Overlap and directional character of the covalent bond

The overall structure of the molecule is a resonance hybrid of the structures I, II, and III. The quantum-mechanical description is a linear combination, 

The coefficients of ij/i and i/ n the composite description are equal, indicating that these two contribute equally to the structure. The coefficient of differs from the other two, indicating that i/ ni contributes differently. The contributions of the three structures in HCl are estimated to have the values I, 26 % II, 26 % III, 48 %. The structures I and II are covalent structures, so we may say that the bond in HCl is 52 % covalent and 48 % ionic. A bond in which the ionic contribution is significant is called a covalent bond with partial ionic character. 

Every covalent bond has more or less ionic character. Even if the two atoms are the same, there is a small contribution of ionic structures, 3% in H2. The bond between two like atoms is usually called a pure covalent bond, nonetheless. 

There are restrictions on the structures that can contribute to the composite structure of a molecule. The structures that can resonate to produce a composite structure must (1) have the same number of unpaired electrons and (2) have the same arrangement of nuclei. For resonance to be effective, the structures should not differ greatly in energy. 

To form a covalent bond two things are needed a pair of electrons with spins opposed, and a stable orbital, an orbital in the valence shell, on each atom. The strength of a bond is qualitatively proportional to the extent of overlap of the charge clouds on the two atoms. The overlap integral 5 is a measure of the overlap of two charge clouds  

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