smiles:CCC(=C=C)C

The one double bond here can take either configuration, and the query did not say which. (E) and (Z) differ in which side of the double bond the substituents sit on, and the bond cannot rotate to interconvert them.

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