9.
Choose the molecule that has polar bonds but does NOT orient within an electric field.
a. HCl
b. Cl2
c. CCl4
d. NH3
CCl4 contains polar bonds but remains nonpolar overall, preventing alignment in an electric field.
Option Analysis
a. HCl features a polar H-Cl bond due to chlorine’s higher electronegativity (3.16 vs hydrogen’s 2.2), creating a permanent dipole moment of ~1.03 D. This diatomic molecule orients toward the positive plate in an electric field. Incorrect; it aligns due to net polarity.
b. Cl2 has nonpolar Cl-Cl bonds since both atoms share identical electronegativities, yielding zero dipole moment. Lacks polar bonds entirely, failing the first condition. Incorrect.
c. CCl4 possesses polar C-Cl bonds (chlorine electronegativity 3.16 > carbon 2.55), but its tetrahedral symmetry causes individual dipoles to cancel, resulting in zero net dipole. Does not orient in an electric field despite polar bonds. Correct answer.
d. NH3 exhibits polar N-H bonds and a lone pair on nitrogen, forming a trigonal pyramidal shape with net dipole ~1.47 D. Orients in electric fields. Incorrect.
Key Concept: Bond vs Molecular Polarity
Polar bonds arise from electronegativity differences (>0.4), but molecular polarity requires a net dipole. Symmetric molecules like CCl4 vector-sum to zero despite polar bonds, unlike asymmetric HCl or NH3. Nonpolar molecules (net dipole = 0) show no torque in uniform electric fields; polar ones align positive end toward negative plate.
CSIR NET Relevance
This tests VSEPR geometry and dipole cancellation, common in chemical bonding units. Visualize CCl4’s 109.5° bond angles ensuring perfect symmetry.
