Question

Identify the correct reason for zero dipole moment.
(A) The molecule may be homoatomic.
(B) There may be a dative bond in the molecule.
(C) The molecule may be heteroatomic and symmetric.
(D) There may be lone pairs in the molecule.

Answer 1

Hint: Recollect the concept of polarity and dipole moment. Read the options carefully and try to write examples of each type of molecule mentioned in the options. Assign the charges on atoms present in each molecule and see in which molecule it is impossible to assign charges.

Complete step by step solution:
- Polarity is the property of an object to possess opposite physical properties at different points. Physical properties can be magnetic properties or electrical charge.
- A molecule is said to be polar if one of the atom or group of atoms has the tendency to attract electrons towards itself (electronegative) thereby, getting a complete or partial negative charge on itself and a complete or partial positive charge will be generated on the atom or group of atoms to which the former is bonded to. For example, HCl or water.
- A bond dipole moment uses the concept of electrical dipole moment in order to measure the extent of polarity of a chemical bond between two atoms of a molecule.
- A molecule is said to have zero dipole moment if the opposite dipole moments cancel each other or there are no partial charges present on the opposite ends of a covalent bond.
- In a homoatomic molecules like ${{H}_{2}}$, ${{O}_{2}}$, etc., there is symmetry. Since the atoms present in the molecule are the same, there is no dipole moment at all as the bond between the two atoms is non-polar. So, a homoatomic molecule always has zero dipole moment.
- A molecule having dative bond is a coordination compound. A dative bond is a coordinate bond formed when the ligand donates its lone pair to the central metal atom forming a bond. Here since ligand is donating electrons, it will get partial positive sign and metal will get partial negative sign. Therefore, this molecule will have a dipole moment in general.
- For a heteroatomic and symmetric molecule like carbon dioxide, oxygen is electronegative so it will pull electrons towards itself and therefore have a partial negative sign. Carbon-dioxide is a linear molecule having symmetry. So, since both oxygen atoms are pulling electrons they will have partial negative signs and carbon will get double partial positive signs. But since the structure is linear $O=C=O$, the negative dipole moments are equal and opposite to each other and so they cancel each other. Therefore, a heteroatomic molecule having symmetry will have zero dipole moment.
- If there are lone pairs in the molecule then also, there will be presence of dipole moment like in case of water molecule or ammonia or HCl.
- Therefore, the molecules having zero dipole moment are homoatomic and heteroatomic molecules having symmetry.

- Therefore, the answer is option (A) and (C).

Note: Remember a molecule is said to have zero dipole moment if the dipole moments are equal and opposite and cancel each other in case of heteroatomic symmetric molecules or if the covalent bond present in the molecule is not polarized at all as in case of homoatomic molecules.