Question

What is the distinction between a bond dipole and a molecular dipole moment?

Short answer

A bond dipole is the electric dipole moment within a single bond, resulting from the difference in electronegativity between two bonded atoms, while a molecular dipole moment is the overall dipole moment of the entire molecule, obtained as the vector sum of all individual bond dipoles. Bond dipoles exist due to charge separation within a bond, while molecular dipole moments depend on both the bond dipoles and the molecular geometry, which may result in a net zero molecular dipole moment even when individual bond dipoles are present.

Step-by-step solution

Definition of bond dipole

A bond dipole is created when two atoms in a molecule have a difference in electronegativity. The more electronegative atom attracts the electrons in the bond, creating a partial negative charge, denoted as "δ-", while the less electronegative atom has a partial positive charge, represented by "δ+". This charge separation leads to the formation of an electric dipole, with the dipole moment \(\vec{\mu}\) pointing from the positive to the negative charge. It is represented as: \[ \vec{\mu} = \delta q \cdot \vec{d} \] Where \(\delta q\) is the magnitude of the partial charge and \(\vec{d}\) is the vector distance between the atoms.

Definition of molecular dipole moment

The molecular dipole moment is the vector sum of all bond dipoles in the molecule. In other words, it is the net dipole moment of the entire molecule, considering the geometry of the molecule and the distribution of partial charges. It is calculated as: \[ \vec{\mu}_{molecule} = \sum_{i} \vec{\mu}_{i} \] Where \(\vec{\mu}_{i}\) represents each bond dipole in the molecule.

Distinction between bond dipole and molecular dipole moment

Bond dipole refers to the electric dipole moment that exists within a single bond, resulting from the difference in electronegativity between the two bonded atoms. In contrast, the molecular dipole moment is the overall dipole moment of the entire molecule, which is the vector sum of all individual bond dipoles. A molecular dipole moment may end up being null if the bond dipoles present in the molecule cancel each other out due to the molecular geometry, even if individual bond dipoles exist.