Question

Which molecule has polar bonding and is nonpolar? A. H2O B. BF3 C. NH3 D. NCl3 E. CH2Cl2

Answer 1

Answer:

B. BF₃

Explanation:

All the molecules have polar bonds, but a molecule will be nonpolar if the molecule has the symmetry that makes the bond dipoles cancel.

To make the decision, we must

1. Draw the Lewis structure
2. Assign the VSEPR electron geometry
3. Determine the molecular shape.
4. Examine the symmetry of the molecule

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A. Water

Lewis structure = H-O-H (2 bonding pairs, 2 lone pairs)

Electron geometry = AX₂E₂ tetrahedral

Molecular geometry = bent

Symmetry (see Figure A): The two O-H bonds are polar, with their negative ends pointing towards the O. The horizontal components of the bond dipoles cancel, but the vertical components reinforce each other and give an upward pointing molecular dipole. This is a polar molecule with polar bonds.

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B. Boron trifluoride

Lewis structure = BF₃ (3 bonding pairs)

Electron geometry = AX₃, trigonal planar

Molecular geometry = trigonal planar

Symmetry (see Figure B): The three B-F bonds are polar, with their negative ends pointing towards the F. The horizontal components of the bond dipoles cancel, but the vertical components of the two downward -pointing dipoles reinforce each other and give a resultant that is equal and opposite to the upward dipole. Thus, the bond dipoles cancel. This is a nonpolar molecule with polar bonds.

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C. Ammonia

Lewis structure = :NH₃ (3 bonding pairs, 1 lone pairs)

Electron geometry = AX₃E, tetrahedral

Molecular geometry = trigonal pyramidal

Symmetry (see Figure C): The three N-H bonds are polar, with their negative ends pointing towards the N. The horizontal components of the bond dipoles cancel, but the vertical components reinforce each other and give an upward pointing molecular dipole. This is a polar molecule with polar bonds.

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D. Nitrogen trichloride

Lewis structure = :NCl₃ (3 bonding pairs, 1 lone pair)

Electron geometry = AX₃E, tetrahedral

Molecular geometry = trigonal pyramidal

Symmetry (see Figure D): The three N-Cl bonds are polar, with their negative ends pointing towards the Cl. The horizontal components of the bond dipoles cancel, but the vertical components reinforce each other and give a downward pointing molecular dipole. This is a polar molecule with polar bonds.

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E. Dichloromethane

Lewis structure = H₂CCl₂ (4 bonding pairs)

Electron geometry = AX₄, tetrahedral

Molecular geometry = tetrahedral

Symmetry (see Figure E): The two C-H bonds are nonpolar, but the two C-Cl bonds are polar with their negative ends pointing towards the Cl. The horizontal components of the bond dipoles cancel, but the vertical components reinforce each other and give a downward pointing molecular dipole. This is a polar molecule with polar bonds.