1-butene: CH2=CH-CH2-CH3
2-butene: CH3-CH=CH-CH3
These are structural isomers - they have the same molecular formula (C4H8), but different structural arrangements of their atoms.
2-butene has 2 stereoisomers: E 2-butene and Z 2-butene. The E isomer has the two methyl groups on the same side of the double bond, and the Z isomer has the two methyl groups on opposite sides (see the link below to see what I mean). The structural arrangement of the atoms is the same in both stereoisomers, but because double bonds can't rotate, these are distinct molecules, with different physical and chemical properties.
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