Question

(a) The \(\mathrm{PH}_{3}\) molecule is polar. Does this offer experimental proof that the molecule cannot be planar? Explain. (b) It turns out that ozone, \(\mathrm{O}_{3}\), has a small dipole moment. How is this possible, given that all the atoms are the same?

Short answer

The polarity of the $\mathrm{PH}_{3}$ molecule suggests that it cannot be planar, as planar and symmetric molecules have their individual dipole moments canceling each other, resulting in nonpolar molecules. Ozone ($\mathrm{O}_{3}$) has a small dipole moment despite having the same type of atom due to its bent structure, which causes an unsymmetrical charge distribution within the molecule. The electronic difference between the single and double bonds creates overall molecular polarity, leading to a small dipole moment.

Step-by-step solution

Understanding molecular polarity

Polarity comes from the distribution of electric charge in the molecule. A molecule has a dipole moment if there is an overall charge separation across the atoms in the molecule, creating one side with an overall partial positive charge and another side with a partial negative charge.

Molecule structure and plane

A molecule is considered planar if all its atoms lie in the same plane. In the case of \(\mathrm{PH}_{3}\), the central atom is Phosphorus (P), which is bonded to three Hydrogen (H) atoms. If the molecule is planar, then these atoms should be positioned in one plane.

(a) Polarity and planar structure of \(\mathrm{PH}_{3}\)

The \(\mathrm{PH}_{3}\) molecule is polar, which means there is a net dipole moment. If the molecule were planar and symmetric, the dipole moments of individual bonds would cancel each other out, making the molecule nonpolar. The fact that \(\mathrm{PH}_{3}\) is polar suggests that the molecule cannot be planar because, in a planar configuration, the dipole moments would cancel each other out, and the molecule would be nonpolar.

(b) Ozone (\(\mathrm{O}_{3}\)) molecule's structure and polarity

In the case of ozone, the molecule is composed of three Oxygen atoms. It has a bent structure with a bond angle of approximately 116° and a bond length of about 128 pm. Despite having the same type of atom, ozone has a small dipole moment because of its bent structure. The central Oxygen atom shares a coordinate bond with one neighboring Oxygen atom and forms a double bond with the other Oxygen atom. This creates an unsymmetrical charge distribution within the molecule. The electronic difference between the single and double bonds causes overall molecular polarity, leading to a small dipole moment.