1. © 2012 Pearson Education, Inc. Chapter 12 Solids and Modern Materials John D. Bookstaver St. Charles Community College Cottleville, MO Lecture Presentation

2. © 2012 Pearson Education, Inc. Bozeman Solids and liquids Metallic bonding Metallic solids Network solids Molecular solidsIonic solids Crash course videos Doing solids Network solids

3. Modern Materials © 2012 Pearson Education, Inc. 12.1 Classification of Solids Four types of solids: Metallic solids share a network of highly delocalized mobile electrons. Ionic solids are sets of cations and anions mutually attracted to one another.

4. Modern Materials © 2012 Pearson Education, Inc. Covalent-network solids are joined by an extensive network of covalent bonds. Molecular solids are discrete molecules that are linked to one another only by van der Waals (IM) forces. 12.1 Classification of Solids

5. 5 All Solids Have a definite shape and volume (expand slightly when heated) KMT: Molecules are close together, in definite crystal arrangement have the strongest forces between molecules Molecules vibrate around fixed positions Allotropes: different molecular forms of the same element Ex: Carbon forms: Diamond, graphite, charcoal Different crystal structure = different properties

6. Modern Materials © 2012 Pearson Education, Inc. 12.2 Solid Structure In Crystalline solids atoms are arranged in a very regular pattern. The Properties depend on the forces (e.g. metallic vs. ionic bonds) Since forces are consistent throughout the crystal, they have a distinct melting point temperature.

7. Modern Materials © 2012 Pearson Education, Inc. Amorphous solids – have no particular order in arrangement of particles The IM forces vary so there is no specific melting point Glass melts over a temperature range

8. Modern Materials © 2012 Pearson Education, Inc. 12.3/4 Metallic solids and Bonding In elemental samples of nonmetals and metalloids, atoms generally bond to each other covalently. Metals, lack valence electrons to share, so form large groups of atoms that share electrons among all.

9. 9 Fe 2+ * Fe 2+ * Fe 2+ * * Fe 2+ * Fe 2+ * Fe 2+ Fe 2+ * Fe 2+ * Fe 2+ * * Fe 2+ * Fe 2+ * Fe 2+ Ex: iron metal * = electron electrons act as mobile glue to hold cations together Metallic “substances” and crystals Formed in pure and mixtures of metals (alloys) Metal atoms all lose their valence (outer) electrons Become “Ions in a sea of mobile electrons”

10. Modern Materials © 2012 Pearson Education, Inc. Electron sea model: In metals, valence electrons are delocalized throughout the solid. Properties: Soft to hard low to high MP; good electrical and thermal conductors; Malleable and ductile; Shiny luster “sea of mobile electrons”

11. 11 Properties of Metallic bond Flexible attraction between positive ions and mobile electrons make metals… …malleable (bendable) and Ductile (wire) …good conductors of electricity as solids. …shiny aka “metallic luster” Chrome -ium -ium Tire rim

12. Modern Materials © 2012 Pearson Education, Inc. In substitutional alloys, a second element takes the place of a metal atom. In interstitial alloys, a second element fills a space in the lattice of metal atoms. Alloys are combinations of two or more elements, the majority of which are metals.

13. Modern Materials © 2012 Pearson Education, Inc. Alloys Carbon Steel is much harder than iron, but is less malleable Pure gold is too soft and is alloyed with other metals to make it durable. carbon atoms reduce the malleability of the iron making the metal harder.

14. Modern Materials © 2012 Pearson Education, Inc. Adding chromium creates stainless steel Chromium oxide creates a protective coating.

15. Modern Materials © 2012 Pearson Education, Inc. Metallic trends: melting point Increases with increasing valence electrons Decrease as atoms get larger

16. Modern Materials © 2012 Pearson Education, Inc. Can you explain this? Melting point increases up to group 6 (Cr and Mo) then decreases. Notice that the S and d sublevel is now half filled with electrons. We need to examine the molecular-orbital theory to explain this.

17. Modern Materials © 2012 Pearson Education, Inc.

18. Modern Materials © 2012 Pearson Education, Inc. A Molecular-Orbital Approach Recall that two atomic orbitals combine and overlap to form one bonding and one antibonding molecular orbital. The bonding orbital has electrons between nuclei and stabilize the bond Anti-bonding orbitals lie outside the internuclear space and destabilize the bond.

19. Modern Materials © 2012 Pearson Education, Inc. In metals, bonding and antibonding orbitals combine to form a continuous band of orbitals Electrons enter bonding orbitals first, followed by the antibonding orbitals.

20. Modern Materials © 2012 Pearson Education, Inc. Metals conduct electricity easily, because bonding and antibonding orbitals overlap, electrons in bonding orbitals can easily move to the unoccupied antibonding orbitals, and move throughout the metal.

21. Modern Materials © 2012 Pearson Education, Inc. Groups 1A to 6B electrons are filling lower energy bonding molecular orbitals so MP increases Groups 7B to 12B, electrons are filling the antibonding molecular orbitals, bonding becomes weaker and MP decreases This also explains the pattern of melting points

22. Modern Materials © 2012 Pearson Education, Inc. In semi-metals (semiconductors) electrons must gain energy to jump a small gap. This also creates a lower valence band and a higher conduction band. Notice these bands overlap in metals. In insulators, this gap is too large for electrons to jump

23. Modern Materials © 2012 Pearson Education, Inc. 12.5 Ionic solids Electrostatic forces Hard, brittle, high MP Poor e - conductors become conductive as liquid or aq Ex: NaCl, etc. Why are salts brittle?

24. 24 Ionic properties Brittle: Like charged ions repel when struck Good electrical conductors when dissolved in water NaCl (aq)

25. 25 Brittle vs. Malleable Ionic Crystal Metallic crystal

26. Modern Materials © 2012 Pearson Education, Inc. Ionic trends Recall that MP increases with increasing lattice energy. MP increases with increasing charge difference, but decreases as radii increase due to coulombs law: E=Q1Q2/d

27. Modern Materials © 2012 Pearson Education, Inc. Attractions in Ionic Crystals In ionic crystals, ions pack themselves so as to maximize the attractions and minimize repulsions between the ions. Notice like charges don’t touch

28. Modern Materials © 2012 Pearson Education, Inc. 12.6 Molecular Solids The physical properties of molecular solids are governed by van der Waals forces. The individual units of these solids are discrete molecules.

29. Modern Materials © 2012 Pearson Education, Inc. Molecular Weak IM forces Soft with low MP’s Poor e - conductors Ex: H 2 O (s), CO 2 (dry ice) I 2, sucrose, wax Dry ice

30. 30 Ex: Water molecules: Hard to decompose H 2 O into H and O (must chemically break bonds) Easy to separate H 2 O molecules (physically) during boiling Vs. Ionic with its strong continuous bonds!

31. Modern Materials © 2012 Pearson Education, Inc. MP depends on types of forces and molecular structure. Can you explain the difference in MP? Benzene packs more efficiently as a solid Phenol’s OH allows for Hydrogen bonding But toluene must have stronger dispersion forces as a liquid

32. Modern Materials © 2012 Pearson Education, Inc. Regents questions: Weak forces between particles nonpolar Not made of charged particles Weak forces between particles Weak forces between particles means molecular = covalent bonds inside Weak IM forces between molecules allow for melting at low temps Unlike metals, Molecules do not have mobile charged electrons.

33. 33 12.7 Covalent Network Solids: Continuous covalent bonds Hard, very high melting points Elements which form 4 bonds Ex: diamond (C), Sand (SiO 2 ) | | | --- C --- C --- C --- | | | --- C --- C --- C --- | | | Molecular: DiscreteParticles Ex: H 2 O Weak forces Between molecules = Low MP

34. Modern Materials © 2012 Pearson Education, Inc. Diamonds are an example of a covalent-network solid, in which atoms are covalently bonded to each other. –They tend to be hard and have high melting points.

35. 35 Quartz diamonds are made from silicon and oxygen Crash course Silicon

36. Modern Materials © 2012 Pearson Education, Inc. In Graphite network layers of atoms are held together with dispersion forces. –They tend to be softer and have lower melting points. –Delocalized bonding allows for e - conductivity Remember resonance? Graphite, a network structure, conducts electricity just like a metal !

37. Modern Materials © 2012 Pearson Education, Inc. Semiconductors In the closely packed molecular orbitals in these compounds, there is a gap between the occupied MOs (valence band) and the unoccupied ones (conduction band).

38. Modern Materials © 2012 Pearson Education, Inc. Semiconductors Among elements, only Group IVA, all of which have 4 valence electrons, are semiconductors. Or in compounds like GaAs, tend to have an average of 4 valence electrons (3 for Ga, 5 for As). Minimal energy is required to move an electron into the conduction band and create an electric current. Voila, and the solar cell is born!

39. Modern Materials © 2012 Pearson Education, Inc. Doping Doping a semiconductor with small numbers of atoms having additional electrons (like phosphorus w/5 val.e - ) add electrons to the conduction band and increase conductivity. These are called n- type (negative) semiconductors

40. Modern Materials © 2012 Pearson Education, Inc. Doping Doping with atoms that have less than 4 electrons (like Al) creates a hole in the valence band. These holes provide places for adjacent electrons to jump to create their own current. These are called P-type (positive) semiconductors

41. Modern Materials © 2012 Pearson Education, Inc. LED Technology: click on the link at the bottom LED refers to Light-Emitting Diodes. Light-Emitting Diodes are semiconductor devices that emit light by only allowing electric current to pass in one direction. LEDs consist of two elements: N-type and P-type semiconductors. The two materials are placed in direct contact as seen in Figure 1: The P-type silicon contains excessive positive charges representing an absence of electrons. The N-type silicon contains a number of negative charges (electrons). When a current is passed through the semiconductor device electrons move from the N-type silicon to the P-type silicon, while the excessive charges move towards the N-type silicon meeting at what is known as a PN Junction (see Figure 2). As the charges meet, energy is released in the form of photons - light. The light released at the PN-Junction in the LED is clean and very efficient. This efficient exchange of electrical energy for light energy results in very little heat output from the device; making LEDs the most energy efficient and environmentally friendly man-made light source known to the world! LED technology video Another one

42. Modern Materials © 2012 Pearson Education, Inc. 12.8 Polymers Polymers are molecules of high molecular mass made by sequentially bonding repeating units called monomers. Crash course polymer video

43. Modern Materials © 2012 Pearson Education, Inc. Addition Polymers Addition polymers are made by coupling the monomers by converting  bonds within each monomer to  bonds between monomers.

44. Modern Materials © 2012 Pearson Education, Inc. Condensation Polymers Condensation polymers are made by joining two subunits through a reaction in which a smaller molecule (often water) is also formed as a by-product. polyester Amino acids join to make proteins

45. Modern Materials © 2012 Pearson Education, Inc. Some Common Polymers

46. Modern Materials © 2012 Pearson Education, Inc. Some Common Polymers PET bottle Polycarbonate

47. Modern Materials © 2012 Pearson Education, Inc. Synthesis of Nylon Nylon is one example of a condensation polymer. The first synthetic polymer

48. Modern Materials © 2012 Pearson Education, Inc. Vulcanizing Rubber …weak cross-link forces are formed between latex polymer strands… …stretchy rubber is formed Latex is a sticky liquid that comes from trees…

49. Modern Materials © 2012 Pearson Education, Inc. Proteins Bozeman Polymer video Ribosomes combine Amino acids to form polypeptides These fold and combine with others to form complexes called proteins the folding creates unique shapes that create function

50. Modern Materials © 2012 Pearson Education, Inc. Enzymes The folding of the protein creates a unique shape that fits the substrate molecule – remember the “lock and key” model? Example: enzymes that break up food molecules into useful nutrients