Lecture 3Lecture 3

Crystal Chemistry Part 2: Crystal Chemistry Part 2:

Bonding and Ionic RadiiBonding and Ionic Radii

Salt, Calcite and Graphite models

Chemical Bonding in Minerals• Bonding forces related to electrically

charged particles – negative attracts positive

• 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

Chemical Bonding in Minerals

•Five general types bonding types: Ionic Covalent

Metallic van der Waals

Hydrogen

Commonly different bond types occur in the same mineral

Chemical BondsChemical BondsElectrical in nature- responsible for most mineral propertiesElectrical in nature- responsible for most mineral properties1) Ionic1) Ionic Na: low 1st Ionization Potential Na: low 1st Ionization Potential 1s2 2s2 2p6 3s11s2 2s2 2p6 3s1

Na Na e e-- + Na + Na++ (Sodium ion has a Neon configuration) (Sodium ion has a Neon configuration) 1s2 2s2 2p6 1s2 2s2 2p6

Cl: high e-neg takes the e- Cl: high e-neg takes the e- Cl Cl-- (Cl(Cl-- ion has Argon configuration) ion has Argon configuration)

Now they have opposite charges & attract = bondNow they have opposite charges & attract = bondBonding is strong Bonding is strong (e.g. Salt has high melting point)(e.g. Salt has high melting point)

But easily disrupted by polarized solvents (e.g. water)But easily disrupted by polarized solvents (e.g. water)Poor electrical conductors; electron strongly held by anionPoor electrical conductors; electron strongly held by anionStrength Strength (1/bond length) & valence (1/bond length) & valenceAlso Also non-directionalnon-directional so symmetrical packing is possible so symmetrical packing is possible

(Isometric crystal system is common in Alkali Metal – Halogen Salts).(Isometric crystal system is common in Alkali Metal – Halogen Salts).

If electronegativity of anion and cation differs by 2.0 or more If electronegativity of anion and cation differs by 2.0 or more will be mostly ionic , say about 70%.will be mostly ionic , say about 70%.

Halite (NaCl)- An Halite (NaCl)- An ExampleExample of Ionic of Ionic BondingBonding

Na+ lost an electron shell, smaller; Cl- gained an electron, repels nucleus, larger

Na+ fits into interstices

Ionic Ionic BondingBonding

Example: NaClExample: NaCl Na (1sNa (1s222s2s222p2p663s3s11) –> Na) –> Na++(1s(1s222s2s222p2p66) + e) + e--

Cl (1sCl (1s222s2s222p2p663s3s223p3p55) + e) + e- - –> Cl–> Cl-- (1s (1s222s2s222p2p663s3s223p3p66) )

Problem 1Problem 1Write down the electron configuration for Write down the electron configuration for

neutral Chlorine Cl and for Chloride Ion Clneutral Chlorine Cl and for Chloride Ion Cl--

using the info from lecture 2.using the info from lecture 2.

Chemical BondsChemical Bonds2) Covalent2) Covalent

Consider 2 close Cl atoms, each = Consider 2 close Cl atoms, each = 1s1s22 2s 2s22 2 p 2 p66 3s 3s22 3p 3p55

If draw closer until overlap an outer orbital, can If draw closer until overlap an outer orbital, can share whereby 2 eshare whereby 2 e-- "fill" the remaining 3p shell of "fill" the remaining 3p shell of each Cleach Cl

Low energy condition causes electrons to stay Low energy condition causes electrons to stay overlapped; results in a strong bond overlapped; results in a strong bond Cl Cl22

This is the covalent or shared electron bondThis is the covalent or shared electron bondUsually stronger than Ionic bondUsually stronger than Ionic bond

Covalent bonding – sharing of Covalent bonding – sharing of valence electronsvalence electrons

Cl:1sCl:1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p55 so 7 electrons in outer shell so 7 electrons in outer shell““The sharing of an electron pair … constitutes a single bond” S&P p54.The sharing of an electron pair … constitutes a single bond” S&P p54.

Chemical BondsChemical Bonds3) Metallic Bonding3) Metallic Bonding

Metals have few, loosely held valence electronsMetals have few, loosely held valence electronsIf closely pack them can get up to 12 nearest neighborsIf closely pack them can get up to 12 nearest neighborsThis causes a high density of valence eThis causes a high density of valence e-- around any given around any given

atom & also a high density of neighbor atoms around the atom & also a high density of neighbor atoms around the loose valence eloose valence e--

These become a sea of mobile electronsThese become a sea of mobile electrons

Metals are excellent conductorsMetals are excellent conductors

Chemical BondsChemical Bonds4) Van der Waals Bonds4) Van der Waals Bonds

Weakest bond – due localized excess chargeWeakest bond – due localized excess chargeUsually between neutral molecules (even large Usually between neutral molecules (even large

ones like graphite sheets)ones like graphite sheets)Weakness of the bond isWeakness of the bond is

apparent in graphite cleavageapparent in graphite cleavage

Caused by momentary correlations in the charge polarity of adjacent atoms

More DetailMore DetailNow let’s look at the bond types in more Now let’s look at the bond types in more

detaildetail

Ionic Bonds Dominate Most Mineral GeometryIonic Bonds Dominate Most Mineral Geometry

Most minerals Most minerals have a strong have a strong ionic component.ionic component.

Mostly covalent Mostly covalent Ion complexes Ion complexes SiOSiO44 -4-4, CO, CO3 3

----, , etc. are ionically etc. are ionically bonded to metal bonded to metal ions to achieve ions to achieve neutrality.neutrality.

Calcite CaCO3

Ionic Bond PropertiesIonic Bond Properties Results in minerals displaying moderate degrees Results in minerals displaying moderate degrees

of hardness and specific gravity, moderately high of hardness and specific gravity, moderately high melting points, high degrees of symmetry melting points, high degrees of symmetry

Poor conductorsPoor conductors Strength of ionic bonds are related to: Strength of ionic bonds are related to:

1) the spacing between ions1) the spacing between ions2) the charge of the ions 2) the charge of the ions

Stronger bond has a higher melting pointStronger bond has a higher melting point

Compound Bond Strength = Melting PointCompound Bond Strength = Melting Point vs. interionic distance, ionic charge vs. interionic distance, ionic charge

+1 cations Li F is an exception

+2 cationsSmall inter-ionic distance = higher melting point

Sodium Na+ with various anionsSmall inter-ionic distance = higher melting point

12203856

3111937

9173553

A (ångström) = 10 -10m

Interionic Distance vs. HardnessInterionic Distance vs. Hardness4

12

20

38 56

22

21

12

11

Closer Interionic Distance =Increased Bond Strength(Hardness)

Covalent BondingCovalent Bonding

formed by sharing of outer formed by sharing of outer shell electronsshell electrons

strongest of all chemical bonds strongest of all chemical bonds most covalent minerals are most covalent minerals are

insoluble in acids insoluble in acids high melting points, high melting points, hard, nonconductive hard, nonconductive have low symmetry due to have low symmetry due to

multi-directional bonding. multi-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, S)the outer shell (e.g. C, Si, Al, S)

Diamond

Tendencies for Ionic vs. Covalent PairingTendencies for Ionic vs. Covalent Pairing

Ionic PairsIonic PairsCovalentCovalent

PairsPairs

Si-O, C-O, S-O, N-O, P-O

Covalent-Ionic continuumCovalent-Ionic continuumDifference in electronegativity of the elements involved tells us if one member is more attractive to electrons i.e. forms ionic bonds. F to Na 4.1 – 1 = 3.1, very different, so Na-F bond very ionic in character.Si-O difference 3.5-1.8 = 1.7 ~ 50% covalent

IonicCovalent

Metallic BondingMetallic Bonding Atomic nuclei and inner filled electron shells Atomic nuclei and inner filled electron shells

in a “sea” of electrons made up of unbound in a “sea” of electrons made up of unbound valence electrons. valence electrons.

Typical of elements with low ionization Typical of elements with low ionization potential. Valence electrons easily stripped.potential. Valence electrons easily stripped.

Yields minerals with minerals that are soft, Yields minerals with minerals that are soft, ductile/malleable, highly conductive (due to ductile/malleable, highly conductive (due to easily mobile electrons). easily mobile 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 depolarized electron cloudsdepolarized electron clouds

commonly occurs around covalently bonded commonly occurs around covalently bonded elementselements

produces solids that are soft, very poor produces solids that are soft, very poor conductors, have low melting points, with low conductors, have low melting points, with low symmetry crystals and strong cleavage.symmetry crystals and strong cleavage.

Hydrogen Bonding example ICEHydrogen Bonding example ICE

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 stronger than Van der WaalsVan der Waals

polarized H2O molecule Ice

Close packing of polarized molecules

Anions

H+

One Hydrogen bond shown as red line above

Summary of Bonding CharacteristicsSummary of Bonding Characteristics

Crystal ChemistryCrystal ChemistryCrystals can be classified into 4 types:Crystals can be classified into 4 types:1. Molecular Crystals1. Molecular Crystals

Neutral molecules held together by weak van der Waals Neutral molecules held together by weak van der Waals bondsbonds

Rare as mineralsRare as mineralsMostly organicMostly organicWeak and readily Weak and readily decompose, melt, decompose, melt, cleave, etc.cleave, etc.Example: Example: graphitegraphite

Crystal ChemistryCrystal Chemistry2. Covalent Crystals2. Covalent Crystals

Atoms of similar high e-neg and toward right side of Atoms of similar high e-neg and toward right side of Periodic TablePeriodic Table

Also uncommon as minerals (but less so than molecular)Also uncommon as minerals (but less so than molecular)Network of strong covalentNetwork of strong covalent bonds with no weak linksbonds with no weak linksDirectional bonds Directional bonds low low symmetry and densitysymmetry and density

Example: Example: diamonddiamond

Crystal ChemistryCrystal ChemistryThe diamond structureThe diamond structure

All carbon atoms in IV coordinationAll carbon atoms in IV coordination

ball-and-stick modelball-and-stick modelpolyhedral modelpolyhedral model blue C onlyblue C only

hard-sphere modelhard-sphere model

FCC unit cellFCC unit cell

Crystal ChemistryCrystal Chemistry3. Metallic Crystals3. Metallic Crystals

Atoms of similar e-neg Atoms of similar e-neg and toward left side of Periodic Tableand toward left side of Periodic Table

Metallic bonds are directionless bonds Metallic bonds are directionless bonds high high symmetry and densitysymmetry and densityPure metals have same sized atomsPure metals have same sized atomsClosest packingClosest packing 12 nearest mutually-touching neighbors 12 nearest mutually-touching neighborsCubic Closest Packing (CCP) abcabcabc stacking = FCC Cubic Closest Packing (CCP) abcabcabc stacking = FCC

cell (face-centered cubic AKA cubic close packed)cell (face-centered cubic AKA cubic close packed)Hexagonal Closest Packing (HCP) ababab = hexagonal Hexagonal Closest Packing (HCP) ababab = hexagonal

cellcellAlso BCC in metals, but this is not Closest PackingAlso BCC in metals, but this is not Closest PackingMore on coordination and closest packing next timeMore on coordination and closest packing next time

Crystal ChemistryCrystal Chemistry4. Ionic Crystals4. Ionic Crystals

Most mineralsMost mineralsFirst approximation: First approximation:

Closest-packed array of oxygen atoms Closest-packed array of oxygen atoms Cations fit into interstices between oxygens, Cations fit into interstices between oxygens,

balance the negative charges. Negative charges balance the negative charges. Negative charges mostly due to oxide ions Omostly due to oxide ions O--

Different types of interstitial sites availableDifferent types of interstitial sites available Cations occupy only certain sites where can fitCations occupy only certain sites where can fit Only enough cations to attain electrical Only enough cations to attain electrical

neutralityneutrality

Multiple Bonding in MineralsMultiple Bonding in Minerals Graphite – covalently bonded Graphite – covalently bonded

sheets of C loosely bound by sheets of C loosely bound by Van der Waals bonds.Van der Waals bonds.

Mica – strongly bonded silica Mica – strongly bonded silica tetrahedra sheets (mixed tetrahedra sheets (mixed covalent and ionic) bound by covalent and ionic) bound by weak ionic and hydrogen weak ionic and hydrogen bondsbonds

Calcite: Calcite: Cleavage planes Cleavage planes commonly correlate to commonly correlate to planes of weak ionic bonding planes of weak ionic bonding versus strong covalent bonds versus strong covalent bonds

inin CO CO33----