Structural isomers are best described as

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Structural isomers are molecules that share the same chemical formula but differ in how the atoms are connected to each other. This difference in connectivity creates distinct skeletal arrangements, such as a straight-chain structure, a branched-chain structure, or a cyclic (ring) structure. That variety in how atoms are linked is the hallmark of structural (constitutional) isomers. The idea that chains can be straight, branched, or rings illustrates this concept of different connectivity. If connectivity were identical, you’d have the same molecule; different atomic weights would mean different elements or isotopes, not just a rearrangement of the same formula.