Resonance structures refer to the structures that occur when writing two or more electron-dot formulas that contain an equal number of electron pairs for the molecule or the ion. Consider the 2 electron dot structure of ozone. The structure on LHS can be converted to the one on the RHS by shifting electron pairs without changing the positions of the oxygen atoms. From the above illustration, this electron-dot structure clearly shows that the bonding in ozone consists of one single coordinate covalent bond and one double covalent bond. Double covalent bonds are usually shorter than single bonds and therefore it is true to conclude that the bonds in ozone are not equal. However, results from experiments show that the bond length is the same in ozone.

Exceptions to the octet rule

The octet rule provides guidance for writing electron dot structure. For some molecules or ions, however, it is impossible to write structures that satisfy the octet rule. This occurs whenever the total number of valence electrons are in an odd number. The nitrogen oxide molecule, for instance, contains a total of seventeen valence electrons. Each oxygen contributes 6 electrons and nitrogen contributes 5.

The two plausible resonance structures can be written for the NO2 molecule as shown below.

An unpaired electron is present in each of these structures, both of which fail to follow the octet rule. It is impossible to write a Lewis dot structure for NO2 that satisfies the octet rule for all atoms. Yet, NO2 does exist as a stable molecule. The concept of electron pairing is vital since it helps us understand the process of bonding and entire properties of molecules. If we consider electrons as small and spinning electric charges, they will generate the magnetic fields as much as current in an electric motor.  In chemistry, substances in which all of the electrons are paired are diamagnetic in nature. In contrast, paramagnetic substances are those that contain one or more unpaired electrons. Such substances show a strong attraction to an external magnetic field. Oxygen is paramagnetic and the measured distance between the oxygen atoms in oxygen molecule shows that the molecule does have some multiple bond character. The idea that the oxygen molecule has some double-bond character is plausible.




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