Why are some molecules polar and others nonpolar?

polar:

water - H2O.

ammonia - NH3.

sulfur dioxide - SO2.

hydrogen sulfide - H2S.

ethanol - C2H6O.

Non polar:

any of the noble gases: He, Ne, Ar, Kr, Xe.

any of the homonuclear diatomic elements: H2, N2, O2, Cl2.

carbon dioxide - CO2.

benzene - C6H6.

carbon tetrachloride - CCl4.

methane - CH4.

ethylene - C2H4.

hydrocarbon liquids, such as gasoline and toluene.

Polar molecules: water - H2O and ammonia - NH3

Nonpolar molecules: carbon dioxide - CO2 and benzene - C6H6.