Question

Which molecule is nonpolar? (a) \(\mathrm{SO}_{3} ;\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\); (c) \(\mathrm{NH}_{3} ;\) (d) FNO.

Short answer

Among the given molecules, only SO3 is nonpolar.

Step-by-step solution

Examine SO3

SO3 molecule has molecular geometry of trigonal planar and is symmetrical, which means the electron pull of each S-O bond in different directions by the same amount creating balance. Therefore, no permanent dipole would form, rendering the molecule nonpolar.

Examine CH2Cl2

Despite the molecule being symmetrical, C-H and C-Cl bonds are polar due to electronegativity difference between C and H and C and Cl, resulting in a net dipole, hence CH2Cl2 is polar.

Examine NH3

NH3 has a ‘pyramidal’ shape that means it is not symmetrical and electron pulls will not cancel each other. This shows it to be polar.

Examine FNO

The FNO molecule is not symmetrical due to presence of a Lone pair of electrons. Therefore, it is polar because the symmetrical shape criterion is not met.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The spatial configuration determines many properties, including how the molecule interacts with other molecules, and whether it is polar or nonpolar. For a molecule to be nonpolar, its geometry must ensure that the electron pair distribution is even.

• An example of symmetrical geometry is the trigonal planar shape that \( \text{SO}_3 \) exhibits. Each angle between the atoms is 120°, allowing the forces to balance out.
• On the other hand, \( \text{NH}_3 \) with its pyramidal shape is asymmetrical. The angles here differ because of the lone pair of electrons on nitrogen, leading to uneven electron distribution.

The symmetry in geometry ultimately decides whether individual bond dipoles cancel each other out. If they do, then the molecule is nonpolar.

Electronegativity

Electronegativity is the ability of an atom to attract bonding electrons towards itself. It's an essential factor when determining if a bond between two atoms is polar or nonpolar,

• Molecules such as \( \text{CH}_2\text{Cl}_2 \) demonstrate the impact of differing electronegativity values. Chlorine is much more electronegative than carbon and hydrogen, resulting in significant dipole formation.
• Conversely, \( \text{SO}_3 \) consists of bonds between sulfur and oxygen where the electronegativity difference isn't as impactful due to the molecule's symmetry.

Higher electronegativity differences between atoms within a molecule can cause polar bonds if the molecule's geometry doesn't cancel these dipoles. Thus, for a molecule like \( \text{FNO} \) which has uneven distribution due to lone pairs, its bonds show noticeable polar characteristics.

Dipole Moment

The dipole moment of a molecule measures the separation of charges within it. It's depicted as a vector quantity, indicating the direction and magnitude of charge separation. To be nonpolar, a molecule must have a net dipole moment of zero.

• For \( \text{SO}_3 \), despite having polar bonds, the molecular geometry ensures that the dipoles cancel. This results in a net dipole moment of zero.
• In molecules like \( \text{CH}_2\text{Cl}_2 \) and \( \text{NH}_3 \), the asymmetrical structures cause the individual bond dipoles to add up rather than cancel, resulting in a net dipole moment.
• The FNO molecule's lack of symmetry due to a lone pair ensures it has a significant dipole moment as well.

A non-zero dipole moment indicates that the molecule is polar. Understanding dipole moments helps in predicting molecular interactions and behavior in different environments.