NCERT Class 11 Science — Chapter 2: Chemical Bonding and Molecular Structure

Correct Option

The correct option is BeF2.

Explanation

Dipole moment is a vector quantity that measures the polarity of a chemical bond or molecule. In polyatomic molecules, the net dipole moment is the vector sum of individual bond dipoles. If a molecule possesses a symmetrical structure where individual bond dipoles are equal in magnitude but opposite in direction, they cancel each other out, resulting in a zero net dipole moment.

Molecule-wise Analysis

• BeF₂ (Beryllium Fluoride):
  • The central atom, Beryllium (Be), has two valence electrons and forms two sigma bonds with Fluorine atoms.
  • According to VSEPR theory, the repulsion between bond pairs results in a linear geometry with a bond angle of 180°.
  • The two Be-F bond dipoles are equal in magnitude but point in exactly opposite directions. Consequently, they cancel each other, leading to a net dipole moment of zero ($\mu = 0$).
• H₂O (Water):
  • The central Oxygen atom has two bond pairs and two lone pairs. This results in a bent or V-shaped structure.
  • The bond dipoles and the orbital dipole due to lone pairs do not cancel out, resulting in a significant net dipole moment ($\mu \neq 0$).
• NH₃ (Ammonia):
  • Nitrogen has three bond pairs and one lone pair, leading to a trigonal pyramidal geometry.
  • The resultant vector of the N-H bonds adds to the dipole moment of the lone pair, resulting in a net non-zero dipole moment ($\mu \neq 0$).
• HF (Hydrogen Fluoride):
  • This is a heteronuclear diatomic molecule. Due to the high electronegativity difference between Hydrogen and Fluorine, the bond is polar.
  • Since there is only one bond, there is no cancellation of dipoles, resulting in a non-zero dipole moment ($\mu \neq 0$).

Key Takeaway: Symmetrical linear molecules of the type AB₂ (such as BeF₂ and CO₂) possess zero dipole moment because the individual bond moments are equal and opposite, effectively canceling each other out.