Types of Crystalline Solids

1. Types of Crystalline Solids • Ionic • Nonpolar Molecular • Polar Molecular • Metallic • Nonbonding atomic • Network Covalent Molecular: Atomic:

2. Determine the type of crystal for each description below: • The substance does not conduct electricity as a solid, but is conductive once dissolved in water. • Extremely high melting point, rigid, does not dissolve in water. • Only slightly soluble in water, nonconducting as a solid and in solution.

3. The mass of the HF molecule is similar to the mass of a neon atom, yet their boiling points are very different. HF’s boiling point is 19.5oC and Neon’s boiling point is -246oC. Explain this difference.

4. Explain the trend in the following melting points.

5. Which would have the higher melting point, KCl or AlCl3? Justify your answer.

6. Types of Solids Nonbonding atomic solids Molecular solids Allotropic Metallic solids

7. Metallic Solids • Pure substances • Alloys • can be two or more metals, or a metal and a nonmetal • some are solid solutions, others are compounds

8. Alloys • combine metals with other substances to improve their properties: • stronger, harder, lighter, etc. • Iron + Carbon = Steel • stronger, harder, rustproof • Aluminum • used to make airplanes • – light, but very soft • + Mg, Mn, Cu = duralumin

9. Alloys • Substitutional Alloys • form between atoms of comparable radius • brass (copper – zinc) • Interstitial Alloys • form between atoms of different radius • Smaller atoms fill spaces between larger atoms • Steel (iron – carbon)

10. Network Covalent Solids • Atoms at points of lattice. • Atoms held together in 3-d network by covalent bonds. • Only formed from nonmetals. • VERY few examples: Carbon, Silicon, • SiO2(quartz), SiC

11. Silicon • Forms 3-d network similar in geometry to diamond • Semiconductor • Conductivity increases as temperature increases • Conductivity increased by doping

12. Band Theory • Orbitals delocalized over the entire crystal. • Valence band – band of occupied MOs • Conduction band – band of unoccupied MOs

13. Band Theory • e- become mobile when they transition from highest occupied molecular orbitals into higher energy empty molecular orbitals.

14. Band Theory • e- become mobile when they transition from highest occupied molecular orbitals into higher energy empty molecular orbitals.

15. Silicon Doping • adding phosphorous increases conductivity. • phosphorous has 5 ve, compared to silicon’s 4 ve. • valence band of silicon already full. • extra e- of added phosphorous atoms enter conduction band. • n-type semiconductor – charge carriers are negatively charged electrons in conduction band

16. Silicon Doping • adding gallium increases conductivity. • gallium has only 3 ve, compared to silicon’s 4 ve. • results in empty spots in valence band. • holes allow for movement of e- in the valence band. • p-type semiconductor – holes are like a positive charge. think of the holes moving in opposition to the electron movement.