Archibald Scott Couper in Paris in 1857 or 1858. Image provided by Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.		Molecular Structure While John Dalton had distinguished compounds on the basis of their molecular composition, the existence of isomers—compounds with identical formulae but different properties—led to the idea of molecular structure. This idea originated with Friedrich Wöhler and Justus von Liebig (1803–1873) and was developed and articulated independently by Archibald Scott Couper (1831–1892) and August Kekulé von Stradonitz (1829–1896). In addition to knowing what atoms made up a molecule and how many, it was now necessary to know just how those atoms were arranged in space and how they were connected. Central to Couper’s and Kekulé’s ideas was the concept of valence—that each kind of atom bonds to a specific number of other atoms.