Chapter 2: INTERMOLECULAR BONDING (4rd session)

Transcription

1 Chapter 2: INTERMOLECULAR BONDING (4rd session) ISSUES TO ADDRESS... Secondary bonding The structure of crystalline solids 1

2 REVIEW OF PREVIOUS SESSION Bonding forces & energies Interatomic vs. intermolecular bonds Interatomic bonds 1) ionic bonding 2) covalent bonding 3) metallic bonding 2

3 INTERATOMIC & INTERMOLECULAR BONDS There different types of primary or chemical bond are found in solids: Ionic bonding Covalent bonding Metallic bonding Secondary or van der Waals bonding (physical forces) Fluctuating induced dipole bonds Polar molecule-induced dipole bonds Permanent dipole bonds 3

4 SECONDARY BONDING Whereas primary bonds involve atom-to-atom attractive forces, secondary bonds involve attraction forces between molecules -No transfer or sharing of electrons -Bonds are weaker than primary bonds Arises from interaction between atomic or molecular dipoles Fig 2.12 Schematic illustration of can der Waals bonding between two dipoles 4

5 POLAR MOLECULE Polar molecules have an asymmetrical electrical structure due to the difference in negative and positive charge found throughout the molecule (unequal distribution of charges) Fig 2.14 Schematic representation of a polar hydrogen chloride molecule. 5

6 PERMANENT DIPOLE BONDS Arises from interaction btw adjacent dipoles-molecule (polar molecules) -general case: secondary bonding Adapted from Fig. 2.15, Callister & Rethwisch 8e. -ex: liquid HCl H Cl secondary bonding H Cl -ex: polymer secondary bonding Hydrogen bonding is a special case of polar molecule bonding 6

7 HYDROGEN BONDING -ex: polymer secondary bonding Hydrogen bond is a special case of polar molecule bonding Nylon 6,6 structure 7

8 HYDROGEN BONDING Source: 8

9 FLUCTUATING INDUCED DIPOLES Small electric dipoles were created because of the constant vibrational motion of atoms Fig 2.13 (a) an electrically symmetric atom and (b) an induced atomic dipole. ex: liquid H 2 H 2 H 2 asymmetric electron clouds secondary bonding H H H secondary bonding H Adapted from Fig Polar molecule-induced dipole bonds 9

10 Type Ionic Summary: BONDING Bond Energy Comments Large! Nondirectional (ceramics) Covalent Metallic Variable large-diamond small-bismuth Variable large-tungsten small-mercury Directional (semiconductors, ceramics polymer chains) Nondirectional (metals) Secondary smallest Directional inter-chain (polymer) inter-molecular 10

11 Summary: Primary Bonds Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Large bond energy large T m large E small a Variable bond energy moderate T m moderate E moderate a Polymers (Covalent & Secondary): Directional Properties Secondary bonding dominates small T m small E large a 11

12 Chapter 3: THE STRUCTURE OF CRYSTALLINE SOLIDS Crystalline materials... atoms pack in repeating or periodic 3D arrays over large atomic distances typical of: -metals -many ceramics -some polymers Noncrystalline materials... atoms have no periodic packing disordered & random atomic distribution occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline crystalline SiO2 Adapted from Fig. 3.23(a), Callister & Rethwisch 8e. Si Oxygen noncrystalline SiO2 Adapted from Fig. 3.23(b), Callister & Rethwisch 8e. 12

13 UNIT CELLS The smallest repeat entities subdivided from the crystal structure (lattice) Unit cell is the basic structural unit or building block of the crystal structure which contains the complete lattice pattern of a crystal Unit sells build up the macroscopic 3-D structure of the lattice (crystal structure) Adapted from Fig. 3.2(c), Callister & Rethwisch 8e. 13

14 UNIT CELLS (con t) Atoms are modeled as identical hard-spheres. Corners of a unit cell coincides with the center of the hard-sphere atoms. In a metal, each atom has the same number of nearestneighbor or touching atoms = coordination number 14

15 METALLIC CRYSTAL STRUCTURES (con t) How can we stack metal atoms to minimize empty space? 2-dimensions vs. Now stack these 2-D layers to make 3-D structures How much can be filled? Pack as tightly as possible 15

16 SIMPLE CUBIC STRUCTURE (SC) Rare due to low packing density (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) (Source: 16

17 ATOMIC PACKING FACTOR (APF) APF = Volume of atoms in unit cell* Volume of unit cell APF for a simple cubic structure = 0.52 a *assume hard spheres close-packed directions contains 8 x 1/8 = 1 atom/unit cell Adapted from Fig. 3.24, Callister & Rethwisch 8e. R=0.5a atoms unit cell APF = 1 volume 4 3 p (0.5a) 3 atom a 3 volume unit cell 17

18 FACE CENTERED CUBIC (FCC) Atoms touch each other along face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. ex: Al, Cu, Au, Pb, Ni, Pt, Ag Coordination # = 12 (Courtesy P.M. Anderson) Adapted from Fig. 3.1, Callister & Rethwisch 8e. 4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8 (Source: 18

19 ATOMIC PACKING FACTOR: FCC APF for a face-centered cubic structure = 0.74 maximum achievable APF 2 a a Adapted from Fig. 3.1(a), Callister & Rethwisch 8e. atoms unit cell APF = Close-packed directions: length = 4R = 2 a Unit cell contains: 6 x 1/2 + 8 x 1/8 = 4 atoms/unit cell p ( 2 a/4 ) 3 a 3 volume atom volume unit cell 19

20 BODY CENTERED CUBIC (BCC) Atoms touch each other along cube diagonals (closed-pack. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. ex: Cr, W, Fe (a), Tantalum, Molybdenum Coordination # = 8 (Courtesy P.M. Anderson) Adapted from Fig. 3.2, Callister & Rethwisch 8e. 2 atoms/unit cell: 1 center + 8 corners x 1/8 (Source: 20

21 ATOMIC PACKING FACTOR: BCC APF for a body-centered cubic structure = a a 2 a Adapted from Fig. 3.2(a), Callister & Rethwisch 8e. atoms unit cell APF = R 2 a 4 3 p ( 3 a/4 ) 3 a 3 Close-packed directions: length = 4R = 3 a volume unit cell volume atom 21

22 THEORETICAL DENSITY, r Density = r = r = Mass of Atoms in Unit Cell Total Volume of Unit Cell n A V C N A where n = number of atoms/unit cell A = atomic weight V C = Volume of unit cell = a 3 for cubic N A = Avogadro s number = x atoms/mol 22

23 Theoretical Density, r Ex: Cr (BCC) A = g/mol R = nm n = 2 atoms/unit cell Adapted from Fig. 3.2(a), Callister & Rethwisch 8e. volume unit cell atoms unit cell r = R a a x a = 4R/ 3 = nm g mol r theoretical r actual atoms mol = 7.18 g/cm 3 = 7.19 g/cm 3 23

24 Densities of Material Classes In general r metals > r ceramics > r polymers Why? Metals have... close-packing (metallic bonding) often large atomic masses Ceramics have... less dense packing often lighter elements Polymers have... low packing density (often amorphous) lighter elements (C,H,O) Composites have... intermediate values r (g/cm 3 ) Metals/ Alloys Platinum Gold, W Tantalum Silver, Mo Cu,Ni Steels Tin, Zinc Titanium Aluminum Magnesium Graphite/ Ceramics/ Semicond Polymers Data from Table B.1, Callister & Rethwisch, 8e. Composites/ fibers B ased on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers in an epoxy matrix). Zirconia Al oxide Diamond Si nitride Glass - soda Concrete Silicon G raphite PTFE Silicone PVC PET PC H DPE, PS PP, LDPE Glass fibers GFRE* Carbon fibers CFRE * A ramid fibers AFRE * Wood 24

25 SUMMARY Atoms may assemble into crystalline or amorphous structures. Common metallic crystal structures are SC, FCC, BCC, and HCP. We can predict the density of a material, provided we know the atomic weight, atomic radius, and crystal geometry (e.g., FCC, BCC, HCP). 25

26 Summary: CHARACTERISTICS OF METALS Crystalline structures in the solid state, almost without exception BCC, FCC, or HCP unit cells Atoms held together by metallic bonding Properties: high strength and hardness, high electrical and thermal conductivity FCC metals are generally ductile

27 Summary: CHARACTERISTICS OF CERAMICS Most ceramics have crystalline structures, while glass (SiO 2 ) is amorphous Molecules characterized by ionic or covalent bonding, or both Properties: high hardness and stiffness, electrically insulating, refractory, and chemically inert

28 Summary: CHARACTERISTICS OF POLYMERS Many repeating mers in molecule held together by covalent bonding Polymers usually carbon plus one or more other elements: H, N, O, and Cl Amorphous (glassy) structure or mixture of amorphous and crystalline Properties: low density, high electrical resistivity, low thermal conductivity, strength and stiffness vary widely

29 ANNOUNCEMENTS Answer: Why water expands upon freezing? Core problems to be solved: 2.23, 3.2, 3.3, 3.7, 3,8, and 3.9. Next lecture: Single crystals & polycrystals Crystal Systems 29