Which of the following molecules is non-polar in nature ?

To determine which of the given molecules is non-polar, we need to analyze their chemical structures, molecular geometries, and individual bond dipoles. A molecule is non-polar if the vector sum of its individual bond dipoles is zero, meaning the dipoles cancel each other out.

Let us evaluate each option step-by-step:

1. POCl₃ (Phosphorus oxychloride):
The central phosphorus atom is bonded to one oxygen atom via a double bond and to three chlorine atoms via single bonds. The geometry is tetrahedral. Since oxygen is more electronegative than phosphorus, there is a net dipole moment along the P=O bond. Similarly, the P-Cl bonds are polar. Because the surrounding atoms are not identical (one oxygen and three chlorines), the individual bond dipoles do not cancel each other out, resulting in a net dipole moment ($\mu \ne 0$). Thus, POCl₃ is polar.

2. CH₂O (Formaldehyde):
The central carbon atom is bonded to two hydrogen atoms and double-bonded to one oxygen atom. The molecular geometry is trigonal planar. Oxygen is highly electronegative compared to carbon and hydrogen, creating a significant dipole moment directed toward the oxygen atom. The asymmetry of the C-H and C=O bonds prevents the dipoles from canceling, giving the molecule a net dipole moment ($\mu \ne 0$). Thus, CH₂O is polar.

3. SbCl₅ (Antimony pentachloride):
The central antimony (Sb) atom is in group 15 and has 5 valence electrons. In SbCl₅, antimony shares all 5 valence electrons to form single bonds with 5 chlorine atoms, leaving no lone pairs on the central atom.
According to VSEPR theory, a molecule with 5 bonding pairs and 0 lone pairs adopts a trigonal bipyramidal geometry.
In this symmetric geometry:
- Three equatorial Sb-Cl bonds lie in a single plane at 120° angles to one another. Their dipoles cancel each other out in the plane (${\mu }_{\text{equatorial}}=0$).
- Two axial Sb-Cl bonds point in opposite directions along a vertical axis (180° to each other). Their dipoles are equal in magnitude and opposite in direction, so they also cancel each other out (${\mu }_{\text{axial}}=0$).
Consequently, the net dipole moment of SbCl₅ is zero ($\mu =0$), making it a non-polar molecule.

4. NO₂ (Nitrogen dioxide):
Nitrogen dioxide is a bent molecule with an odd number of valence electrons (one unpaired electron on the nitrogen atom). The bent geometry ensures that the polar N-O bond dipoles do not cancel each other out, resulting in a net dipole moment ($\mu \ne 0$). Thus, NO₂ is polar.

Therefore, the only non-polar molecule among the choices is antimony pentachloride (SbCl₅) due to its highly symmetrical trigonal bipyramidal geometry.