A colourless flammable gaseous hydrocarbon, C2H4; m.p. –169°C; b.p. –103.7°C. It is the first member of the alkene series of hydrocarbons. It is made by cracking hydrocarbons from petroleum and is now a major raw material for making other organic chemicals (e.g. ethanal, ethanol, ethane-1,2-diol). It can be polymerized to polyethene. It occurs naturally in plants, in which it acts as a growth substance promoting the ripening of fruits.