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8/11/2019 Kuliah 2b Kimia Kristal
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Lecture 3 (9/13/2006)
Crystal Chemistry
Part 2:
Bonding and Ionic Radii
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Chemical Bonding in Minerals
Bonding forces are electrical in nature (related tocharged particles)Bond strength controls most physical and
chemical properties of minerals
(in general, the stronger the bond, the harderthe crystal, higher the melting point, and thelower the coefficient of thermal expansion)
Five general types bonding types:
Ionic Covalent Metallicvan der Waals HydrogenCommonly different bond types occur in thesame mineral
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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: NaClNa (1s22s22p63s1)> Na+(1s22s22p6) + e-
Cl (1s22s22p63s23p5) + e-> Cl-(1s22s22p63s23p6)
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Properties of Ionic Bonds
Results in minerals displaying moderatedegrees of hardness and specific gravity,
moderately high melting points, highdegrees of symmetry, and are poorconductors of heat (due to ionic stability)
Strength of ionic bonds are related:
1) the spacing between ions
2) the charge of the ions
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Cation Bond Strengthf(IA distance, ionic charge)
+1 cations
+2 cations
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Covalent Bonding formed by sharing of outer shell
electrons
strongest of all chemical bonds
produces minerals that areinsoluble, high melting points,hard, nonconductive (due tolocalization of electrons), havelow symmetry (due to
directional bonding). common among elements with
high numbers of vacancies inthe outer shell (e.g. C, Si, Al, S)
Diamond
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Tendencies for Ionic vs. Covalent Pairing
Ionic Pairs
CovalentPairs
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Ionic-Covalent GradationThese bond types share characteristics of each other
The degree of ionic character (exchangerather than sharing) can be estimatedfrom the contrasting electronegativity(ability to attract electrons) of the
elements involved.
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Metallic Bonding
atomic nuclei and inner filled electronshells in a sea of electrons made up of
unbound valence electronsYields minerals with minerals that are soft,
ductile/malleable, highly conductive (dueto easily mobile electrons).
Non-directional bonding produces highsymmetry
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van der Waals (Residual) Bonding
created by weak bonding of oppositelydipolarized electron clouds
commonly occurs around covalently bondedelements
produces solids that are soft, very poorconductors, have low melting points, lowsymmetry crystals
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Hydrogen Bonding
Electrostaticbonding between anH+ ion with an anion
or anionic complexor with a polarizedmoleculesWeaker than ionic
or covalent;stronger than vander Waals
polarized H2Omolecule Ice
Close packing of
polarized molecules
Anions
H+
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Summary of Bonding Characteristics
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Multiple Bonding in Minerals
Graphitecovalently bondedsheets of C loosely bound byvan der Waals bonds.
Micastrongly bonded silica
tetrahedra sheets (mixedcovalent and ionic) bound byweak ionic and hydrogenbonds
Cleavage planes commonlycorrelate to planes of weakionic bonding in an otherwisetightly bound atomic structure
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Atomic Radii
Absolute radius of an atom based onlocation of the maximum density ofoutermost electron shell
Effective radius dependent on thecharge, type, size, and number ofneighboring atoms/ions
- in bonds between identical atoms, thisis half the interatomic distance
- in bonds between different ions, the
distance between the ions is controlledby the attractive and repulsive forcebetween the two ions and their charges
F = k [(q+
)(q-
)/d2
] Coulombs law
Ch d i C l
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Charge and Attractive Force Control onEffective Ionic Radii
Eff t f C di ti N b d
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Effect of Coordination Number andValence on Effective Ionic Radius
Metallic Ionic Radii (CN-12) (Table 3.10)Na1.91K2.50Ca1.97Rb2.50
IncreasingI
onic
radii
DecreasingIonic radii
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Control of CN
(# of nearestneighbors) onionic radius
Reflectsexpansion ofcations intolarger pore
spacesbetween anionneighbors
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Next Lecture
Crystal Chemistry III
Coordination of Ions
Paulings Rules
Crystal Structures
Read p. 69 - 90
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