Butene has two isomers„one with methyl groups on the same side of the double bond and the other with methyl groups on the opposite side. what kind of isomers are these?

 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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