# Summary of session 4

1. In Lewis structures, each covalent bond is represented by a shared electron pair. Double bonds require two shared pairs; a triple bond requires three. These allocations often leave some atoms with non-bonded electron pairs.
2. Generally this operation provides each atom with a noble gas shell structure, especially if dative bonds in which both electrons are contributed by one atom are introduced. But in some cases, such as PF5, it does not.
3. Sometimes the bond lengths in a molecule are such that it cannot be represented by a single Lewis structure or structural formula. It is better described as a resonance hybrid-an average or superposition of two or more structural formulae called 'resonance structures'.

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OpenLearn - Discovering chemistry
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