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Lecture 3 (9/13/2006) Crystal Chemistry Part 2: Bonding and Ionic Radii

Lecture 3 (9/13/2006) Crystal Chemistry Part 2: Bonding and Ionic Radii. Chemical Bonding in Minerals. Bonding forces are electrical in nature (related to charged particles) Bond strength controls most physical and chemical properties of minerals

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Lecture 3 (9/13/2006) Crystal Chemistry Part 2: Bonding and Ionic Radii

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  1. Lecture 3 (9/13/2006)Crystal ChemistryPart 2: Bonding and Ionic Radii

  2. Chemical Bonding in Minerals Bonding forces are electrical in nature (related to charged particles) Bond strength controls most physical and chemical properties of minerals (in general, the stronger the bond, the harder the crystal, higher the melting point, and the lower the coefficient of thermal expansion) Five general types bonding types: Ionic Covalent Metallic van der Waals Hydrogen Commonly different bond types occur in the same mineral

  3. Ionic Bonding Common between elements that will... • easily exchange electrons so as to stabilize their outer shells (i.e. become more inert gas-like) • create an electronically neutral bond between cations and anions Example: NaCl Na (1s22s22p63s1) –> Na+(1s22s22p6) + e- Cl (1s22s22p63s23p5) + e- –> Cl- (1s22s22p63s23p6)

  4. Properties of Ionic Bonds • Results in minerals displaying moderate degrees of hardness and specific gravity, moderately high melting points, high degrees of symmetry, and are poor conductors of heat (due to ionic stability) • Strength of ionic bonds are related: 1) the spacing between ions 2) the charge of the ions

  5. Cation Bond Strengthf (IA distance, ionic charge) +2 cations +1 cations

  6. Covalent Bonding • formed by sharing of outer shell electrons • strongest of all chemical bonds • produces minerals that are insoluble, high melting points, hard, nonconductive (due to localization of electrons), have low symmetry (due to directional bonding). • common among elements with high numbers of vacancies in the outer shell (e.g. C, Si, Al, S) Diamond

  7. Tendencies for Ionic vs. Covalent Pairing Ionic Pairs Covalent Pairs

  8. Ionic-Covalent Gradation These bond types share characteristics of each other The degree of ionic character (exchange rather than sharing) can be estimated from the contrasting electronegativity (ability to attract electrons) of the elements involved.

  9. Metallic Bonding • atomic nuclei and inner filled electron shells in a “sea” of electrons made up of unbound valence electrons • Yields minerals with minerals that are soft, ductile/malleable, highly conductive (due to easily mobile electrons). • Non-directional bonding produces high symmetry

  10. van der Waals (Residual) Bonding • created by weak bonding of oppositely dipolarized electron clouds • commonly occurs around covalently bonded elements • produces solids that are soft, very poor conductors, have low melting points, low symmetry crystals

  11. Hydrogen Bonding • Electrostatic bonding between an H+ ion with an anion or anionic complex or with a polarized molecules • Weaker than ionic or covalent; stronger than van der Waals H+ Close packing of polarized molecules Anions polarized H2O molecule Ice

  12. Summary of Bonding Characteristics

  13. Multiple Bonding in Minerals • Graphite – covalently bonded sheets of C loosely bound by van der Waals bonds. • Mica – strongly bonded silica tetrahedra sheets (mixed covalent and ionic) bound by weak ionic and hydrogen bonds • Cleavage planes commonly correlate to planes of weak ionic bonding in an otherwise tightly bound atomic structure

  14. Atomic Radii • Absolute radius of an atom based on location of the maximum density of outermost electron shell • Effective radius dependent on the charge, type, size, and number of neighboring atoms/ions - in bonds between identical atoms, this is half the interatomic distance - in bonds between different ions, the distance between the ions is controlled by the attractive and repulsive force between the two ions and their charges F = k [(q+)(q-)/d2] Coulomb’s law

  15. Charge and Attractive Force Control on Effective Ionic Radii

  16. Effect of Coordination Number and Valence on Effective Ionic Radius Metallic Ionic Radii (CN-12) (Table 3.10) Na – 1.91 K – 2.50 Ca – 1.97 Rb – 2.50 Increasing Ionic radii Decreasing Ionic radii

  17. Control of CN (# of nearest neighbors) on ionic radius Reflects expansion of cations into larger “pore spaces” between anion neighbors

  18. Next Lecture • Crystal Chemistry III Coordination of Ions Pauling’s Rules Crystal Structures • Read p. 69 - 90

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