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How can chemical isomers have the same molecular formula but different physical and chemical properties?
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isomerism
Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)
Chemical isomers possess the same molecular formula but exhibit different physical and chemical properties because their atoms are organized into distinct structures. While they contain the same number and types of atoms, the specific way these atoms are bonded or oriented in space dictates the molecule's unique characteristics.123
Isomers are generally categorized into two types based on how their structures differ:
- Constitutional Isomers: These molecules have different connectivity, meaning the atoms are linked in a different order. For example, ethyl alcohol and methyl ether share the same molecular formula but differ in how their atoms are bonded, resulting in different chemical behaviors. Similarly, butane and isobutane differ by their straight-chain versus branched-chain structures, which leads to significant differences in their boiling and freezing points.123
- Stereoisomers: In these molecules, the connectivity is identical, but the atoms are oriented differently in space. A common example is enantiomers, which are mirror images of each other, much like a pair of hands. While their physical and chemical properties are often identical, they differ in their interaction with polarized light, a property known as optical rotation.14
Ultimately, because a molecule's properties are fundamentally linked to its spatial distribution and structural arrangement, even minor variations in atomic configuration result in distinct substances.23