Which is the most polar?
a) PF3 b) PH3 c) PI3 d) PBr3 e) PCl3
The Correct Answer and Explanation is:
The most polar molecule among the options is e) PCl3 (phosphorus trichloride).
Explanation:
To determine which molecule is the most polar, we must consider the electronegativity differences between the atoms involved, the molecular shape, and how these factors contribute to the overall dipole moment of the molecule.
- Electronegativity:
The polarity of a molecule arises from the difference in electronegativity between the atoms involved. Electronegativity is a measure of an atom’s ability to attract bonding electrons. The larger the electronegativity difference between the atoms in a bond, the more polar the bond will be. In the molecules listed, the electronegativities of the atoms are as follows:
- Phosphorus (P): 2.19
- Fluorine (F): 3.98
- Chlorine (Cl): 3.16
- Bromine (Br): 2.96
- Iodine (I): 2.66
- Hydrogen (H): 2.20
- Analysis of Each Molecule:
- PF3 (Phosphorus trifluoride): The electronegativity difference between P (2.19) and F (3.98) is large, suggesting a highly polar bond. However, the molecule has a trigonal pyramidal shape, which leads to a net dipole moment, but the large size and electronegativity difference of F relative to P result in less symmetry in the molecular geometry, reducing the net polarity.
- PH3 (Phosphine): The electronegativity difference between P (2.19) and H (2.20) is very small, leading to weakly polar bonds. The trigonal pyramidal shape does create some dipole moment, but the minimal difference in electronegativity results in a molecule that is almost nonpolar.
- PI3 (Phosphorus triiodide): The electronegativity difference between P (2.19) and I (2.66) is smaller than with F or Cl. Although PI3 has a trigonal pyramidal shape, the smaller electronegativity difference makes the molecule less polar compared to others.
- PBr3 (Phosphorus tribromide): The electronegativity difference between P (2.19) and Br (2.96) is moderate, leading to a trigonal pyramidal structure with a noticeable dipole moment, but not as large as the PCl3 molecule.
- PCl3 (Phosphorus trichloride): The electronegativity difference between P (2.19) and Cl (3.16) is significant, but not as extreme as in PF3. The trigonal pyramidal shape leads to a net dipole moment, and the substantial electronegativity difference between P and Cl results in a highly polar molecule.
Conclusion:
While PF3 has very electronegative F atoms, the net polarity of PCl3 is higher because of the favorable balance of electronegativity differences and molecular shape. The chlorine atoms, while not as electronegative as fluorine, still contribute to a significant dipole moment, making PCl3 the most polar molecule in the given list.