Moorpark College - PSY M01

CHEMISTRY 171: PROBLEM SET 5 (10/25/19)

1)The internuclear distances in gas-phase ionic molecules are considerably smaller than those in the corresponding crystals. For example, the internuclear distance in NaCl(g) is 2.3606 Å, whereas in NaCl(s) the shortest Na-Cl distance is 2.814 Å. Explain why this should be so when the binding energy is larger in the crystal.

2. Magnesium oxide and sodium fluoride are isoelectronic and have the same crystal structure as NaCl. Explain the observation that MgO is nearly twice as hard as NaF and has a much higher melting point than NaF (2800 ?C compared to 993 ?C).

3. Draw and label band diagrams for:

a. cobalt          b. diamond                   c. n-type Si                    d. Ga doped Si

4. What are the major carriers of electricity in each compound listed in problem #3?

5    For Si and As doped Si, make a plot of 1n ? vs 1/t.

6. How many sigma bonds are there in a C60 molecule?

7. What is the carbon-carbon bond order in C60?

8. Which has stronger carbon-carbon bonds: graphite, diamond or C60? Please explain.

9. How many carbon atoms are in the unit cell of Buckminsterfullerene, C60?

10. Describe the unit cell of the superconductors K3C60 (Tc = 19K) and Rb3C60 (Tc = 30K).

11. What types of forces hold molecules together in crystals and liquids?

12. Why are nonmetallic network solids usually quite hard?

13. What happens to the conductivity of a semiconductor as temperature is increased? What happens to its conductivity as pressure increased? Why?