ABSTRACT
According to standard valence bond theory,1 the essential components of a molecular structure are the atoms and bonds composing the molecule. Two atomic properties are required-the atomic symbol or atomic number and the formal charge on the atom. Other atomic properties might be useful in many applications but are not considered as essential for the discussion in this chapter. The two atoms participating in a bond define the bond. A bond type, namely single, double, or triple, completes the definition of a bond. It is frequently convenient to use the notion of aromaticity2 to further classify bonds and even the atoms composing that bond. The concept of a hydrogen bond is not considered here.
