An alkene is an unsaturated hydrocarbon. Unsaturated means that it contains at least one double bond and hence, does not have maximum number of hydrogen atoms.  

General formula for alkenes: CnH₂n
If (n = 3), the compound would be:
C₃H₂₍₃₎ = C₃H₆

Every alkene’s name ends in ‘-ene’.

An alkene’s prefix always changes according the number of carbon atoms present in the compound. 

There cannot be an alkene with only 1 carbon atom. There needs to be a minimum of two carbon atoms for there to be a double bond present. So there is no alkene named methene. 

Name of AlkeneNo. of Carbon atomsFormula
Ethene2C₂H₄
Propene3C₃H₆
Butene4C₄H₈
Pentene5C₅H₁₀
Hexene6C₆H₁₂
Heptene7C₇H₁₄
Octene8C₈H₁₆
Nonene9C₉H₁₈
Decene10C₁₀H₂₀