Resonance structures and curly arrows

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

In our brief introduction to Lewis structures (see Section 2.2), we paid particular attention to valency, the number of bonds an atom could make to other atoms via the sharing of electrons. We must now broaden this idea to consider atoms in a molecule that are no longer neutral, but which carry a formal positive or negative charge. This means we are considering cations and anions, as in ionic bonding. 

We can deduce the charge associated with ammonia and water by simply considering that the component atoms are neutral, that all we have done is share the electrons, so the molecules must also be neutral. The formal charge on an individual atom can be assessed more rigorously by subtracting the number of valence electrons assigned to an atom in its bonded state from the number of valence electrons it has as a neutral free atom. Electrons in bonds are considered as shared equally between the atoms, whereas unshared lone pairs are assigned to the atom that possesses them. 

Other systems are less familiar, and will therefore have to be assessed carefully. For example, what charge is associated with the structure shown on the left below  

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