Chapter 13 Reading Notes 01 14 2015 13 1 Solids unlike gases and liquids cannot change their relative postions Unit cell smallest repeating unit that has all of the symmetry characteristics of the ways the atoms ions or molecules are arranged in the solid Lattice points corners of the cube geometric object that constitutes the unit cell Crystal lattice 3D assemblage of unit cells Cubic unit cell 1 of the 7 3D unit cells and the most common Primitive cubic pc 1 8 of each corner atom inside cell and 8 corners 1 Body centered cubic bcc 1 atom in the center and 1 8 around each atom total corner 2 atoms total Face centered cubic fcc 8 corner atoms with 1 8 within unit cell 6 atoms on 6 faces each atom is way in unit cell 4 atoms total 13 2 Structures and formulas of ionic solids Ionic compounds are often made by placing ions of opposite charges into holes with in the lattice structure Rules for types of holes 1 M n ions occupy the cubic hole in a primitive cubic x n lattice CsCl 2 M n ions in all the octahedral holes in a face centered cubic X n lattice ex NaCl 3 M n ions occupying of the tetrahydral holes in a face centered cube lattice of X n ions ZnS 13 3 MO theory can describe metallic bonding Delocalized bonding electrons are associated with all atoms in a crystal and not a specific bond between 2 atoms Band theory energy level diagram shows bonding and antibonding MO blending together into a band of MO The individual MO are so close together in energy they are not distinguishable Band is composed of as many MO as atomic orbitals In metals there aren t enough electrons to fill all MO The lowest energy for a system occurs when all electrons are in orbitals with lowest possible energy which is at 0K The highest filled energy level at 0K is the Fermi level Above 0K some electrons will occupy above the Fermi level because of For every electron above the Fermi level there is a positive hole below thermal energy which causes conductivity When the electric field is applied to a metal negative electrons move towards the positive side and positive holes move toward the negative side Band of energy levels is essentially continuous Metals can absorb any wavelength which causes reflexivity and luster Metals are also malleable and ductile Semiconductors have a lower level VB and higher level conduction band that are separated in a band gap Group 4A orbitals filled but conduction band is empty Semiconductors can conduct current because thermal energy allows a few elctrons to the conduction band Electrons in conduction band determine by temp and magnitude of band Extrinsic controlled by adding small amounts of different atoms dopants P type semiconductor positive holes are created ex silicon 4 VE doped with aluminum 3 valence electrons The bond forms a discrete but empty band at an energy level higher than the valence band but lower than the conduction band called the acceptor level because it can accept electrons from the valence band Gap between valance band and acceptor level is usually small s electrons can be promoted readily to acceptor level N type semiconductor negative charge carriers ex silicon 4VE doped gap with phosphorous 5 VE phosphorus atom N type have a negative charge because there is one extra VE for each N type semiconductors have a distrete partially filled donor leel just below the conduction band Electrons can be promoted from the donor band to the conduction band and electrons in the conduction band carry the charge 13 4 It is also possible for group 2B and 6A elements to form semiconducting compounds but the further away elements are on the periodic table the more ionic the bonding becomes As the ionic character increases the band gap increases which causes material to become a insulator Ionic compounds usually have high melting points an indication of the strength of bonding in the crystal lattice Measure of this is in lattice energy Interactions between positive and negative ions is related to Coulombs law q1 q2 d 2 Uionpair C N e N e d C constant D distance between ion centers E charge on electron The enery will always have a negative value This equation shows Energy depends directly on the charges on the ions and inversely on the distance between them When we take into account all interactions between ions in a lattice we calculate lattice energy Lattice energy the energy of formation of one mole of solid crystalline ion compound when ions in the gas phase combine Lattice enthalpy often used when dealing with ionic compounds Same trends are seen in lattice energy and enthalpy Numerical values nearly identical also As given by Coulombs law higher ion charges the greater the attraction between oppositely charged ions o Delta H has a larger neg value for more highly charged ions Also as Coulombs law shows lattice built from smaller ions generally leads to a more neg value for the lattice enthalpy Lattice enthalpies can be calculated with Born Haber cycle an application of Hess s law The Steps 1a Enthalpy of Formation of the neg ion 1b Delta H of the neg ion 2a Enthalpy of formation of cation 2b Delta H of cation 3 Combine standard enthalpy of formation values found from first two steps to calculate Delta H which is the lattice enthalpy 13 5 Molecular Solids Ex H2O CO2 Exist as solids under appropriate conditions Molecules pack in a regular fashion in a 3D crystal lattice The way that the molecules are arranged depends on shape of molecules and IMFs They tend to pack the most efficient way possible and align in ways that will maximize their IMFs Network Solids EX Graphite and Diamond o Graphite carbon atoms bonded in flat sheets that cling weakly to one another o Diamond hardest material and best conductor of heat known These are composed entirely of a 3D array of covalently bonded atoms Most network solids are hard and rigid Have high melting points and boiling points Amorphous Solids Solids that do not have a regular structure EX glass o When heated it softens over a variable temperature range o When glass breaks it leaves randomly shaped pieces 13 6 Melting point of solids the temperature that the lattice collapses and solid is converted to a liquid Melting requires energy endothermic enthalpy of fusion kJ mol Freezing gives off energy exothermic enthalpy of crystallization kj mol Low melting point low enthalpy of fusion and vice versa Transition metals high enthalpies of fusion In general nonpolar subtances that form molecular solids have low MP o Melting points