Lewis structures

   Lewis presented a simple, but useful, method of describing the arrangement of valence electrons in molecules. The approach uses dots (or dots and crosses) to represent the number of valence electrons, and the nuclei are indicated by appropriate elemental symbols. A basic premise of the theory is that electrons in a molecule should be paired; the presence of a single (odd) electron indicates that the species is a radical. Diagram 1.1 shows the Lewis structure for H₂O with the O-H bonds designated by pairs of dots (electrons); an alternative representation is given in structure 1.2 where each line stands for one pair of electrons, i.e. a single covalent bond. Pairs of valence electrons which are not involved in bonding are lone pairs.

  The Lewis structure for N₂ shows that the N-N bond is composed of three pairs of electrons and is a triple bond (structures 1.3 and 1.4). Each N atom has one lone pair of electrons. The Lewis structures 1.5 and 1.6 for O₂ indicate the presence of a double bond, with each O atom bearing two lone pairs of electrons.

  Lewis structures give the connectivity of an atom in a molecule, the bond order and the number of lone pairs and these may be used to derive structures using the valenceshell electron-pair repulsion model .