smiles:CC1=CCCC1

Same atoms, joined differently. Every one of these is a separate compound with its own boiling point, smell and reactions. These are the ones PubChem has on file, which for a larger formula is fewer than the number that could exist on paper.

• Cyclohexene
• 1,5-Hexadiene
• 1,4-Hexadiene
• 2-Hexyne
• 1-Hexyne
• 1,3-Pentadiene, 2-methyl-
• 2,4-Hexadiene
• 3,3-Dimethyl-1-butyne
• 2,3-Dimethyl-1,3-butadiene
• 3-Hexyne
• 1,3-Hexadiene
• 3-Methylcyclopentene
• 1-Methylcyclopentene
• Cyclopentane, methylene-
• 2-Methyl-1,4-pentadiene
• 4-Methyl-1,3-pentadiene
• 1,4-Pentadiene, 3-methyl-
• 3-Methylene-1-pentene
• Isopropenylcyclopropane
• 3-Methyl-1-pentyne
• 4-Methyl-1-pentyne
• 4-Methyl-2-pentyne
• (3E)-3-methylpenta-1,3-diene
• Bicyclo(3.1.0)hexane
• 4-Methylcyclopentene
• 3-Methyl-1,2-pentadiene
• Bicyclo[2.1.1]hexane
• 1,2-Hexadiene
• Ethylidenecyclobutane
• 2,3-Pentadiene, 2-methyl-
• Cyclopropane, (1-methylethylidene)-
• Bicyclopropyl
• 1,2-Pentadiene, 4-methyl-
• trans-1-Propenylcyclopropane
• Cyclopropane, 1-ethenyl-1-methyl-
• 2,3-Hexadiene
• Spirohexane
• Cyclobutane, ethenyl-
• 1,3,3-Trimethylcyclopropene
• Cyclopropane, 1,1-dimethyl-2-methylene-
• 2-Propen-1-ylcyclopropane
• Cyclobutene, 3,3-dimethyl-
• 1-Methyl-3-methylidenecyclobutane
• 2-Ladderane
• 1,3-Dimethylbicyclo(1.1.0)butane
• Cyclobutene, 1,2-dimethyl-
• cis-2-Methyl-1-vinylcyclopropane
• Trans-3,4-dimethylcyclobutene