CHEM 106 1nd Edition Lecture 2 Outline of Last Lecture I. Solids, metallic bonds, unit cells, and alloys.Outline of Current Lecture II. Fitting atoms in hole structuresa. StoichiometryIII. Molecular SolidsIV. Ionic SolidsCurrent LectureChapter 13: Organic Chemistry (continued)What size can fit in the holes?Radius ratio:rnon−hostr host(non-host is smaller)Tetrahedral hole: r = 0.22 – 0.41 smallerOctahedral hole: r = 0.41 – 0.73 largerStoichiometry:1 octahedral hole per host atom2 tetrahedral holes per host atomEx: C (r = 77 pm) in FeC in octahedral holes (other steels incorporate C in homogenously)12.5 Crystalline nonmetalsAllotropes of C diamond + graphite Fig. 12.27Diamond: hard, thermal conductor Graphite: lubricantBoth are network solids (covalent)Molecular Solids – individual covalent molecules crystallized togetherP4S8H2O B12C6012.6 Ionic solids (salts)Unit cell must have the stoichiometry of the compoundAnions are typically much larger than cations, following radius ratio rules, the cation can be in tetrahedral or octahedral holesEx: NaClr Cl−¿rNa+¿¿¿ = 102 pm181 pm = 0.56Na+ fits in the octahedral holes of a face centered cubic closest packed Cl- lattice Fig 12.32Stoichiometry check:Cl-FCC 8 corners * 1/8 = 1 6 faces * ½ = 3 = 4 Cl-Na+12 edges * ¼ = 3 1 inside cell = 1 = 4 Na+= NaClAlternatively, the number of octahedral holes equals the number of lattice (Cl-) atoms, thus 4 Cl−¿4Na+¿¿¿ = NaClEach Na+ is an octahedral hole “touching” 6 nearest neighbor Cl- ions in an octahedral geometryLikewise, each Cl- has an octahedron of Na+ nearest neighbors as wellEx: ZnSS2-Face centered cubic Fig. 12.34Zn2+ in half of the tetrahedral holes (4 neighbors)Ex: CaF2 FluoriteCa2+Face centered cubicF- in all of the tetrahedral holes12.7 Ceramics *skip this section*12.8 X-ray Diffraction *skip this
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