which of the following tetrahedral molecules is polar? please explain

Question

CH4
CF4
CH3F

thanks

Answer 1

CH4 is not.
CF4 is not
CH3F is.

All three molecules are tetrahedral shapes. The C-H bonds are slightly polar BUT the tetrahedral shape is symmetrical and the individual bond polarities cancel so the overall polarity is zero for the molecule. Same argument for CF4. However, for CH3F that C-F bond doesn't cancel out the resultant CH3 bonds so the molecule is polar. Said another way the CH3F molecule is not symmetrical. It's the same kind of thing, but easier to see, that the O=C=O molecule has polar bonds but the molecule is not polar because the two polar bonds cancel each other. But O=C=S would be polar because the O=C doesn't quite cancel the C=S bond.