Complex sentence: It is based on methane, except that two of the four hydrogen atoms have been replaced by fluorine atoms, hence the formula is CH 2 F 2 instead of CH 4 for normal methane.
Split into simpler sentences: It is based on methane, except that two of the four hydrogen atoms have been replaced by fluorine atoms. Hence the formula is CH 2 F 2 instead of CH 4 for normal methane.