In each group below, which has the least polar bonds?
SiCl4, PCI3, SCI2
NCI3, PCI3, AsCl3
Sketch the structure for KI. Label the partial charges and draw the dipole moment.
The correct answer and explanation is :
Let’s first break down the groups to analyze the polarity of the bonds.
Group 1: SiCl4, PCl3, SCI2
- SiCl4 (Silicon tetrachloride): Silicon (Si) is less electronegative than chlorine (Cl). However, because of the symmetry of the molecule (tetrahedral), the bond dipoles cancel out, and the molecule is nonpolar overall. The bonds, however, are polar because Cl is much more electronegative than Si.
- PCl3 (Phosphorus trichloride): Phosphorus (P) and chlorine (Cl) have a significant difference in electronegativity, making the P-Cl bonds polar. PCl3 has a trigonal pyramidal geometry due to a lone pair on phosphorus, which doesn’t allow the dipoles to cancel out, making the molecule polar overall.
- SCI2 (Sulfur dichloride iodide): Sulfur (S) is less electronegative than chlorine and iodine, and the polarities of the bonds depend on the relative electronegativity of chlorine and iodine. The electronegativity difference between sulfur and iodine is less than between sulfur and chlorine, but since SCI2 has an asymmetric geometry (with iodine being larger), the dipoles do not cancel out, making the molecule polar.
Least Polar Bonds: SiCl4 has the least polar bonds in the group. The bond between silicon and chlorine is polar, but due to the molecule’s symmetry, the overall polarity is canceled out.
Group 2: NCl3, PCl3, AsCl3
- NCl3 (Nitrogen trichloride): The N-Cl bond is polar due to the electronegativity difference between nitrogen and chlorine. The molecule itself is pyramidal, and since it is asymmetrical, it is overall polar.
- PCl3 (Phosphorus trichloride): Like in Group 1, the P-Cl bonds are polar due to the electronegativity difference, and the molecule itself is also pyramidal. It is overall polar.
- AsCl3 (Arsenic trichloride): The As-Cl bonds are polar, and the molecule has a trigonal pyramidal structure, so it is overall polar.
Least Polar Bonds: AsCl3 has the least polar bonds because arsenic (As) is less electronegative than nitrogen (N) and phosphorus (P), meaning the electronegativity difference between As and Cl is smaller than that between N or P and Cl.
Sketch for KI (Potassium iodide)
KI is an ionic compound with potassium (K) and iodine (I). Potassium has a low electronegativity, while iodine is much more electronegative. The bond between K and I is ionic rather than covalent, so the electrons are not shared but are transferred from K to I. In this case, iodine becomes negatively charged (I⁻), and potassium becomes positively charged (K⁺). This creates a dipole moment that points from the positive K⁺ to the negative I⁻, indicating the direction of the electron flow.