bonds tends to go up with molecular size, larger molecules can exist as solids, but they usually have low melting points and poor mechanical strength. Waxes, polymers (plastics), and many biological tissues are all familiar examples of large molecular substances.