AP Chemistry · Free-response skill

From Lewis structure to a property

The move. Build a molecule's Lewis structure, use it to predict shape and polarity, and connect polarity to a measured property.

How to do it

  1. Count the valence electrons first, then place them: bonds, then outer octets, then any leftover pairs on the central atom.
  2. Count the domains on the central atom, including lone pairs, to get the arrangement; name the shape from the atoms only.
  3. For polarity, say whether the bonds are polar and whether the shape lets their pulls cancel.
  4. To explain a property, name the force between molecules that differs, and say that a stronger attraction takes more energy to overcome.

Practice prompt Try it

Two chlorides of period 2 and period 3 elements

CompoundFormulaBoiling point (°C)
Boron trichlorideBCl₃12.6
Phosphorus trichloridePCl₃76.1

A student compares the two compounds in the table. Each B–Cl and P–Cl bond is polar. Answer each part.

  1. Give the total number of valence electrons in a PCl₃ molecule, and say how many lone pairs are on the phosphorus atom in its Lewis structure.
  2. Name the molecular geometry of PCl₃ and of BCl₃. (Boron has no lone pairs in BCl₃.)
  3. Explain why BCl₃ is nonpolar even though each B–Cl bond is polar.
  4. PCl₃ is polar. Using that fact, explain why PCl₃ boils at a higher temperature than BCl₃.

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Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 6 points

Model answer, annotated

(a) Phosphorus brings 5 valence electrons and each chlorine brings 7, so PCl₃ has 5 + 3(7) = 26. Three P–Cl bonds use 6, and three lone pairs on each Cl use 18, which leaves 2 electrons: one lone pair on phosphorus.

  • Point 1 Adds the valence electrons of every atom to get 26.
  • Point 2 Places the 2 leftover electrons on phosphorus as one lone pair.

(b) PCl₃ has four electron domains on phosphorus, one of them a lone pair, so its shape is trigonal pyramidal. BCl₃ has three domains and no lone pairs on boron, so it is trigonal planar.

  • Point 3 Uses four domains with one lone pair to name trigonal pyramidal.
  • Point 4 Uses three domains and no lone pair to name trigonal planar.

(c) In trigonal planar BCl₃ the three identical B–Cl bonds point 120° apart, so their dipoles are equal in size and cancel. The molecule has no net dipole.

  • Point 5 Ties the cancelling of the dipoles to the symmetric planar shape.

(d) Because PCl₃ is polar, its molecules attract one another by dipole-dipole forces as well as London dispersion forces. Nonpolar BCl₃ molecules have only London forces. (PCl₃ also has 66 electrons against 56 in BCl₃, so its London forces are somewhat stronger too; both effects point the same way.) The stronger total attraction between PCl₃ molecules takes more energy to overcome, so PCl₃ boils at a higher temperature.

  • Point 6 Names the dipole-dipole force that only PCl₃ has and links it to more energy to boil.