Multiple Choice
Which of the following best describes the molecular polarity of phosphorus pentachloride (PCl_5)?
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13. Liquids, Solids & Intermolecular Forces
Molecular Polarity
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Which of the following best describes the molecular polarity of NH_3 (ammonia)?
A
NH_3 is a polar molecule due to its trigonal pyramidal shape and lone pair on nitrogen.
B
NH_3 is polar only when dissolved in water.
C
NH_3 is nonpolar because its molecular geometry is linear.
D
NH_3 is a nonpolar molecule because all the N-H bonds are identical.
Verified step by step guidance1
Identify the molecular geometry of NH_3 by considering the number of bonding pairs and lone pairs on the central nitrogen atom. Nitrogen has three bonding pairs with hydrogen atoms and one lone pair, which leads to a trigonal pyramidal shape.
Recall that molecular polarity depends on both the polarity of individual bonds and the overall shape of the molecule. Each N-H bond is polar because nitrogen is more electronegative than hydrogen, creating a dipole moment.
Analyze how the lone pair on nitrogen affects the molecular shape and dipole moments. The lone pair pushes the bonding pairs downward, creating an asymmetrical shape that does not cancel out the dipole moments of the N-H bonds.
Conclude that because of the trigonal pyramidal shape and the presence of the lone pair, the dipole moments do not cancel, making NH_3 a polar molecule.
Compare this with the incorrect options: NH_3 is not linear, so it cannot be nonpolar due to linear geometry; it is polar regardless of being dissolved in water; and identical N-H bonds do not guarantee nonpolarity if the molecular shape is asymmetrical.
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