Structural isomers are molecules that have:

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The same molecular formula but different arrangements of atoms. Structural isomers (also called constitutional isomers) are molecules that have the same molecular formula but different structural arrangements of their atoms. This means they contain the same numbers of each type of atom but the atoms are connected in different ways. For example, butane (CH₃CH₂CH₂CH₃) and isobutane (or 2-methylpropane, (CH₃)₃CH) both have the molecular formula C₄H₁₀, but butane has a straight chain while isobutane has a branched chain.

Having the same structural formula but different molecular formulas is contradictory - if the structure is the same, the molecular formula must also be the same. Having different molecular formulas and different structures describes entirely different compounds, not isomers. Isomers must share the same molecular formula. Identical structures with different physical states would describe the same compound in different phases (solid, liquid, gas), not isomers.

Isomerism is a crucial concept in organic chemistry because isomers can have significantly different physical and chemical properties despite having the same molecular formula. Beyond structural isomers, other types include geometric (cis-trans) isomers, which differ in the spatial arrangement of groups around a double bond or ring, and optical isomers (enantiomers), which are mirror images of each other. Understanding the different types of isomerism and how molecular structure affects properties is important for the CLEP Chemistry exam.

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