The theory of valency was developed in the 19th century. It determines the ability of an element to combine with other elements, and is dictated by the number of electrons in the outermost shell of an atom. Each atom therefore can combine with a definite and characteristic number of other atoms, of the same or other elements, to form molecules. Carbon was determined to have a valency of 4. This explained the structure of many organic molecules, where carbon atoms bonded with each other in long chains, but failed to account for benzene (C6H6). These models represent early attempts to solve this problem, which was resolved in 1865 when Friedrich August Kekule determined that benzene had a ring structure.
© Science Museum / Science & Society Picture Library