Limitations with Lewis Structures - QS Study
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Lewis structures are very useful to predict whether ionic or covalent bonds will form between certain elements; and in what ratio they will combine. VSEPR predicts molecular shape well. However, they assume that pairs of electrons are all localised into bonds. Occasionally the true shape of a molecule (discovered by X-ray crystallography) shows that some electrons must be delocalised over more atoms.

Resonance Structures

Sometimes a molecule or ion cannot be accurately represented by one electron-sharing Lewis diagram. E.g. carbonate ion CO3-.

Two or more resonance structures can be drawn, the true structure being a resonance hybrid (an imagined blending) of the different, non-existent resonance structures. X ray crystallography shows that all three bonds are the same length.

Lewis structures assume that electrons are either entirely located on one atom ( in ionic compounds) or shared equally between two atoms (covalent bonding). Ionic bonding and perfect covalency (perfect sharing) are two extremes. Most compounds are neither perfectly ionic nor perfectly covalent, but somewhere in between. Small highly charged cations (e.g. Mg2+, Al3+) can polarise the electrons in large anions giving covalent nature to some ionic compounds.