See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Isomerism: Required Similarity **Definition of Isomers:** Isomers are compounds that have the **same molecular formula** but **different structural arrangements**, resulting in different properties. **Analysis of each option:** **(1) Molecular formula** ✓ **CORRECT** - All isomers must have identical molecular compositions (e.g., $C_4H_{10}$) - This is the fundamental requirement that defines isomerism - Without same molecular formula, compounds are different substances entirely **(2) Structure formula** ✗ - Isomers have **different** structures, not the same - Different structures are what make them isomers **(3) Physical formula** ✗ - Not a standard chemistry term - Physical properties of isomers differ (boiling point, melting point, etc.) **(4) Chemical formula** ✗ - This is vague and overlaps with molecular formula - "Chemical formula" alone doesn't specify the same composition requirement **Example:** - $n$-butane: $CH_3-CH_2-CH_2-CH_3$ - $iso$-butane: $CH_3-CH(CH_3)-CH_3$ Both have molecular formula $C_4H_{10}$ but different structures and properties. **Answer: (1) Molecular formula** — isomers must have identical molecular composition.