Chapter 12 Metals. crystalline, in which particles are in highly ordered arrangement. (Have MP.)

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1 Chapter 12 Metals 12.1 Classification of Solids Covalent Ionic Molecular Metallic Solids Solids Solids Solids Molecular consist of molecules held next to each other by IMF s. Relatively low to moderate MP s and soft. (Weak IMF s.) Ex. CO2(s) (dry ice), H2O(s), glucose(s) (C6H12O6). Covalent Network Atoms held together by covalent bonds in a huge network. Ex. Diamond, graphite, quartz (SiO2). Very hard, very high MP. Ionic Solids consist of ions held together by ionic bonds (strong electrostatic attractions). High MP s, brittle, do not conduct electricity. Metallic Solids Consist of metal + cores held together by a sea of electrons. Conduct electricity, range of MP s. Malleable (changing position of ionic cores does not affect bonding) Structures of Solids We have two main groups: crystalline, in which particles are in highly ordered arrangement. (Have MP.) amorphous, in which there is no particular order in the arrangement of particles. (No definite MP.) Crystal Lattices and Unit Cells The arrangement of the particles in a crystalline solid is called the crystal lattice. The smallest unit that shows the pattern of arrangement for all the particles is called the unit cell. Starting anywhere within the crystal results in the same unit cell. Unit cells are 3-dimensional: usually containing 2 or 3 layers of particles. Unit Cells Unit cells are repeated over and over to give the macroscopic crystal structure of the solid. Each particle in the unit cell is called a lattice point. Lattice planes are planes connecting equivalent points in unit cells throughout the lattice. 1

2 Crystal Lattices Within each major lattice type, additional types are generated by placing lattice points in the center of the unit cell or on the faces of the unit cell. Unit Cells The number of other particles each particle is in contact with is called its coordination number. Higher coordination number means more interaction, therefore stronger attractive forces holding the crystal together. The packing efficiency is the percentage of volume in the unit cell occupied by particles: the higher the coordination number, the more efficiently the particles are packing together 12.3 Metallic Solids Motif refers to an atom or group of atoms associated with each lattice point. The structures of many metals conform to one of the cubic unit cells. Cubic Structures One can determine how many atoms are within each unit cell which lattice points the atoms occupy. Cubic Unit Cells All 90 angles between corners of the unit cell. The length of all the edges are equal. If the unit cell is made of spherical particles: ⅛ of each corner particle is within the cube ½ of each particle on a face is within the cube ¼ of each particle on an edge is within the cube V cube = l 3 ; V sphere = 4 3 πr3 Edge length in terms of r: Simple Cubic l = 2r Coordination Number is 6. 2

3 Edge length in terms of r: Body-Centered Cubic c 2 = b 2 + l 2 b 2 = l 2 + l 2 = 2l 2 c = 4r (4r) 2 = 2l 2 + l 2 = 3l 2 l 2 = (4r)2 3 l = 4r 3 Coordination Number is 8. Edge length in terms of r: Face-Centered Cubic b 2 = l 2 + l 2 = 2l 2 b = 4r (4r) 2 = 2l 2 l 2 = (4r)2 2 l = 4r 2 = 2r 2 Coordination Number is 12. Packing Efficiency It is not possible to pack spheres together without leaving some void spaces between the spheres. Packing efficiency is the fraction of space in a crystal that is actually occupied by the atoms. Packing efficiency = V atoms in cell V cell Simple Cubic: V sphere = 4 3 πr3 l = 2r ; V simple cubic = l 3 = 8r 3 Packing Efficiency = 1 atom per cell 4 3 πr3 8r 3 Packing Efficiency = 0.52 or 52% 3

4 Cubic Cell Name Atoms per Unit Cell Structure Coordination Number Edge length in terms of r Packing efficiency Simple Cubic 1 6 2r 52 % 4r Body-centered Cubic % Face-centered Cubic r 74 % Q. Calculate the density of Al if it crystallizes in a fcc and has a radius of 143 pm. Closest-Packed Structures First Layer With spheres, it is more efficient to offset each row in the gaps of the previous row than to line up rows and columns. Closest-Packed Structures Second Layer The second layer atoms can sit directly over the atoms in the first layer called an AA pattern. Or the second layer can sit over the holes in the first layer called an AB pattern. Closest-Packed Structures Third Layer with Offset 2 nd Layer The third layer atoms can align directly over the atoms in the first layer called an ABA pattern. Or the third layer can sit over the uncovered holes in the first layer called an ABC pattern. 4

5 Alloys Alloys are combinations of two or more elements, the majority of which are metals: a base metal and alloying materials Alloys show metallic properties. Most common physical properties of alloys are often averages of the component metals. But engineering properties may be quite different than the components (may be tailored ): like tensile strength and shear strength Most melt over a large temperature range rather than having a fixed melting point. Alloy Composition Some alloys are solid solutions with variable composition: steel = Fe, C, and other metals brass = Cu and Zn bronze = Cu and Sn Some have fixed composition like a compound: intermetallic compounds are solids with different crystal structures than any of their components alnico used for its magnetic properties NiTi memory metal Types of Alloys In substitutional alloys one metal atom substitutes for another: crystal structure may stay the same or change brass In interstitial alloys an atom fits in-between the metal atoms in the crystal: usually a small nonmetal atom at low concentrations it is best described as a solution at high concentrations it is better described with a stoichiometric formula Interstitial Alloys H, B, N, C can often fit in the holes in a closest packed structure. Formula of alloy depends on the number and type of holes occupied. Interstitial alloys are often harder and denser than the base metals because more of the space in the crystal is occupied. Octahedral Hole: an atom with maximum radius 41.4% of the metal atom s radius can fit in an octahedral hole. 5

6 12.4 Metallic Bond Metals are shiny, malleable, ductile and good conductors of heat and electricity. They actually feel cold because of the high heat conductivity. All these properties are predicted by the electron-sea model. Electron-sea model The low ionization energy of metals allows them to lose electrons easily. Metal atoms release their valence electrons to be shared by all to atoms/ions in the metal: an organization of metal cation islands in a sea of electrons electrons delocalized throughout the metal structure Bonding results from attraction of the cations for the delocalized electrons. According to the Electron Sea Model Since e s are able to move through the structure, this model predicts metallic solids should conduct electricity well (metals do!). If T increases, the metal ions vibrate faster and it is more difficult for the e s to travel through the crystal, and electrical conductivity should decrease (this does happen!). Heat (K.E.) is conducted as particles move and collide with one another. Light e s moving through the solid transfer K.E. faster, so metals are good conductors of heat. The bonding does not depend on the exact positioning of the atoms, which makes metals malleable and ductile. The model does work However The e s sea model doesn t explain ALL properties of metals. For instance: It predicts that more valence e s higher m.p. due to stronger attractions, but this is NOT the case. In actuality, the m.p. increases and then decreases as the MM (and # of valence electrons) increases. Molecular-Orbital Approach When two atomic orbitals combine they produce both a bonding and an antibonding molecular orbital. As the number of atoms in a chain increases, the energy gap between molecular orbitals (MOs) essentially disappears, and continuous bands of energy states result. 6

7 Reality is a bit more complex This is the electronic structure of Nickel. Notice that: 1. There is a band structure. 2. Not all molecular orbitals are filled. For metals: Occupied orbitals are very close in energy to unoccupied orbitals. e s are easily excited to higher levels. When e s are in unoccupied orbitals, they are free to move. Unlike other metals, group 6B (Cr, Mo, W) have all their bonding orbitals completely filled: this leads to maximum bonding interactions (most stable): highest mp, hardest, etc. Transition metals before the 6B group have bonding orbitals unfilled (less interactions), and those after the 6B group have electrons in antibonding orbitals (detracting from bonding). The difference in energy between the valence band and conduction band is called the band gap: the larger the band gap, the fewer electrons there are with enough energy to make the jump As the temperature rises, some of the electrons may acquire enough energy to jump to the conduction band. The more electrons at any one time that a substance has in the conduction band, the better conductor of electricity it is. Types of Band Gaps and Conductivity Diamond band gap = 580 kj/mol (huge). Doesn t conduct requires to much energy to excite e. Metalloids (Si, Ge) have smaller band gap. They may conduct in the presence of an energy source (heat, light) Doping Semiconductors Doping is adding impurities to the semiconductor s crystal to increase its conductivity. Goal is to increase the number of electrons in the conduction band. n-type semiconductors do not have enough electrons themselves to add to the conduction band, so they are doped by adding electron-rich impurities. p-type semiconductors are doped with an electron-deficient impurity, resulting in electron holes in the valence band. Electrons can jump between these holes in the valence band, allowing conduction of electricity. Diodes When a p-type semiconductor adjoins an n-type semiconductor, the result is an p-n junction. Electricity can flow across the p-n junction in only one direction this is called a diode. This also allows the accumulation of electrical energy called an amplifier. 7