Ch. 13 The Chemistry of Solids 13.1 Crystal Lattices and Unit Cells In gases and liquids, molecules move continually and randomly, but in solids they cant change their relative positions and so a regular repeating pattern within the structure is a characteristic of most solids Properties of Solids molecules, atoms, or ions locked into a crystal lattice particles are CLOSE together these exhibit strong intermolecular forces highly ordered, rigid, incompressible Crystal Lattices unit cells are the smallest repeating internal unit that has the symmetry characteristic of the solid Cubic Unit cells there are 7 basic crystal systems, but we will only be concerned with the cubic form All angles are 90 degrees All sides are equal length of each atom on a corner is within the cube of each atom on a face is within the cube of each atom on a side is within the cube Unit cells are important because (1) a # of metals, ionic solids, and intermetallic cpds crystallize in cubic unit cells and (2) it is relatively easy to do calculations with these unit cells. So we can easily calculate the volume if given the cell edge (a) because a3 is the volume of the cube We shall look at the three variations of the cubic crystal system (simple or primitive, body-centered, face-centered) Simple or primitive the balls represent the positions of atoms, ion, or molecule in a simple cubic unit cell each atom, ion, or molecule at a corner is shared by 8 unit cells (8[]=1 atom / cell) Body-Centered Cubic has an additional atom, ion, or molecule in the center of the unit cell On a body-centered cubic unit cell there are 8 corners + 1 particle in the center contains 2 particles Face Centered Cubic has a cubic unit cell structure with an extra atom, ion, or molecule in each face 8 corners + 6 faces= 4 atoms / unit cell # of atoms/Unit cellsvolradius

Simple cubic1 atom/unit cellCell edge = 2 atomic radiia3a/ 2 = r

Body-centered2 atoms/unit cells3 x a = 4 atomic radiia3a3/ 4 = r

Face-centered4 atoms/unit cell2 x a = 4 atomic radiia3a2/4= r

13.2 Structures and Formulas of Ionic Solids CsCl Ionic Solids have ions that occupy the positions in the unit cell Octahedral Holes: Na + are said to be in octahedral holes because surrounded by 6 Cl- and makes an 8 sided crystal Tetrahedral holes: ion surrounded by 4 oppositely charged ions 13.3 Bonding in Metals and Semiconductors Molecular Orbital theory was introduced in ch 9 to rationalize covalent bonding in molecules MO theory can also be used to describe metallic bonding metals can be thought of as a supermolecule metallic bonding is described as delocalized: the electrons are associated with all the atoms in the crystal are not associated with specific bonded atoms this theory of metallic bonding is called bond theory Bonding in Metals and Semiconductors An energy level diagram shows the bonding and antibonding molecular orbitals blending together into a band of molecular orbitals Band Theory Fermi level: highest filled level at OK Molecular orbitals are constructed from the valence orbitals on each atom and are delocalized over all the atoms. When sufficient energy is added, electrons are excited to the conduction band. Band of energy levels in a metal is essentially continuous that is the energy gap between levels are extremely small. So electron can move to higher energy state (absorb energy) then can emit a photon to return electron to original level. This rapid and efficient absorption and emission of light make polished metal surfaces be reflective and shiny Metals are shiny (lustrous), malleable, and ductile Semiconductors Band gap: a barrier to the promotion of electrons to a higher energy level GROUP 4A: orbitals of valence band are completely filled, but the conduction band is empty Intrinsic semiconductors: naturally occurring property of the pure material Extrinsic semiconductors: conductivity of material is changed by adding/doping the material with a different element p-type: positive holes are created n-type: negative holes are created 13.4 Bonding in Ionic Compounds:Lattice Energy Reminder: Ionic compounds have high melting points because ion-ion interactions are so strong Lattice Energy Ionic cpds held together by extensive attractions between ions of opposite charge and repulsion between ions of like charges(Coulombs law) Dissolution of Solids in Liquids The energy released when a mole of formula units of a solid is formed from its constituent ions in the gas phase is called the crystal lattice energy the crystal lattice energy is a measure of the attractive forces in a solid the crystal lattice energy increases as the charge density increases (ionic charge/radius) always negative more negative = more strong attractions Calculating a Lattice Enthalpy from Thermodynamic Data Hesss law 13.5 The Solid State: Other Types of Solid Materials Solids can be classified on the basis of the bonds that hold the atoms or molecules together molecular network (covalent) ionic metallic Molecular characterized by relatively strong intramolecular bonds between the atoms that form the molecules the intermolecular forces between these molecule are much weaker than the bonds because the intermolecular forces are relatively weak, molecular solids are often soft with low melting points Network Solids (Covalent) conventional chemical bonds hold the chemical subunits together the bonding between chemical subunits is identical to that within the subunits resulting in a continuous network of chemical bonds 3 examples are diamond, graphite, and quartz cannot detect discrete molecules composed of a 3D array of covalently bonded atoms most are hard and rigid and have high mp and bp Ionic Solids salts that are held together by the strong force of attraction between ions of opposite charge because this force of attraction depends of the square of the distance between the positive and negative charges, the strength of the bond depends on the radii of the ions that form the solid as these ions become larger, the bond becomes weaker Metallic Solids metal atoms dont have enough electrons to fill their valence shells by sharing electrons with their immediate neighbors electrons in the valence shell are therefore shared by many atoms instead of just two in effect, the valence electrons are delocalized over many many atoms. because these electrons arent tightly bound to individual atoms, they are free to migrate through the metal, as a result, metals are good conductors Amorphous Solids do not have a regular structure glass; melt over a range of temperature and breaks into randomly shaped pieces 13.6 Phase Changes Involving Solids Melting: Solid Liquid the melting point of a solid is the temperature at which the lattice collapses into a liquid. Like any phase change, melting requires energy (enthalpy of fusion) Trends: Mp increases within a series of related molecules as the size and mass increases nonpolar substances that form molecular solids have low mp. ionic have higher due to strong ion-ion intermolecular forces Sublimation: Conversion of Solid to Vapor solid vapor without passing through the liquid phase 13.7 Phase Diagrams Used to show the relationship between phases of matter and the P and T Phase diagrams only apply to closed systems Triple point: the condition at which all phases coexist P vs T: displays all of the different phase transitions of a substance Triple point: one point at which all 3 phases of a substance solid, liquid, gas can coexist at equilibrium Critical T: is the T above which a gas cannot be liquefied, i.e. the T above which the liquid and gas do not exist as distinct phases Critical P: is the P required to liquefy a gas at its critical T Critical point: is the combination of critical T and critical P Supercritical fluid: a substance at a T above its critical T