Isomerism — Chemistry, 14–17
Some molecules have the same molecular formula but different arrangements of atoms. Those different structures can have different shapes, properties and uses.
Same ingredients, different arrangement
Isomers are different molecules that contain the same numbers of each kind of atom. The atoms are connected or arranged differently, so the molecules may behave differently. A formula tells you what is present, but not always how the structure is built.
Why does structure need its own idea?
A molecular formula alone sometimes predicts too little. Chemists found that compounds with the same formula could have different boiling points, smells or reactions. Isomerism was needed to explain this problem: the order of the atoms matters, not only the atom count.
Two structures for C₄H₁₀
Start with C₄H₁₀: four carbon atoms and ten hydrogen atoms. They can form a straight chain, CH₃–CH₂–CH₂–CH₃, called butane. Or one carbon can sit in a branch, giving CH₃–CH(CH₃)–CH₃, called 2-methylpropane. Both formulas match, but the structures differ.
The formula is not the whole molecule
It is reasonable to think that identical formulas must mean identical substances: in many calculations, the formula is exactly what you need. But C₄H₁₀ can describe two isomers, and their different shapes change how closely molecules pack and how they boil.
Isomers in medicines
A medicine may work because its molecule fits a particular biological target, a little like a key fitting a lock. An isomer with the same atoms can have a different three-dimensional arrangement and fit poorly, or act differently. Drug designers therefore test structure, not just formula.
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