AIPMT PRELIMS 2004 Chemistry Molecular Geometry MCQ Question

To understand why H₂O is dipolar while BeF₂ is not, we need to delve into the concepts of molecular geometry, polarity, and electronegativity.

Explanation of the Correct Answer (C)

Correct Answer: C) H₂O is angular and BeF₂ is linear

1. Molecular Geometry:

   • Water (H₂O) has a bent or angular molecular shape with a bond angle of approximately 104.5 degrees. The oxygen atom is at the center, and the two hydrogen atoms are bonded to it. The presence of lone pairs on the oxygen atom distorts the shape, leading to a dipole moment.
   • Beryllium fluoride (BeF₂), however, has a linear molecular structure due to the arrangement of its bonding pairs. The geometry allows the dipole moments of the two Be-F bonds to cancel each other out, resulting in a non-polar molecule.

2. Polarity:

   • In H₂O, the difference in electronegativity between oxygen (3.44) and hydrogen (2.20) leads to polar O-H bonds. The bent shape ensures that the dipole moments do not cancel out, giving water its dipolar character.
   • In BeF₂, while the Be-F bonds are polar due to the electronegativity difference (fluorine is more electronegative at 3.98), the linear geometry means that the dipoles oppose each other directly and cancel, resulting in no net dipole moment.

Why Other Options Are Incorrect

• Option A: H₂O involves hydrogen bonding whereas BeF₂ is a discrete molecule:

  • This statement is inaccurate in the context of explaining dipolarity. While H₂O indeed has hydrogen bonding due to its polarity, the key reason for its dipolar nature compared to BeF₂ lies in their molecular geometries, not the presence or absence of hydrogen bonding.

• Option B: H₂O is linear and BeF₂ is angular:

  • This statement is factually incorrect. H₂O is angular (bent) and not linear. BeF₂ is correctly described as linear. The incorrect description of H₂O's shape leads to an incorrect conclusion.

• Option D: The electronegativity of F is greater than that of O:

  • While it is true that fluorine is more electronegative than oxygen, this fact alone does not explain the dipolarity of H₂O versus the non-polar nature of BeF₂. The key factor is the molecular geometry and how the bond dipoles interact, which is not addressed in this option.

Summary

In summary, the correct answer is based on the molecular geometries of H₂O and BeF₂. H₂O's angular shape results in a net dipole moment, making it dipolar, while BeF₂'s linear geometry causes the bond dipoles to cancel each other out, rendering it non-polar. Understanding these geometric arrangements and their implications for molecular polarity is crucial in the study of chemical bonding.