The formation and function of molecules depend on chemical bonding between atoms. Atoms with incomplete valance shells can interact with certain other atoms so the partner completes the shell: either share or transfer valance electrons. Covalent bond: the sharing of a pair of valence electrons by two atoms. Molecule: two or more atoms held together by covalent bonds. Single bond: a single covalent bond; the sharing of a pair of valence electrons by two atoms. Double bond: a double covalent bond; the sharing of two pairs of valence electrons by two atoms. Valence: the bonding capacity of a given atom; usually equals the number of unpaired electrons required to complete the atom"s outermost (valence) shell. Nonpolar covalent bond: a type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity. Polar covalent bond: a covalent bond between atoms that differ in electronegativity.