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Electronic structure Bonding State of aggregation Octet stability Primary: 1.Ionic 2.Covalent 3.Metallic 4.Van der Waals Secondary: 1.Dipole-dipole 2.London dispersion 3.Hydrogen Gas Liquid Solid

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GAS LIQUID SOLID The particles move rapidly Large space between particles The particles move past one another The particles close together Retains its volume The particles are arranged in tight and regular pattern The particles move very little Retains its shape and volume

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CLASSIFICATION OF SOLID BY ATOMIC ARRANGEMENT Ordered regular long-range crystalline “crystal” transparent Disordered random* short-range* amorphous “glass” opaque Atomic arrangement Order Name

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EARLY CRYSTALLOGRAPHY ROBERT HOOKE (1660) : canon ball Crystal must owe its regular shape to the packing of spherical particles (balls) packed regularly, we get long- range order. NEILS STEENSEN (1669) : quartz crystal All crystals have the same angles between corresponding faces, regardless of their sizes he tried to make connection between macroscopic and atomic world. If I have a regular cubic crystal, then if I divide it into smaller and smaller pieces down to an atomic dimension, will I get a cubic repeat unit?

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RENĖ-JUST HAŪY (1781): cleavage of calcite Common shape to all shards: rhombohedral Mathematically proved that there are only 7 distinct space-filling volume elements 7 CRYSTAL SYSTEMS

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CRYSTALLOGRAPHIC AXES 3 AXES4 AXES yz = xz = xy = yz = 90 xy = yu = ux = 60

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THE SEVEN CRYSTAL SYSTEMS

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(rombhohedral)

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SPACE FILLING TILING

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AUGUST BRAVAIS (1848): more math How many different ways can we put atoms into these 7 crystal systems and get distinguishable point environment? He mathematically proved that there are 14 distinct ways to arrange points in space 14 BRAVAIS LATTICES

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The Fourteen Bravais Lattices

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Simple cubic Body-centered cubic Face-centered cubic 1 1 3 3 2 2

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4 4 Simple tetragonal Body-centered tetragonal 5 5

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Simple orthorhombicBody-centered orthorhombic 6 6 7 7

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8 8 9 9

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10 12 11

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14 13 Hexagonal

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A point lattice Repeat unit

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z x y A unit cell O b a c a, b, c , , Lattice parameters

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CRYSTAL STRUCTURE (Atomic arrangement in 3 space) BRAVAIS LATTICE (Point environment) BASIS (Atomic grouping at each lattice point)

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EXAMPLE: properties of cubic system *) BRAVAIS LATTICE BASISCRYSTAL STRUCTURE EXAMPLE FCCatomFCCAu, Al, Cu, Pt moleculeFCCCH 4 ion pair (Na + and Cl - ) Rock saltNaCl Atom pairDC (diamond crystal) Diamond, Si, Ge C C 109 *) cubic system is the most simple most of elements in periodic table have cubic crystal structure

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CRYSTAL STRUCTURE OF NaCl

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CHARACTERISTIC OF CUBIC LATTICES SCBCCFCC Unit cell volumea3a3 a3a3 a3a3 Lattice point per cell124 Nearest neighbor distancea a 3 / 2a/ 2 Number of nearest neighbors (coordination no.) 6812 Second nearest neighbor distance a2a2 aa Number of second neighbor1266 a = f(r)2r 4/ 3 r2 2 r or 4r = a4a4a3a3a2a2 Packing density0.520.680.74

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EXAMPLE: FCC FCC 74% matter (hard sphere model) 26% void

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In crystal structure, atom touch in one certain direction and far apart along other direction. There is correlation between atomic contact and bonding. Bonding is related to the whole properties, e.g. mechanical strength, electrical property, and optical property. If I look down on atom direction: high density of atoms direction of strength; low density/population of atom direction of weakness. If I want to cleave a crystal, I have to understand crystallography.

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CRYSTALLOGRAPHIC NOTATION POSITION: x, y, z, coordinate, separated by commas, no enclosure O: 0,0,0 A: 0,1,1 B: 1,0,½ B A z x y Unit cell O a

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DIRECTION: move coordinate axes so that the line passes through origin Define vector from O to point on the line Choose the smallest set of integers No commas, enclose in brackets, clear fractions OB1 0 ½[2 0 1] AO0 -1 -1 B A z x y Unit cell O

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Denote entire family of directions by carats e.g. all body diagonals:

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all face diagonals: all cube edges:

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For describing planes. Equation for plane: where a, b, and c are the intercepts of the plane with the x, y, and z axes, respectively. Let: so that No commas, enclose in parenthesis (h k l) denote entirely family of planes by brace, e.g. all faces of unit cell: {0 0 1} etc.

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MILLER INDICES a b c Intercept at Intercept at a/2 Intercept at b Miller indices: (h k l) (2 1 0) Parallel to z axes (h k l) [h k l] [2 1 0] (2 1 0)

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Miller indices of planes in the cubic system (0 1 0) (0 2 0)

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Many of the geometric shapes that appear in the crystalline state are some degree symmetrical. This fact can be used as a means of crystal classification. The three elements of symmetry: Symmetry about a point (a center of symmetry) Symmetry about a line (an axis of symmetry) Symmetry about a plane (a plane of symmetry)

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SYMMETRY ABOUT A POINT A crystal possesses a center of symmetry when every point on the surface of the crystal has an identical point on the opposite side of the center, equidistant from it. Example: cube

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If a crystal is rotated 360 about any given axis, it obviously returns to its original position. If the crystal appears to have reached its original more than once during its complete rotation, the chosen axis is an axis of symmetry. SYMMETRY ABOUT A LINE

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DIAD AXIS TRIAD AXIS TETRAD AXIS HEXAD AXIS AXIS OF SYMMETRY Rotated 180 Twofold rotation axis Rotated 120 Threefold rotation axis Rotated 90 Fourfold rotation axis Rotated 60 Sixfold rotation axis

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THE 13 AXES OF SYMMETRY IN A CUBE

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A plane of symmetry bisects a solid object in a such manner that one half becomes the mirror image of the other half in the given plane. A cube has 9 planes of symmetry: SYMMETRY ABOUT A PLANE THE 9 PLANES OF SYMMETRY IN A CUBE

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Cube (hexahedron) is a highly symmetrical body as it has 23 elements of symmetry (a center, 9 planes, and 13 axis). Octahedron also has the same 23 elements of symmetry.

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ELEMENTS OF SYMMETRY

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Combination forms of cube and octahedron

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IONIC COVALENT MOLECULAR METALLIC SOLID STATE BONDING Composed of ions Held by electrostatic force Eg.: NaCl Composed of neutral atoms Held by covalent bonding Eg.: diamond Composed of molecules Held by weak attractive force Eg.: organic compounds Comprise ordered arrays of identical cations Held by metallic bond Eg.: Cu, Fe

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Two or more substances that crystallize in almost identical forms are said to be isomorphous. Isomorphs are often chemically similar. Example: chrome alum K 2 SO 4.Cr 2 (SO 4 ) 3.24H 2 O (purple) and potash alum K 2 SO 4.Al 2 (SO 4 ) 3.24H 2 O (colorless) crystallize from their respective aqueous solutions as regular octahedral. When an aqueous solution containing both salts are crystallized, regular octahedral are again formed, but the color of the crystals (which are now homogeneous solid solutions) can vary from almost colorless to deep purple, depending on the proportions of the two alums in the solution.

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CHROME ALUM CRYSTAL

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A substance capable of crystallizing into different, but chemically identical, crystalline forms is said to exhibit polymorphism. Different polymorphs of a given substance are chemically identical but will exhibit different physical properties, such as density, heat capacity, melting point, thermal conductivity, and optical activity. Example:

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ARAGONITE

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CRISTOBALITE

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Polymorphic Forms of Some Common Substances

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Material that exhibit polymorphism present an interesting problem: 1.It is necessary to control conditions to obtain the desired polymorph. 2.Once the desired polymorph is obtained, it is necessary to prevent the transformation of the material to another polymorph. Polymorph 1 Poly- morph 2 Poly- morph 2 Polymorphic transition

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In many cases, a particular polymorph is metastable Transform into more stable state Relatively rapid infinitely slow Carbon at room temperature Diamond (metastable) Graphite (stable)

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POLYMORPH MONOTROPIC ENANTIOTROPIC One of the polymorphs is the stable form at all temperature Different polymorphs are stable at different temperature The most stable is the one having lowest solubility

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In nature perfect crystals are rare. The faces that develop on a crystal depend on the space available for the crystals to grow. If crystals grow into one another or in a restricted environment, it is possible that no well-formed crystal faces will be developed. However, crystals sometimes develop certain forms more commonly than others, although the symmetry may not be readily apparent from these common forms. The term used to describe general shape of a crystal is habit.

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Crystal habit refers to external appearance of the crystal. A quantitative description of a crystal means knowing the crystal faces present, their relative areas, the length of the axes in the three directions, the angles between the faces, and the shape factor of the crystal. Shape factors are a convenient mathematical way of describing the geometry of a crystal. If a size of a crystal is defined in terms of a characteriza- tion dimension L, two shape factors can be defined: Volume shape factor: V = L 3 Area shape factor: A = L 2

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Some common crystal habits are as follows. Cubic - cube shapes Octahedral - shaped like octahedrons, as described above. Tabular - rectangular shapes. Equant - a term used to describe minerals that have all of their boundaries of approximately equal length. Fibrous - elongated clusters of fibers. Acicular - long, slender crystals. Prismatic - abundance of prism faces. Bladed - like a wedge or knife blade Dendritic - tree-like growths Botryoidal - smooth bulbous shapes

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Internal structureExternal habit TabularPrismaticAcicular External shape of hexagonal crystal displaying the same faces

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Crystal habit is controlled by: 1.Internal structure 2.The conditions at which the crystal grows (the rate of growth, the solvent used, the impurities present) Variation of sodium chlorate crystal shape grown: (a) rapidly; (b) slowly

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(a)(b) Sodium chloride grown from: (a) pure solution; (b) Solution containing 10% urea

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