The VSEPR model allows us to determine with great accuracy the geometry of a molecule. Lewis's theory allows us to understand how a covalent bond is formed through the pairing of electrons. However, we cannot with these theories explain the lengths and energies of the bonds formed. Thus, an HH bond is very strong and short (436.4 kJ/mol and 74 pm), compared to the much weaker and long FF bond (150.6 kJ/mol and 142 pm).

To explain these properties it is necessary to use theories based on quantum mechanics, such as the valence bond theory, which considers electrons in atomic orbitals, which when overlapping form bonds. The characteristics of these bonds depend on the type of orbitals that overlap (size and geometry), which explains the great difference between the properties of HH and FF bonds.