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On the Pauling electronegativity scale, fluorine and chlorine are 3.98 and 3.16, respectively.

Since the dipole moment is dependent on electronegativity, why is the dipole moment of chloromethane larger than the dipole moment of fluoromethane?

I guess that the smaller size of the s orbital in the fluorine atom has something to do with this.

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Dipole moment is not just about charges, it also has $L$ term. Bond length of $\ce{C-Cl}$ is greater than $\ce{C-F}$ and in this case, that is more dominating factor.

The dipole moment is in order

$$\ce{CH3Cl} \gt \ce{CH3F} \gt \ce{CH3Br} \gt \ce{CH3I}$$

You can see that electronegativity plays a more dominating role in $\ce{CH3X}$ when $\ce{X}$ is $\ce{Br}$/$\ce{I}$.

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