Factors Affecting the Occurrence of Simple Ionic StructuresSlide 2Calculating Radius Ratio When CN = 3Determining Structure From Radius RatioFig. 12.31Factors Determining Occurrence of Simple Ionic StructuresProblemExplanation of ProblemMacromolecular SolidsTable 12.6Slide 11Slide 12Molecular Liquids & SolidsTunable SolidsSlide 15Slide 16Uses of Tunable SolidsPreparation of Tunable SolidsFig. 12.44Table 12.7Factors Affecting the Occurrence of Simple Ionic StructuresAX compounds CN of cation & anion the same to get electroneutrality AX2 compounds CN = 2 for cation wrt to anion, so AX2 is linearAX3 compounds CN = 3 for cation, so AX3 is trigonal planarAX4 compounds CN = 4 for cation, so AX4 is tetrahedralAX6 compounds CN = 6 for cation, so AX6 is octahedralThese are structures with minimal repulsionFactors Affecting the Occurrence of Simple Ionic StructuresAX8 compounds For minimal repulsion when CN of cation = 8,should get square antiprism structure, butsquare antiprism structure can’t extend indefinitely in 3 dimensions because it is structurally unstable, so a cubic structure is adoptedCalculating Radius Ratio When CN = 3 B CA E DFor CN = 3, get a stable structure when the 3 anions touch the central cation. If cation gets smaller relative to anions, then anions touch each other, and below this limiting lower value for the radius ratio, the structure is unstable because all 3 anions can’t touch the cation. So, the minimum radius ratio for which this structure is stable is:BE/BD = cos 30ora/(ra + rc) = 3/2 rc/ra = 2/3 – 1 = (2 - 3)/3 = (2 – 1.732)/1.732 2 ra = 3 rc + 3 rarc/ra = 0.268/1.732 = 0.1553 rx = 2 ra - 3 ra See Table 6.5 (Limiting Radius Ratios For Various rx = 2/ 3 ra – raStructures) on page 3 of handoutrx = ra(2/3 – 1)Determining Structure From Radius RatioCalculate radius ratio of LiF (see p. 322 of text for ionic radii values)rc+/ra- = 60 pm/136 pm = 0.44 Therefore, we predict LiF has rocksalt (sodium chloride) structure and octahedral coordination of F to Li, i.e., CN of Li is 6 (see Table 6.6 (Radius Ratio Values and Structures of Some Ionic Compounds) on page 3 of handoutFig. 12.31Factors Determining Occurrence of Simple Ionic Structures5. So ionic solids form by close-packing (either hcp or ccp) of anions with cations occupying holes. There are 3 types of holes:a. Trigonal holes (so small they are never occupied in binary ionic solids)b. Td holesc. Oh holes6. Order of hole size: trigonal  Td  Oh7. Whether Td or Oh holes are occupied depends on relative sizes of anion & cation (see Table 10.1 on p. 4 of handout)ProblemMelting Points of Fluorides of Second-Row ElementsNaF MgF2 AlF3 SiF4 PF5 SF6988oC 1266oC 1291oC -90oC -94oC -50oCEXPLAIN THIS TRENDExplanation of Problem1. Marked decrease in ionic character as we move left to right across Period 2 is not the reason for the enormous and sudden change to lower melting points (beginning at SiF4)2. Abrupt change occurs because of a change from an infinite lattice structure in which each aluminum is coordinated to six F atoms to a lattice of discrete SiF4 molecules held together by van der Waals interparticle forces.3. Radius ratio is critical factor in determining structureMacromolecular SolidsProperties1. High m. pts. (often above 1000oC)2. Insoluble in all common solvents3. Poor electrical conductorsExamples1. Carbon(a) exhibits allotropy (allotropes are different physical forms of the same element) 1. Cdiamond 2. Cgraphite 3. C60(b) Diamond & graphite are both macromolcular solidsTable 12.6Macromolecular Solids2. Si & Ge(a) Each has only 1 form, the diamond form3. Silica (silicon(IV) oxide)(a) 3 polymorphs (different physical forms of the same compound), each with a high-temp. & a low-temp. modification 870oC 1470oC -quartz -tridymite -cristobalite -quartz -tridymite -cristobalite 573oC120oC – 160oC200oC – 275oCMacromolecular Solids (b) amorphous silica (not macromolecular)silica powder + H2O  silica gel © glass 1. soft glass SiO2 + Na2CO3 + CaCO3  SiO2/Na2O/CaO + CO2 7:1:1 ratio of oxides 2. hard glassSiO2 + B2O3 + Al2O3 + Na2CO3 + K2CO3  SiO2/B2O3/Al2O3/Na2O/K2O + CO2Pyrex or KimaxMolecular Liquids & Solids•Except for noble gases, all substances that are gases or liquids at 25oC consist of covalently-bonded molecules.•Molecular substances tend to be: 1. volatile w/ appreciable vapor pressures at room T 2. insoluble in H2O but soluble in nonpolar solvents 3. nonconductors of electricity when pure 4. low melting and low boiling, so they are likely either liquids or gases at room T & P•Have very weak IP forces (LDF or dipole-dipole) (Exceptions: those with H-bonds, e.g., H2O, NH3, & HF)Tunable Solids•Have substitutional stoichiometry e.g, when Si & H react, result is a discrete molecular substance, SiH4, with invariable stoichiometry—this is not a tunable solid but when Si & Ge react, result is a substance with variable stoichiometry, SixGe1-x (0<x<1); this is a tunable solid As x varies, many of the physical properties of the solid, such as colors of light absorbed & emitted, varyTunable Solids•SixGe1-x is a disordered substitutional solid, i.e., the Ge & Si randomly replace each other e.g. Si0.283Ge0.717 means there is a 28.3% chance that the atom is Si and a 71.7% chance the atom is Ge•Other substitutional solids 1. cation type e.g. MgxFe1-xO (0<x<1) 2. anion type e.g. CdSxSe1.x (0<x<1)Tunable Solids•Minerals as tunable solids 1. MgxFe2-xSiO4 Extreme examples are:a. Mg2SiO4 (forsterite) b. Fe2SiO4 (fayalite) 2. Ca5(PO4)3(OH)xF1-x Extreme examples are: a. Ca5(PO4)3OH or tooth enamel b. Ca5(PO4)3F or fluoroapatiteUses of Tunable Solids•Tunable solids are used in: 1. semiconductors used in light-emitting diodes and diode lasers 2. metal alloys (e.g., brass) 3. battery electrodesPreparation of Tunable Solids•Tunable solids are prepared by: 1. comelting 2. coprecipitation 3. codeposition from gas or solution phases 4. codeposition of volatile precursor molecules (chemical vapor deposition)Fig. 12.44Table