Question

Explain why HCI has a smaller dipole moment than HF,even though the H -CI bond is longer than the H-F bond.

Short answer

HF has a larger electronegativity difference compared to that of HCI and thus it has a smaller dipole moment despite its longer bond length.

Step-by-step solution

Understanding the Dipole Moment and its relation with bond length and electronegativity

The dipole moment is a vector quantity measured in terms of electronegativity. It is the measurement of separation of two opposite charges.

With the increase in electronegativity, the difference in dipole moment of a molecule increases. With the increase in bond length, the dipole moment of a molecule increases.

Comparing HF and HCI

Fluorine is more electronegative than chlorine thus, the electronegativity difference of HF is more compared to HCI .

On the other hand, the bond length is greater in the case of H-CI compared to H-F.

Bond length ∝ 1/△EN

Here, △EN is the electronegativity difference between the two atoms. The higher difference in electronegativity overcomes the bond length factor thus giving the H-F molecule a greater dipole moment than HCl.