This second edition was updated to include some of the recent developments, such as increased-valence structures for 3-electron-3-centre bonding, benzene, electron conduction and reaction mechanisms, spiral chain O4 polymers and recoupled-pair bonding. The author provides qualitative molecular orbital and valence-bond descriptions of the electronic structures for primarily electron-rich molecules, with strong emphasis given to the valence-bond approach that uses increased-valence structures. He describes how long-bond Lewis structures as well as standard Lewis structures are incorporated into increased-valence structures for electron-rich molecules. Increased-valence structures involve more electrons in bonding than do their component Lewis structures, and are used to provide interpretations for molecular electronic structure, bond properties and reactivities. Attention is also given to Pauling 3-electron bonds , which are usually diatomic components of increased-valence structures for electron-rich molecules.