Lecture 3 (9/13/2006) Crystal Chemistry Part 2: Bonding and Ionic Radii.
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Transcript of Lecture 3 (9/13/2006) Crystal Chemistry Part 2: Bonding and Ionic Radii.
Lecture 3 (9/13/2006)Lecture 3 (9/13/2006)
Crystal ChemistryCrystal Chemistry
Part 2: Part 2:
Bonding and Ionic RadiiBonding and Ionic Radii
Chemical Bonding in Chemical Bonding in MineralsMinerals
Bonding forces are electrical in nature Bonding forces are electrical in nature (related to (related to charged particles)charged particles) Bond strength controls most physical and Bond strength controls most physical and
chemical properties of mineralschemical properties of minerals(in general, the stronger the bond, the (in general, the stronger the bond, the
harder harder the crystal, higher the melting the crystal, higher the melting point, and the point, and the lower the coefficient of lower the coefficient of thermal expansion)thermal expansion) Five general types bonding types: Five general types bonding types:
IonicIonic CovalentCovalent MetallicMetallic van der Waalsvan der Waals HydrogenHydrogenCommonly different bond types occur in Commonly different bond types occur in
the the same mineralsame mineral
Ionic BondingIonic Bonding
Common between elements that will... Common between elements that will...
1)1) easily easily exchangeexchange electrons so as to stabilize electrons so as to stabilize their outer shells (i.e. become more inert their outer shells (i.e. become more inert gas-like)gas-like)
2)2) create an electronically neutral bond create an electronically neutral bond between cations and anionsbetween cations and anions
Example: NaClExample: NaCl Na (1sNa (1s222s2s222p2p663s3s11) –> Na) –> Na++(1s(1s222s2s222p2p66) + e) + e--
Cl (1sCl (1s222s2s222p2p663s3s223p3p55) + e) + e- - –> Cl–> Cl-- (1s(1s222s2s222p2p663s3s223p3p66) )
Properties of Ionic BondsProperties of Ionic Bonds
Results in minerals displaying Results in minerals displaying moderate degrees of hardness and moderate degrees of hardness and specific gravity, moderately high specific gravity, moderately high melting points, high degrees of melting points, high degrees of symmetry, and are poor conductors of symmetry, and are poor conductors of heat (due to ionic stability)heat (due to ionic stability)
Strength of ionic bonds are related: Strength of ionic bonds are related: 1) the spacing between ions1) the spacing between ions2) the charge of the ions 2) the charge of the ions
Cation Bond StrengthCation Bond Strengthff (IA distance, ionic charge) (IA distance, ionic charge)
+1 cations
+2 cations
Covalent BondingCovalent Bonding formed by sharing of outer formed by sharing of outer
shell electronsshell electrons strongest of all chemical strongest of all chemical
bonds bonds produces minerals that are produces minerals that are
insoluble, high melting points, insoluble, high melting points, hard, nonconductive (due to hard, nonconductive (due to localization of electrons), have localization of electrons), have low symmetry (due to low symmetry (due to directional bonding). directional bonding).
common among elements with common among elements with high numbers of vacancies in high numbers of vacancies in the outer shell (e.g. C, Si, Al, the outer shell (e.g. C, Si, Al, S)S) Diamond
Tendencies for Ionic vs. Covalent Tendencies for Ionic vs. Covalent PairingPairing
Ionic Pairs
CovalentPairs
Ionic-Covalent Ionic-Covalent GradationGradation
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 BondingMetallic Bonding
atomic nuclei and inner filled electron atomic nuclei and inner filled electron shells in a “sea” of electrons made up shells in a “sea” of electrons made up of unbound valence electronsof unbound valence electrons
Yields minerals with minerals that are Yields minerals with minerals that are soft, ductile/malleable, highly soft, ductile/malleable, highly conductive (due to easily mobile conductive (due to easily mobile electrons). electrons).
Non-directional bonding produces high Non-directional bonding produces high symmetrysymmetry
van der Waals (Residual) Bondingvan der Waals (Residual) Bonding
created by weak bonding of oppositely created by weak bonding of oppositely dipolarized electron cloudsdipolarized electron clouds
commonly occurs around covalently commonly occurs around covalently bonded elementsbonded elements
produces solids that are soft, very poor produces solids that are soft, very poor conductors, have low melting points, low conductors, have low melting points, low symmetry crystalssymmetry crystals
Hydrogen BondingHydrogen Bonding
Electrostatic Electrostatic bonding between bonding between an H+ ion with an an H+ ion with an anion or anionic anion or anionic complex or with a complex or with a polarized polarized moleculesmoleculesWeaker than Weaker than ionic or covalent; ionic or covalent; stronger than van stronger than van der Waalsder Waals
polarized H2O molecule Ice
Close packing of polarized molecules
Anions
H+
Summary of Bonding CharacteristicsSummary of Bonding Characteristics
Multiple Bonding in MineralsMultiple Bonding in Minerals Graphite – covalently Graphite – covalently
bonded sheets of C loosely bonded sheets of C loosely bound by van der Waals bound by van der Waals bonds.bonds.
Mica – strongly bonded Mica – strongly bonded silica tetrahedra sheets silica tetrahedra sheets (mixed covalent and ionic) (mixed covalent and ionic) bound by weak ionic and bound by weak ionic and hydrogen bondshydrogen bonds
Cleavage planes commonly Cleavage planes commonly correlate to planes of weak correlate to planes of weak ionic bonding in an ionic bonding in an otherwise tightly bound otherwise tightly bound atomic structureatomic structure
Atomic RadiiAtomic Radii Absolute radiusAbsolute radius of an atom based on of an atom based on
location of the maximum density of location of the maximum density of outermost electron shelloutermost electron shell
Effective radiusEffective radius dependent on the dependent on the charge, type, size, and number of charge, type, size, and number of neighboring atoms/ionsneighboring atoms/ions- in bonds between identical atoms, - in bonds between identical atoms, this is half the interatomic distancethis is half the interatomic distance- in bonds between different ions, - in bonds between different ions, the distance between the ions is the distance between the ions is controlled by the attractive and controlled by the attractive and repulsive force between the two repulsive force between the two ions and their chargesions and their charges
F = k [(qF = k [(q++)(q)(q--)/d)/d22] Coulomb’s law] Coulomb’s law
Charge and Attractive Force Charge and Attractive Force Control on Effective Ionic RadiiControl on Effective Ionic Radii
Effect of Coordination Number Effect of Coordination Number and Valence on Effective Ionic and Valence on Effective Ionic
RadiusRadiusMetallic Ionic Radii (CN-12) (Table 3.10)Na – 1.91K – 2.50Ca – 1.97Rb – 2.50
Incr
easi
ng Ionic
rad
ii
DecreasingIonic radii
Control of CN(# of nearest neighbors) on ionic radius
Reflects expansion of cations into larger “pore spaces” between anion neighbors
Next LectureNext Lecture
Crystal Chemistry IIICrystal Chemistry III
Coordination of IonsCoordination of Ions
Pauling’s RulesPauling’s Rules
Crystal StructuresCrystal Structures
Read p. 69 - 90 Read p. 69 - 90