1. The Four General Types of Crystalline Solid At an early stage in the study of chemistry we learn that there are four general types of crystalline solid and associated material type:four general types of crystalline solid ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

2. The Four Types Metallic materials Ionic materials Network covalent materials Semiconductors & Metalloids ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

3. Metallic Materials Metals have a characteristic luster and some are excellent conductors of electricity. There are two models of bonding in metals. –Drude modelDrude model –band theoryband theory ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

4. Drude model The classical Drude model of conductivity has a lattice of cations immersed in a sea of mobile valence electrons delocalized over the entire crystal.Drude model –The positive cations attract the delocalized electrons, and vice versa. –The mobile electrons are the agents responsible for the conduction of electricity and heat: ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

5. Drude model ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

6. Metals can also be modeled using band theory band theory The structure of metals results in many near energy molecular orbitals which gives rise to both luster and conductivity. Because orbitals are close in energy, electrons are easily promoted to higher energy, vacant orbitals, giving rise to electical conductivity. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

7. Metal Properties and Temperature The thermal and electrical conductivities of a metal are proportional to each other, but increasing the temperature increases the thermal conductivity and decreases the electrical conductivity, a behavior quantified by the Wiedemann-Franz Law. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

8. Why two models? From Wikipedia: "The simple classical Drude model provides a very good explanation of DC and AC conductivity in metals, the Hall effect, and thermal conductivity (due to electrons) in metals. The model also explains the Wiedemann-Franz law of 1853.Wikipedia "However, the Drude model greatly overestimates the electronic heat capacities of metals. In reality, metals and insulators have roughly the same heat capacity at room temperature. Although the model can be applied to positive (hole) charge carriers, as demonstrated by the Hall effect, it does not predict their existence." Hall effect ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

9. Other Properties of Metals Metals are often ductile and exhibit a huge range of melting points, from mercury (-39 °C) to tungsten (3200 °C). ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

10. Ionic Materials Ionic materials, such as sodium chloride, NaCl, have an extended crystal lattice with non-metal anions electrostatically attracted to adjacent metal cations and metal cations electrostatically attracted to adjacent non-metal anions: ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

11. Ionic Crystal Lattice ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

12. Ionic Materials Most ionic materials are insulators as solids, but are electrical conductors when molten and when dissolved in aqueous solution. Ionic materials may dissolve in water (and sometimes in dipolar aprotic solvents such as DMSO), but they are insoluble in non-polar solvents like hexane or chloroform. Ionic binary materials have moderately high melting points, usually in the 300-1000 °C range. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

13. Molecular van der Waals Materials Discrete molecules, such as methane, CH 4, are held together internally by strong intramolecular (within molecule) shared electron pair covalent bonds, but when forming condensed solid or liquid phases, the molecules interact via weak intermolecular (between molecule) van der Waals forces: ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

14. Molecular van der Waals Materials ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

15. Intermolecular Attraction There are several types of van der Waals attraction:van der Waals dipole/dipole, dipole/induced-dipole, and spontaneous-dipole/induced-dipole. It is tempting to consider these forces to be of different strengths, but it is the distance range that is more important. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

16. London Dispersion Forces The spontaneous-dipole/induced-dipole attractions – also known as London dispersion forces (LDF) – are surprisingly strong but only act at very short range. These are contact attractions: the surface of even neutral, non-polar molecules like methane are 'sticky'. All molecules have London dispersion forces and the strength increases with the size/surface area of the molecule. This logic is used to explains the increasing boiling and sublimation temperatures of the halogens: F 2 < Cl 2 < Br 2 < I 2. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

17. dipole-dipole In addition, some molecules have dipole-dipole, hydrogen bonding, etc., which increase the total amount of interaction between the molecules. Consider iodine chloride, ICl and bromine, Br 2. –Both are 70-electron systems, but ICl is polar and Br 2 is non-polar, yet they have rather similar boiling points of 97 °C and 59 °C, respectively, showing that the dipole/dipole attraction makes only a minor contribution. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

18. Hydrogen bonding Molecular materials may also be hydrogen bonded: – where a hydrogen bond involves a proton being shared between two Lewis bases, usually with oxygen, nitrogen or fluorine atomic centers. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

19. Molecular Materials Molecular materials exhibit a vast array of properties, but they are generally –mechanically weak, –have zero electrical conductivity, –have low melting and boiling points, and/or a susceptibility to sublime. –Molecular materials are usually soluble in (or miscible with) non-polar solvents. –Hydrogen bonded molecular solids are often soluble in water. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

20. Network Covalent Materials Network covalent materials, such as diamond, C, silicon, Si, and silicon dioxide (quartz and sand), SiO 2, have atoms arranged in an extended lattice of strong, shared electron pair covalent bonds. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

21. Network Covalent Lattice ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

22. Network covalent materials are –hard, –refractory, –brittle, –usually electrical insulators, and –they are not soluble in any solvent. –Materials have very high melting points, >1500 °C ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

23. Semiconductors & Metalloids Semiconductors are network covalent materials that conduct electricity, albeit with high resistance when pure.Semiconductors The electrical conductivity, modeled using band theory, can be modified by doping, and this is the basis of the transistor and silicon chip – integrated circuit – technology.band theorytransistor integrated circuit Semiconductor elements and binary materials include: silicon, gallium arsinide & indium phosphide.silicongallium arsinideindium phosphide ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

24. Best of both worlds Several elements have pairs of allotropes, one of which is molecular and insulating, and the other which is metalloid: –hard, brittle, with a network covalent structure, a metallic luster and electrical conductivity. –These include: carbon (C60 & graphite), tin (gray & white) & arsenic (yellow & grey).C60 & graphitegray & whiteyellow & grey ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

25. Ionic Materials Ionic salts like: sodium chloride, NaCl Lattice of electrostatically attracted anions and cations. Usually soluble in water to some extent. Insulators when solid. Conduct electricity when molten and when in aqueous solution: ElectrolytesElectrolytes Intermediate melting points ~300 – 1000 °C ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

26. Metallic Materials Metals like aluminium and alloys like brass Lattice of metal cations in sea of electrons Conduct electricity & heat as solid and liquid Metallic lustre and ductility Huge range of melting points: mercury –39 °C tungsten 3407 °C Metals may, or may not, alloy with each other ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

27. Molecular van der Waals Materials Molecular materials like methane, CH 4 Small molecules Strong intramolecular – within molecule – covalent bonds Weak intermolecular – between molecule – bonds: van der Waals forces Generally low melting and boiling points: liquids and gases at 25 °C Insulators Soluble in polar or non-polar solvents ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

28. Network Covalent Materials Network of strong covalent bonds Diamond Very high melting point, >1500 °C Insoluble, insulators Refractory materials ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht

29. A bonding continuum ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyrig ht