Fluorine is more electronegative than chlorine (Figure 7.4)...

Fluorine is more electronegative than chlorine (Figure 7.4$)$ yet fluoromethane $\left(\mathrm{CH}_{3} \mathrm{F} ; \mu=1.86 \mathrm{D}\right)$ has a smaller dipole moment than chloromethane $\left(\mathrm{CH}_{3} \mathrm{Cl} ; \mu=1.90 \mathrm{D}\right) .$ Explain.

Key Concepts

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. Although fluorine is more electronegative than chlorine, this property alone does not determine the size of a molecule's dipole moment, which is a vector sum of individual bond dipoles.

Bond Dipole and Bond Length

The bond dipole is determined not only by the difference in electronegativity between two bonded atoms but also by the distance between them. In the case of fluoromethane versus chloromethane, the differences in bond lengths cause variations in the magnitude of the individual bond dipoles, which can result in a smaller net dipole moment for fluoromethane despite its highly electronegative fluorine atom.

Molecular Geometry

Molecular geometry influences how individual bond dipoles add up vectorially to produce the overall dipole moment. Even if one bond is very polar, the spatial arrangement of bonds (such as in a tetrahedral geometry) can lead to partial cancellation or enhancement of these dipoles, affecting the net molecular dipole moment.

Polarizability

Polarizability relates to how easily the electron cloud of an atom or group can be distorted. Chlorine, being larger, is more polarizable than fluorine. This greater polarizability can lead to a more pronounced separation of charges in chloromethane, contributing to its slightly larger dipole moment even though chlorine’s electronegativity is lower.

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