104.Resonance(1)

From this post onwards, we shall start discussing a phenomenon, which manifests itself in many compounds and which is responsible for deviations in structure, properties and reaction mechanisms of organic and inorganic species.

Many a times , it is not possible to define structure of a molecule using just one Lewis structure. There are many contributing structures which together can completely explain the structure of that species.Resonance is the description of these structures and their connection to each other. Thus,

Resonance is the description of the electronic structure of a molecule by means of several schemes of pairing electrons, with features of each scheme contributing in the description.

In simple words, this means that a species(molecule / ion) can have many resonating structures, each contributing to the overall structure of the molecule.Thus, in species where a single Lewis structure is inadequate to describe the species totally , there are many resonating structures, which together make that species.

e.g. What is the structure of Ozone (O3)?

1041

The above structure suggests that ozone has one single and one double bond.However, when the bond lengths are calculated experimentally , the value for both bonds is the same and the bond length value lies intermediate between a single and double bond!

Single bond length for O – O →  1.48Aº
Double bond length for O=O → 1.21 Aº
Bond length in ozone → 1.28 Aº 

This observation suggests that, both bonds are similar and show character between single and double bond.This can be shown by drawing two resonating structures for ozone as follows –

1042.jpg

As seen above,

resonating structure  1 → a single bond between first and second oxygen and a double bond between second and third oxygen atoms.

resonating structure  2 → a double bond between first and second oxygen and a single bond between second and third oxygen atoms.

Both these structures contribute to the overall structure of ozone ,which can be shown as a hybrid structure , as follows –

1043.jpg
Hybrid structure of Ozone.

  • Resonance should be pictured as blending of structures , not as flickering alterations between them. In quantum mechanical terms, for a molecule with n resonating structures,

Ψ(molecule) = Ψ(resonating structure 1) + Ψ(resonating structure 2) + …….. +                                               Ψ(resonating structure n).

  • Resonance has two main effects-
  • 1)It averages the bond characteristics over a molecule.
    2)Energy of resonance hybrid structure is lower than any of the single contributing structures.


     

FORMAL CHARGE

Before to proceed to draw resonating structures for molecules , we must know how to calculate formal charges(FC) on atoms in a molecule. Ions either have a positive or negative charge. Neutral species carry zero charge.Knowing which atoms in a polyatomic species carries what charge can help us understand the charge distribution in that species. We know that the valence electrons do not belong to only one atom specifically and these electrons are not always shared equally. The distribution of electrons depends on the electronegativity of atoms in the species. A more electronegative element will pull the electrons more towards itself than a less electronegative element. Calculating formal charges help us to exactly know-

i)which atom carries what charge
ii)how much charge.

The formula for calculating FC is –

FC = VE – LPE – (BE/2)
where ,

FC ⇒ Formal Charge.
VE ⇒ Valence Electrons.
LPE ⇒ Lone Pair Electrons.
BE ⇒ Bonding Electrons.

Let us understand this concept with some examples.

  1. Methane (CH4) – First draw the dot structure of the methane. How to draw dot structures? Read post # 53.0.jpgThus, the formal charge on carbon in methane is zero i.e the molecule is neutral.
  2. Ammonium ion (NH4+) – In this ion, there are 5 electrons on N atom and no lone pairs. So the Formal charge on the central N atom can be calculated as follows –

0.jpg

 

We will continue our discussion on resonance in the next post too. Till then ,

  • Be a perpetual student of life and keep learning…Good day !

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