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