Which statement justifies that phosphine (PH3) is a polar molecule?
The Correct Answer and Explanation is:
Correct Statement:
Phosphine (PH₃) is a polar molecule because it has a trigonal pyramidal molecular geometry resulting in an asymmetric distribution of electron density, causing a net dipole moment.
Explanation
Phosphine (PH₃) is a chemical compound made up of one phosphorus atom and three hydrogen atoms. To determine whether PH₃ is polar, we must examine its molecular geometry and the distribution of its electrons.
The central phosphorus atom in PH₃ has five valence electrons. When it bonds with three hydrogen atoms, three of these electrons form covalent bonds, and the remaining two electrons form a lone pair. This lone pair is key in determining the shape of the molecule. Due to the presence of this lone pair, PH₃ adopts a trigonal pyramidal geometry rather than a flat, trigonal planar shape. The lone pair repels the bonding pairs of electrons, pushing the hydrogen atoms downward and creating a three-dimensional, non-symmetrical shape.
Electronegativity also plays a role. Phosphorus and hydrogen have relatively similar electronegativity values, but not identical. The electronegativity difference is small, which means the P–H bonds are only slightly polar. However, because of the molecule’s shape, the bond dipoles do not cancel each other out. Instead, they combine to produce a net dipole moment pointing from the hydrogen atoms toward the lone pair on the phosphorus atom.
The asymmetry caused by the lone pair and the geometry ensures that the charge distribution is uneven. Even though each individual bond is only weakly polar, the shape of the molecule causes the overall dipole to remain. As a result, PH₃ is classified as a polar molecule.
In contrast, a molecule with similar atoms but a symmetrical shape, such as BH₃, would be nonpolar due to the cancellation of dipole moments. Therefore, the polar nature of PH₃ is due to both its molecular geometry and the presence of a lone pair on phosphorus.
