Just as early chemists were beginning to understand the nature of chemical compounds, a new twist complicated the picture. They found that the molecular formula for two different compounds can be the same. Because the atoms are connected in different ways, a single formula can represent more than one compound, each with its own distinct physical and chemical properties. For example, a molecule containing 6 atoms of carbon and 12 of hydrogen (C₆ H ₁₂) can form a number of hydrocarbons, such as cyclohexane and hexene-1, each of which behaves quite differently.

This concept, called isomerism, was first noted by French chemist Joseph Gay-Lussac in 1814. He raised the issue again 10 years later, when he was editing a scientific journal and noticed that two papers used the same formula to refer to different chemicals, yet both were correct. Gay-Lussac concluded that it was the arrangement of a compound's particles that determined the...