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Electronegativity, Fajan’s rule and dipole moment: NEET MCQs with answers

90 questions on this topic of Chemical Bonding and Molecular Structure (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Consider the following two statements regarding molecular dipole moments: Statement I: Carbon dioxide (CO₂) and sulfur dioxide (SO₂) both contain polar bonds and have zero dipole moment. Statement II: The net dipole moment of a molecule depends on both the individual bond polarities and the molecular geometry. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
Show answer and explanation

Answer: (C) Statement I is incorrect but Statement II is correct.

Statement I is incorrect because SO₂ is a bent (V-shaped) molecule with a permanent non-zero dipole moment (~1.63 D), although CO₂ is linear with μ = 0. Statement II is correct because the net dipole moment is the vector sum of individual bond moments, which depends on spatial arrangement (geometry).

medium

2. Which among the following molecules has a dipole moment equal to zero?

  1. XeF₄
  2. SF₄
  3. ClF₃
  4. PCl₃
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Answer: (A) XeF₄

XeF₄ has square planar geometry (sp³d² hybridisation with two axial lone pairs opposing each other). The individual bond moments and lone pair moments cancel out completely, giving a net dipole moment of zero. SF₄, ClF₃, and PCl₃ are all asymmetrical with non-zero dipole moments.

hard

3. The experimentally measured dipole moment of a molecule MX₂ is 1.5 D. If the individual M-X bond dipole is 1.5 D, the bond angle (X-M-X) in the molecule is:

  1. 60°
  2. 90°
  3. 180°
  4. 120°
Show answer and explanation

Answer: (D) 120°

The resultant dipole moment is given by μ_net = 2μ cos(θ/2). Here, 1.5 = 2(1.5) cos(θ/2), which gives cos(θ/2) = 0.5. Thus θ/2 = 60°, and θ = 120°.

medium

4. Which among the following alkaline earth metal carbonates undergoes thermal decomposition at the lowest temperature?

  1. BaCO₃
  2. SrCO₃
  3. CaCO₃
  4. BeCO₃
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Answer: (D) BeCO₃

According to Fajan's rules, Be²⁺ has the smallest ionic size and therefore the highest polarizing power among the group 2 cations. It strongly polarizes the carbonate anion, weakening the C-O bond and making BeCO₃ thermally unstable, decomposing at the lowest temperature.

easy

5. According to the Pauling scale, the electronegativity difference (Δχ) between two atoms A and B is related to the resonance energy Δ (in kJ mol⁻¹) by the relation:

  1. χ_A - χ_B = 0.102 × √Δ
  2. χ_A - χ_B = 0.208 × √Δ
  3. χ_A - χ_B = 0.059 × √Δ
  4. χ_A - χ_B = 0.182 × Δ
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Answer: (A) χ_A - χ_B = 0.102 × √Δ

When bond energies are expressed in kJ mol⁻¹, the formula for the Pauling electronegativity scale is χ_A - χ_B = 0.102 × √Δ. If energies are in kcal mol⁻¹, the coefficient is 0.208.

hard

6. Given the dipole moments of ortho-, meta-, and para-dichlorobenzene, which of the following sequences correctly represents their values in decreasing order?

  1. ortho > meta > para
  2. para > meta > ortho
  3. ortho > para > meta
  4. meta > ortho > para
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Answer: (A) ortho > meta > para

The resultant dipole moment of disubstituted benzenes depends on the angle θ between the two C-Cl bond dipoles: μ_net = √(μ₁² + μ₂² + 2μ₁μ₂cosθ). For ortho, θ = 60° (highest μ); for meta, θ = 120° (intermediate μ); for para, θ = 180° (cos 180° = -1, yielding μ = 0). Thus, the order is ortho > meta > para.

easy

7. According to Fajan's rules, which of the following halides is expected to have the lowest melting point?

  1. CaF₂
  2. CaI₂
  3. CaBr₂
  4. CaCl₂
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Answer: (B) CaI₂

As the size of the halide anion increases from F⁻ to I⁻, its polarisability increases significantly. High polarisability leads to greater covalent character, which lowers the lattice energy and consequently results in the lowest melting point for CaI₂.

easy

8. Which of the following orders is correct for the increasing covalent character of the given alkaline earth metal chlorides?

  1. SrCl₂ < BaCl₂ < MgCl₂ < CaCl₂
  2. MgCl₂ < CaCl₂ < SrCl₂ < BaCl₂
  3. CaCl₂ < MgCl₂ < BaCl₂ < SrCl₂
  4. BaCl₂ < SrCl₂ < CaCl₂ < MgCl₂
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Answer: (D) BaCl₂ < SrCl₂ < CaCl₂ < MgCl₂

According to Fajan's rules, smaller cations have higher polarizing power. The size of alkaline earth metal cations follows the order Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺, hence polarizing power and covalent character decrease down the group.

medium

9. Which molecule has zero dipole moment?

  1. HCl
  2. CO₂
  3. H₂O
  4. NH₃
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Answer: (B) CO₂

CO₂ is linear, so its two equal C=O bond dipoles cancel. H₂O and NH₃ are bent/pyramidal, and HCl is polar.

NCERT Class 11

medium

10. Which of the following statements is INCORRECT regarding Fajan's rules and polarization?

  1. A cation with an 18-electron valence shell has lower polarizing power than one with an 8-electron shell of similar size.
  2. Polarizability of an anion increases with an increase in its ionic size.
  3. Higher charge on the cation increases its polarizing power.
  4. Greater polarization of the electron cloud of an anion leads to increased covalent character.
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Answer: (A) A cation with an 18-electron valence shell has lower polarizing power than one with an 8-electron shell of similar size.

A cation with a pseudo-noble gas configuration (18 valence electrons, like Cu⁺ or Ag⁺) has a higher polarizing power than a noble gas configuration cation (8 valence electrons, like Na⁺ or K⁺) of comparable size because the d-electrons shield the nuclear charge poorly.

easy

11. According to Fajan's rules, which of the following halides has the maximum ionic character?

  1. LiF
  2. KF
  3. CsF
  4. RbCl
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Answer: (C) CsF

Ionic character increases with larger cation size and smaller anion size. Cs⁺ has the largest ionic radius among alkali metal cations, and F⁻ is the smallest halide anion, leading to minimum polarization and maximum ionic character for CsF.

medium

12. Consider the following two statements: Statement I: The dipole moment of CH₃F is greater than that of CH₃Cl. Statement II: The electronegativity of fluorine is greater than that of chlorine, but the C-Cl bond length is considerably larger than the C-F bond length.

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because the experimentally determined dipole moment of CH₃Cl (1.860 D) is greater than that of CH₃F (1.60 D). Statement II is correct because dipole moment is the product of charge (q) and distance (d); the significantly longer C-Cl bond distance outweighs the smaller electronegativity difference, giving CH₃Cl a higher dipole moment.

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