Cliffs AP Chemistry, 3rd Edition

(b) Restatement: Explain halogen trend.

• Observed melting point order: I > Br > Cl > F


• All halogens are nonmetals


• Intramolecular forces =covalent bonding


• Intermolecular forces =dispersion (van der Waals) forces, which exist between
  molecules


• Dispersion forces result from “temporary” dipoles caused by polarization of
  electron clouds


• As one moves up the group, the electron clouds become smaller


• Smaller electron clouds result in higher charge density


• As one moves up the group, electron clouds are less readily polarized


• Less readily polarized clouds result in weaker dispersion forces holding molecules
  to other molecules


• Therefore, attractive forces between molecules decrease as one moves up the
  group, resulting in lower melting points
  (c) Restatement: Predict melting point order (high to low) CsI, NaCl, LiF, and KBr and
  explain.


• LiF > NaCl > KBr > CsI


• All compounds contain a metal and a nonmetal


• Predicted order has ionic bonds


• Larger ionic radius results in lower charge density


• Lower charge density results in smaller attractive forces


• Smaller attractive forces result in lower melting point

Questions Commonly Asked About the AP Chemistry Exam

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