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 properties
1) 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 = bond
Bonding 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 1
Write 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 bond
Usually 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 neighbors
This 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 charge
Usually 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 Detail
Now 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 ionic component.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 ions
2) 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
12
20
38
56
3
11
19
37
9
17
35
53
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 CrystalsNeutral molecules held together by weak van der Waals Neutral molecules held together by weak van der Waals
bondsbonds
Rare as mineralsRare as minerals
Mostly organicMostly organic
Weak 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 links
Directional bonds Directional bonds low low
symmetry and densitysymmetry and density
Example: Example: diamonddiamond
Crystal ChemistryCrystal Chemistry
The diamond structureThe diamond structureAll 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 density
Pure metals have same sized atomsPure metals have same sized atoms
Closest packingClosest packing 12 nearest mutually-touching neighbors 12 nearest mutually-touching neighbors
Cubic 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 cellcell
Also 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 minerals
First 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 versus strong covalent bonds bonds
inin CO CO33----