Lecture Notes (PDF) - University of Bristol

Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Ions and Ionic Compounds
Chemistry for Earth Scientists,
DM Sherman
University of Bristol
Stable Electronic Configurations
Atoms like to adopt closed-shell configurations. To do this, they
may gain or lose electrons to become ions
NaNa: 0 : open shell
Na + Na : closed + : closed shell shell
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Stable Electronic Configurations (Cont.)
The most stable ionization state of Si is the Si 4+ ion. The semiclosed
shell Si 2+ ion does exist in interstellar gas, however.
Si 0 : open shell
Si 4+ : closed shell
Stable Electronic Configurations (Cont.)
Iron cannot adopt a closed-shell configuration...
Fe 0 : open shell
Fe 3+ : open shell
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Electronegativity
Solar (“Cosmic”) Abundance of Elements!
The most abundant element in the rocky planets is
oxygen which is (almost) the most electronegative
element.
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Ions and Electronic Configurations!
Whether an atom will gain electrons
to become an anion or lose
electrons to become a cation
depends on its electronegativity
relative to other atoms.
Mg + O → Mg 2+ + O 2-
Atoms with high electronegativity
tend to become anions. More
precisely, if two neutral atoms come
together, the more electronegative
atom will receive electrons and
become the anion.
Stable Ions and Oxidation states
The full ionic charge is only
realized in completely ionic
compounds. Otherwise it is only
the formal oxidation state.
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Attraction between ions:
The force between two point
charges a and b is
F =
q a q b
4πε 0
r 2
€
But ions are not point charges;
as they get closer, the nuclei
repel each other as do the
electron “clouds” on adjacent
ions.
Ionic Bonds
• Formed between atoms of very different
electronegativity.
• The less electronegative atom completely donates
one or more electron to the more electronegative
atom. The resulting ions are held together by
electrostatic attraction.
• Most important for
bonding between O
and Mg, Ca, Si, Al, Na,
K.
• Hence, the primary
bonding type in silicate
and oxide minerals.
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Ionic Radii
• An ion with a closed-shell configuration will have a spherical
electron distribution.
• To a first
approximation, the
ionic radius will be
constant.
• A better approximation
gives different radii for
each coordination
number.
• The radius of an ion
can be a very useful
predictor of an ion’s
geochemical behaviour.
Ionic Radii and Solid Solutions
Ions with similar radii can substitute for each other (even if charges
are different).
Example: the amphibole hornblende
(Na,Ca) 2 (Mg,Fe,Al) 5 [(Al,Si) 4 O 11 ] 2 (OH) 2
has a complex composition due to solid
solutions.
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Pauling’s Rules for Ionic Structures!
Rule 1: The coordination number of a
cation A by an anion B will be
determined by the radius ratio of
the ions A and B:
Radius Ratio CN Examples*
R A /R B < 0.16 3 C 4+
0.16 > R A /R B < 0.41 4 Si 4+ , Al 3+
0.41 > R A /R B < 0.73 6 Fe 2+ , Mg 2+
0.73 > R A /R B < 1.0 8 Ca 2+ , Na+
1.0 > R A /R B 12 K +
*when coordinated by O 2- .
Pauling’s Rules for Ionic Structures (cont.)
Rule 2: For an anion to be
stable, the sum of the
strengths* of the electrostatic
bonds that reach an anion
from its coordination of cations
will equal the charge on the
anion. This is the
electrostatic valency
principle.
(*The “Pauling bond strength” or “Bond Valence” that a cation gives to
an anion is the cation charge/coordination number).
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Volatiles in the Mantle (?): Prediction from
Paulings 2 nd Rule
Wadsleyite (β-Mg 2 SiO 4 )
Replace Mg with 2H +
• Octahedral (yellow) sites for Mg.
• Si tetrahedra in corner-shared pairs
• One oxygen has a ∑PBS of only 4/3.
Pauling’s Rules for Ionic Structures
Rule 3: The sharing of edges and faces by
coordination polyhedra decreases the stability of a
structure.
Edge-sharing
Corner-sharing
Face-sharing
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Pauling’s Rules for Ionic Structures (cont.)
Rule 3: The sharing of edges and faces by coordination polyhedra
decreases the stability of a structure.
Anatase
Rutile
There are two polymorphs of TiO 2 . Rutile is more stable (by 6 kJ/mole)
than anatase because there is less edge-sharing in the structure.
Breakdown of Pauling’s Rules
Pauling’s rules will fail when bonds are not ionic. Sulfide minerals
tend to have covalent bonds with S. Metallic bonding is also found is
metals and many sulfides.
Mackinawite (FeS)
Triolite (FeS)
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Breakdown of Pauling’s Rules
At high pressure, Pauling’s first rule break down as
cations adopt high coordination numbers.
Si in 4-fold
coordination
Olivine Mg 2 SiO4
Mg in 6-fold
coordination
Si in 6-fold
coordination
MgSiO 3 Perovskite
Mg in 12-fold
coordination
Chemical bonding: ionic vs. covalent
When electrons are completely transferred between
atoms to yield cations and anions, the atoms will be
held together by ionic bonds.
If atoms have similar
electronegativities, they
adopt closed-shell
configurations by sharing
electrons with each other;
the atoms are held together
by covalent bonds.
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Chemistry for Earth Scientists
DM Sherman, University of Bristol
2012/2013
Ionic Potential and Ion Hydration!
Large cations with low
charge will be weakly
hydrated.
Small cations with
high charge will form
oxyanions (or
oxycations).
Summary
Be able to:
work out the electronic configurations via the
Pauli exclusion principle.
predict the stable ions in geochemistry (e.g.,
Mg +2 , Si +4 , Al +3 ..).
• Ionic bonds form between atoms of different
electronegativity.
• The concept of ionic radius is a useful picture for
geochemistry. Ions with similar radii can often
substitute for each other.
• Pauling’s rules give a predictive model for ionic
structures.
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