Q.Linear form of carbon dioxide molecule has two polar bonds, yet the molecule has zero dipole moment. Why?

When a covalent bond forms between two atoms of DIFFERENT electronegativity, the more electronegative atom pulls the shared electron pair more strongly toward itself, distorting the electron cloud and creating partial charges — δ+ on the less electronegative atom, δ− on the more electronegative one. Such a bond is called polar covalent (as opposed to the purely non-polar covalent bond found between two identical atoms, e.g. H₂, where the shared pair sits exactly midway). Hydrogen fluoride is the standard example: electronegativities H = 2.1, F = 4.0, so fluorine pulls the shared pair roughly twice as strongly, giving Hδ+—Fδ−.

Dipole moment quantifies this polarity: $\mu = q \times 2d$, the product of the (partial) charge magnitude and the distance separating the two charge centres. It is a VECTOR, conventionally drawn pointing from the positive end of the bond toward the negative end. Its SI unit is the coulomb-metre, but it is usually expressed in the practical unit Debye (D), where 1 D = 3.336 × 10⁻³⁰ C m. Homonuclear diatomics (H₂, O₂, F₂) have zero dipole moment (nonpolar); heteronuclear diatomics (HF, HCl, CO, NO) have non-zero dipole moments (polar). …