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) Bonding forces are electrical in nature (related to charged particles) Bond strength controls most physical and chemical properties of minerals 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: Five general types bonding types: IonicCovalentMetallic van der WaalsHydrogen van der WaalsHydrogen Commonly different bond types occur in the same mineral

Ionic Bonding Common between elements that will... 1) easily exchange electrons so as to stabilize their outer shells (i.e. become more inert gas-like) 2) create an electronically neutral bond between cations and anions Example: NaCl Na (1s 2 2s 2 2p 6 3s 1 ) –> Na + (1s 2 2s 2 2p 6 ) + e - Cl (1s 2 2s 2 2p 6 3s 2 3p 5 ) + e - –> Cl - (1s 2 2s 2 2p 6 3s 2 3p 6 )

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) 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: Strength of ionic bonds are related: 1) the spacing between ions 2) the charge of the ions

Cation Bond Strength f (IA distance, ionic charge) +1 cations +2 cations

Covalent Bonding formed by sharing of outer shell electrons formed by sharing of outer shell electrons strongest of all chemical bonds 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). 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) common among elements with high numbers of vacancies in the outer shell (e.g. C, Si, Al, S) Diamond

Tendencies for Ionic vs. Covalent Pairing Ionic Pairs Covalent Pairs

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.

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

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

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 polarized H 2 O molecule Ice Close packing of polarized molecules Anions H+H+

Summary of Bonding Characteristics

Multiple Bonding in Minerals Graphite – covalently bonded sheets of C loosely bound by van der Waals bonds. 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 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 Cleavage planes commonly correlate to planes of weak ionic bonding in an otherwise tightly bound atomic structure

Atomic Radii Absolute radius of an atom based on location of the maximum density of outermost electron shell 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 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 - )/d 2 ] Coulomb’s law F = k [(q + )(q - )/d 2 ] Coulomb’s law

Charge and Attractive Force Control on Effective Ionic Radii

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

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

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