Lecture 3 (9/13/2006)
Crystal Chemistry
Part 2:
Bonding and Ionic Radii
Chemical Bonding in Minerals
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Bonding forces are electrical in nature (related to
charged particles)
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)
Five general types bonding types:
Ionic
Covalent
van der Waals
Metallic
Hydrogen
Commonly different bond types occur in the
same mineral
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: NaCl Na (1s22s22p63s1) –> Na+(1s22s22p6) + eCl (1s22s22p63s23p5) + e- –> Cl- (1s22s22p63s23p6)
Properties of Ionic Bonds
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Results in minerals displaying moderate
degrees of hardness and specific gravity,
moderately high melting points, high
degrees of symmetry, and are poor
conductors of heat (due to ionic stability)
Strength of ionic bonds are related:
1) the spacing between ions
2) the charge of the ions
Cation Bond Strength
f (IA distance, ionic charge)
+2 cations
+1 cations
Covalent Bonding
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formed by sharing of outer shell
electrons
strongest of all chemical bonds
produces minerals that are
insoluble, high melting points,
hard, nonconductive (due to
localization of electrons), have
low symmetry (due to
directional bonding).
common among elements with
high numbers of vacancies in
the outer shell (e.g. C, Si, Al, S)
Diamond
Tendencies for Ionic vs. Covalent Pairing
Ionic Pairs
Covalent
Pairs
Ionic-Covalent Gradation
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 Bonding
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atomic nuclei and inner filled electron
shells in a “sea” of electrons made up of
unbound valence electrons
Yields minerals with minerals that are soft,
ductile/malleable, highly conductive (due
to easily mobile electrons).
Non-directional bonding produces high
symmetry
van der Waals (Residual) Bonding
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created by weak bonding of oppositely
dipolarized electron clouds
commonly occurs around covalently bonded
elements
produces solids that are soft, very poor
conductors, have low melting points, low
symmetry crystals
Hydrogen Bonding
Electrostatic
bonding between an
H+ ion with an anion
or anionic complex
or with a polarized
molecules
Weaker than ionic
or covalent;
stronger than van
der Waals
H+
Close packing of
polarized molecules
polarized H2O
molecule
Anions
Ice
Summary of Bonding Characteristics
Multiple Bonding in Minerals
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Graphite – covalently bonded
sheets of C loosely bound by
van der Waals bonds.
Mica – strongly bonded silica
tetrahedra sheets (mixed
covalent and ionic) bound by
weak ionic and hydrogen
bonds
Cleavage planes commonly
correlate to planes of weak
ionic bonding in an otherwise
tightly bound atomic structure
Atomic Radii
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Absolute radius of an atom based on
location of the maximum density of
outermost electron shell
Effective radius dependent on the
charge, type, size, and number of
neighboring atoms/ions
- in bonds between identical atoms, this
is half the interatomic distance
- in bonds between different ions, the
distance between the ions is controlled
by the attractive and repulsive force
between the two ions and their charges
F = k [(q+)(q-)/d2] Coulomb’s law
Charge and Attractive Force Control on
Effective Ionic Radii
Increasing Ionic radii
Effect of Coordination Number and
Valence on Effective Ionic Radius
Metallic Ionic Radii (CN-12) (Table 3.10)
Na – 1.91
K – 2.50
Ca – 1.97
Rb – 2.50
Decreasing
Ionic radii
Control of CN
(# of nearest
neighbors) on
ionic radius
Reflects
expansion of
cations into
larger “pore
spaces”
between anion
neighbors
Next Lecture
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Crystal Chemistry III
Coordination of Ions
Pauling’s Rules
Crystal Structures
Read p. 69 - 90