The alkenes are straight-chain hydrocarbons with one double bond per molecule

Alkenes, CnH2n
     
ethene C2H4 CH2=CH2
propene C3H6 CH2=CHCH3
but-1-ene C4H8 CH2=CHCH2CH3
pent-1-ene C5H10 CH2=CHCH2CH2CH3
hex-1-ene C6H12 CH2=CHCH2CH2CH2CH3
hept-1-ene C7H14 CH2=CHCH2CH2CH2CH2CH3
oct-1-ene C8H16 CH2=CHCH2CH2CH2CH2CH2CH3
non-1-ene C9H18 CH2=CHCH2CH2CH2CH2CH2CH2CH3
dec-1-ene C10H20 CH2=CHCH2CH2CH2CH2CH2CH2CH2CH3

Sometimes long chain formulas are abbreviated
e.g. CH2=CHCH2CH2CH2CH2CH2CH3 may be written CH2=CH(CH2)5CH3