Polythene is chemically bonded monomer units of ethene. The ethene's molecular formula is C2H4, or the two atom of carbon combining with four atoms of hydrogen with molar weight of 12*2+1*4 =28gm/mole.
CH2=CH2 or C2H4 is ethylene or ethene. CH2 is not compound , but coming under naming mythelene. CH4 is methane and C2H6 is called ethane.