A Level Chemistry OCR Practice Exam

What are structural isomers?

1. Molecules with identical structural arrangements

2. Molecules with the same molecular formula and different structural arrangements

3. Molecules that contain only single bonds

4. Molecules that can form double bonds

The correct answer is: Molecules with the same molecular formula and different structural arrangements

Structural isomers are defined by having the same molecular formula but differing in their structural arrangements. This means that while the number and types of atoms in the molecules are identical, the way these atoms are connected or arranged differs, leading to distinct chemical and physical properties. For instance, in butane (C4H10), there are two structural isomers: n-butane and isobutane. They both have the same number of carbon and hydrogen atoms but are structured differently. The other options do not accurately describe structural isomers. Identical structural arrangements would refer to the same compound, not differing ones. Solely containing single bonds or being able to form double bonds pertains to the types of bonds present in a molecule rather than its structural isomerism. Thus, the correct understanding of structural isomers centers on their differing arrangements with equivalent molecular formulas.