Question

how do you determine which compound would have the smallest dipole moment in the gas phase?

Answer 1

The dipole moment of a compound in gas phase depends on the following factors:

1. Electronegativity difference: Higher the electronegative difference between two bonded atoms, higher is the dipole moment. Ionic compounds having the highest elecetronegativity difference between atoms,have the highest dipole moment followed by covalent molecules.

If the electronegativity difference is 0, then the molecule has 0 dipole moment. e.g: Cl2, O2, Br2 etc. have zero diple moment.

2. Direction of dipole moment: If there are more than 2 elements in a compound, then each pair of bonded atoms have different value of dipole moment. In this the dipole moment of the compound is the vector sum of the dipole moment of each pair.

e.g: in NH3, the dipole moment of the whole molecule is the vector sum of the diple moment of 3 N-H bonds and the lone pair. This is equal to 1.42 D.

3: Symmetry of molecule: In case of symmeric molecules the dipole moment of each pair cancel each other making the resultant dipole moment equals to 0. e.g: CCl4, CO2 etc.

Hence symmetric compounds and molecules/compounds made of same atoms have smallest dipole moment which is equal to 0.