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
Draw the Lewis structure
Assign the VSEPR electron geometry
Determine the molecular shape.
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.
