Classes of materials

Transcription

1 Classes of materials POLYMERS CERAMICS METALS DUCTILITY Varies Poor Good CONDUCTIVITY (ELECTRICAL & THERMAL) Low Low High HARDNESS/STRENGTH Low medium Very high Medium high CORROSION RESISTANCE Fair good Good Fair poor STIFFNESS Low High Fair FRACTURE TOUGHNESS Low medium Low High MACHINABILITY Good Poor Good

2 Why study bonding? Because the properties of materials (strength, hardness, conductivity, etc..) are determined by the manner in which atoms are connected. Also by how the atoms are arranged in space Crystal. Structure What determines the nature of the chemical bond between atoms? Electronic structure (distribution of electrons in atomic orbitals) Number of electrons and electronegativity (tendency for an atom to attract an electron)

3 BOHR ATOM orbital electrons: n = principal quantum number n=3 2 1 Adapted from Fig. 2.1, Callister 6e. Nucleus: Z = # protons = 1 for hydrogen to 94 for plutonium N = # neutrons Atomic mass A Z + N

4 ELECTRON ENERGY STATES Electrons... have discrete energy states tend to occupy lowest available energy state. Increasing energy n=4 n=3 n=2 n=1 4s 3s 2s 1s 4p 3p 2p 3d s, p,d and f signify the subshells which the electrons occupy. Different types of subshells have different numbers of energy states Within each energy state there are two possible spin orientations Remember that n is the principal quantum number

5 Stable electron configurations... have complete s and p subshells are unreactive. Z Element Configuration 2 He 1s 2 10 Ne 1s 2 2s 2 2p 6 18 Ar 1s 2 2s 2 2p 6 3s 2 3p 6 Adapted from Table 2.2, Callister 6e. 36 Kr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 Valence electrons are the electrons that occupy the outermost filled shell.

6 Most elements: Electron configuration not stable. Element Hydrogen Helium Lithium Beryllium Boron Carbon... Neon Sodium Magnesium Aluminum... Argon... Krypton SURVEY OF ELEMENTS Atomic # Electron configuration 1s 1 1s 2 (stable) 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p s 2 2s 2 2p 6 (stable) 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 1s 2 2s 2 2p 6 3s 2 3p 1... Adapted from Table 2.2, Callister 6e. 1s 2 2s 2 2p 6 3s 2 3p 6 (stable)... 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s (stable) Why? Valence (outer) shell usually not filled completely.

7 Increasing Electronegativity

8 give up 1e give up 2e give up 3e H Li Be Na K Rb Cs Fr Mg Ca Sr Ba Ra Sc Y THE PERIODIC TABLE Columns: Similar Valence Structure Metal Nonmetal Intermediate accept 2e accept 1e O S Se Te Po F Cl Br I At inert gases He Ne Ar Kr Xe Rn Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions.

9 ELECTRONEGATIVITY Ranges from 0.7 to 4.0, Large values: tendency to acquire electrons. Smaller electronegativity Larger electronegativity

10 IONIC BONDING Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Na (metal) unstable electron Cl (nonmetal) unstable Na (cation) stable Stable because the s and p subshells are filled! + - Coulombic Attraction Cl (anion) stable

11 EXAMPLES: IONIC BONDING Predominant bonding in Ceramics H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 NaCl MgO CaF2 CsCl Ni 1.8 Zn 1.8 As 2.0 O 3.5 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He - Ne - Ar - Kr - Xe - Rn - Give up electrons Acquire electrons Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

12 COVALENT BONDING Requires shared electrons Example: CH4 C: has 4 valence e, needs 4 more CH4 H shared electrons from carbon atom H: has 1 valence e, needs 1 more Electronegativities are comparable. H C H H Adapted from Fig. 2.10, Callister 6e. shared electrons from hydrogen atoms

13 EXAMPLES: COVALENT BONDING H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 H2 Ti 1.5 Cr 1.6 Fe 1.8 H2O C(diamond) SiC Ni 1.8 Zn 1.8 Ga 1.6 column IVA C 2.5 Si 1.8 Ge 1.8 Sn 1.8 Pb 1.8 As 2.0 GaAs Molecules of nonmetals Molecules of metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA) O 2.0 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He - Ne - Ar - Kr - Xe - Rn - F2 Cl2 It is possible for bonds to be partially covalent and partially ionic in nature. Look in Chapter 2 to see how to evaluate this aspect of bonds

14 METALLIC BONDING Arises from a sea of donated valence electrons (1, 2, or 3 from each atom) Adapted from Fig. 2.11, Callister 6e. Primary bond for metals and their alloys 12

15 SECONDARY BONDING Arises from interaction between dipoles Fluctuating dipoles asymmetric electron ex: liquid H2 clouds H2 H secondary Permanent dipoles-molecule induced -general case: bonding + - secondary bonding H H Adapted from Fig. 2.13, Callister 6e. + - H secondary bonding H -ex: liquid HCl secondary H Cl bonding H Cl -ex: polymer secondary bonding

16 SUMMARY: BONDING Type Ionic Covalent Metallic Secondary Bond Energy Large! Variable large-diamond small-bismuth Variable large-tungsten small-mercury smallest Comments Nondirectional (ceramics) Directional semiconductors, ceramics polymer chains) Nondirectional (metals) Directional inter-chain (polymer) inter-molecular 14

17 Bonding Forces and Energies Bond length, r repulsive, attractive, and net forces repulsive, attractive, and net energies Net force is given by the sum of an attractive force and a repulsive force Potential is given by the integral of the net force curve with respect to distance: E= F dr Note: equilibrium separation occurs where the net force = 0 and the energy is at a minimum. F r F

18 Bonding Forces and Energies Bonding energy: Minimum of the potential vs. distance curve. Indicates how much energy must be supplied to completely disassociate the two atoms Depth of the potential well indicates bonding strength Deep well strongly bonded Shallow well weakly bonded Energy (r) unstretched length r o r Eo= bond energy

19 PROPERTIES FROM BONDING: T M Bond length, r F r F Melting Temperature, Tm Energy (r) Bond energy, Eo Energy (r) r o smaller Tm r unstretched length r o r Eo= bond energy larger Tm Tm is larger if Eo is larger. 15

20 Example: Bonding energy and T M Use the data below to estimate the bonding energy of copper which has a melting temperature of 1084 C. E 0, ev/atom T M, C Hg Al Fe W tungsten

21 Melting Temperature (C) Solution: Plot the data E 0 =3.6 ev/atom Bonding Energy (ev/atom) With this analysis we estimate E 0 of copper = 3.6 ev/atom. The measured value is 3.5 ev/atom.

22 PROPERTIES FROM BONDING: E Elastic modulus, E length, Lo undeformed deformed ΔL cross sectional area Ao F F ΔL A = E o L o Elastic modulus E ~ curvature at ro Energy unstretched length r o r E is larger if Eo is larger. smaller Elastic Modulus larger Elastic Modulus 16

23 PROPERTIES FROM BONDING: α Coefficient of thermal expansion, α length, Lo unheated, T1 heated, T2 ΔL coeff. thermal expansion ΔL Lo = α (T 2 -T 1 ) α ~ symmetry at ro Energy r o larger α r α is larger if Eo is smaller. smaller α 17

24 Bonding Types: Summary A comparison of the type of bonding found in different materials: For brass, the bonding is.. metallicsince it is a metal alloy. For rubber, the bonding is covalent with some. Van der Waals Rubber is composed primarily of carbon and hydrogen atoms secondary bonding For BaS, the bonding is predominantly ionic.. (but with some covalent character) on the basis of the relative positions of Ba and S in the periodic table. For solid xenon, the bonding is Van. der Waals since xenon is an inert gas. For nylon, the bonding is covalent.. with perhaps some.. Van der Waals Nylon is composed primarily of carbon and hydrogen For AlPthe bonding is predominantly covalent (but with some ionic character) on the basis of the relative positions of Al and P in the periodic table.

25 SUMMARY: PRIMARY BONDS Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Large bond energy large Tm large E small α Variable bond energy moderate Tm moderate E moderate α Polymers (Covalent & Secondary): secondary bonding Directional Properties Secondary bonding dominates small T small E large α 18