Cliffs AP Chemistry, 3rd Edition

2. As one moves down the halogen column, one notices that the boiling point increases.
   However, when examining the alkali metal family, one discovers that the melting point
   decreases as one moves down the column.

(a) Account for the increase in boiling point of the halogens as one moves down the

column.

(b) Account for the decrease in melting point of the alkali metals as one moves down the

column.

(c) Rank Cs, Li, KCl, I 2 , and F 2 in order of decreasing melting point, and explain your

reasoning.

Answer

This answer might best be done in the bullet format.

2. (a)Restatement: Explain increase in B.P. of halogens as one moves down column.
   
   
   • Halogens are nonmetals.
   
   
   • Halogens are diatomic.
   
   
   • Bonding found within halogen molecule is covalent — formed as a result of sharing
     of electrons.
   
   
   • Forces found between halogen molecules are van der Waals forces, which are due to
     temporarily induced dipoles caused by polarization of electron clouds.
   
   
   • Moving down the column, one would expect greater nuclear charge.
   
   
   • Moving down the column, one would expect larger electron clouds due to higher en-
     ergy levels being filled as well as greater atomic numbers and hence a greater number
     of electrons.
   
   
   • Moving down halogen family, shielding effect and greater distance from nucleus
     would cause easier polarization of electron cloud.
   
   
   • Therefore, greater polarization of electron cloud would cause greater attractive force
     (van der Waals force), resulting in higher boiling points.
   
   
   • Furthermore, one must consider the effect of the molecular weight on the B.P. As the
     individual molecules become more and more massive, they need higher and higher
     temperatures to give them enough kinetic energy and velocity to escape from the sur-
     face and “boil.”
     (b)Restatement: Explain decrease in melting point of alkali metals as one moves down
     column.
   
   
   
   

• Alkali metal family are all metals.


• Metals have low electronegativity.


• Metals have low ionization energies.


• Metals exist in definite crystal arrangement — cations surrounded by “sea of
  electrons.”

Part II: Specific Topics