This second edition was first published in 1959. It presents a compact elementary account of classical and modern conceptions of valency.This second edition was first published in 1959. The first four chapters present a compact elementary account of classical and modern conceptions of valency, suitable for the more general reader and for students preparing for university examinations. The concluding chapters are more advanced: chapter five shows how the electronic theory is applied to the heavier elements and their principal compounds; and the last chapter is devoted to current developments, and theories still in their early stages.This second edition was first published in 1959. The first four chapters present a compact elementary account of classical and modern conceptions of valency, suitable for the more general reader and for students preparing for university examinations. The concluding chapters are more advanced: chapter five shows how the electronic theory is applied to the heavier elements and their principal compounds; and the last chapter is devoted to current developments, and theories still in their early stages.This second edition was first published in 1959. The first four chapters present a compact elementary account of classical and modern conceptions of valency, suitable for the more general reader and for students preparing for university examinations. The concluding chapters are more advanced: chapter five shows how the electronic theory is applied to the heavier elements and their principal compounds; and the last chapter is devoted to current developments, and theories still in their early stages.1. Historical introduction; 2. Development of the doctrine of valency before the rise of electronic theories; 3. Methods for determining structure and valency; 4. The electronic spin theory of valency; 5. The spin theory applied to the heavier elements; 6. Special topics.