NEET 2020 Chemistry Question 98 Solution

Dipole moment is a vector quantity (magnitude $$\mu = q \times d$$) pointing from the positive to the negative center of charge. A polyatomic molecule has a net zero dipole moment only when its individual bond dipoles cancel each other symmetrically in space.

- $$\text{BF}_3$$: $$sp^2$$ hybridized, trigonal planar geometry ($$120^\circ$$ bond angles). The vector resultant of any two B-F bonds is equal and opposite to the third B-F bond, so $$\mu_{\text{net}} = 0$$. - $$\text{BeF}_2$$: $$sp$$ hybridized, linear ($$180^\circ$$). The two Be-F bond dipoles oppose and cancel, so $$\mu_{\text{net}} = 0$$. - $$\text{CO}_2$$: $$sp$$ hybridized, linear ($$180^\circ$$). The two C=O bond dipoles cancel, so $$\mu_{\text{net}} = 0$$. - 1,4-dichlorobenzene ($$p$$-dichlorobenzene): The two C-Cl dipole vectors are oriented directly opposite each other at $$180^\circ$$ across the aromatic ring, canceling out completely ($$\mu_{\text{net}} = 0$$).

- $$H_2O$$: $$sp^3$$ hybridized bent geometry with two lone pairs, $$\mu = 1.85\text{ D}$$. - $$NH_3$$: Trigonal pyramidal with one lone pair, $$\mu = 1.47\text{ D}$$. - $$NF_3$$: Trigonal pyramidal, $$\mu = 0.23\text{ D}$$. - $$HF$$: Strongly polar linear diatomic, $$\mu = 1.78\text{ D}$$. - 1,3-dichlorobenzene ($$m$$-dichlorobenzene): The two C-Cl dipoles meet at a $$120^\circ$$ angle, giving a substantial net dipole moment ($$\mu \neq 0$$).

Only the fourth set consists exclusively of non-polar molecules with $$\mu = 0$$.