Metallic bonds
Metallic bond and Plasmons
Metallic bond – sea of electrons with positive charge embedded in them
Plasma - plasma is a state of matter similar to gas in which a certain portion
of the particles is ionized
Plasma oscillations - oscillations of the valence electrons of the metal
Plasmon frequency – frequency of oscillation of the valence electrons on the
surface
Light of frequency below the plasma frequency is reflected, because the
electrons in the metal screen the electric field of the light.
Light of frequency above the plasma frequency get transmitted
In most metals, the plasma frequency is in the ultraviolet, making them
shiny (reflective) in the visible range.
Metals such as copper and gold have interband transitions that match the frequency
Of visible light hence the certain part of the visible light energy is absorbed - color
There are some materials like indium tin oxide (ITO) that are metallic
conductors (actually degenerate semiconductors) for which this threshold is in
the infrared,[9] which is why they are transparent in the visible, but good mirrors
in the IR.
States of Matter - Solids
A solid is a nearly incompressible state of
matter with a well-defined shape. The units
making up the solid are in close contact and
in fixed positions.
– Solids are characterized by the type of force
holding the structural units together.
– In some cases, these forces are intermolecular,
but in others they are chemical bonds (metallic,
ionic, or covalent).
Solid State
– Molecular (Van der Waals forces) Examples
include solid neon, solid water (ice), and solid carbon
dioxide (dry ice).
– Metallic (Metallic bond) A metallic solid is
a solid that consists of positive cores of atoms
held together by a surrounding “sea” of
electrons (metallic bonding).
– Ionic (Ionic bond)
– An ionic solid is a solid that consists of cations
and anions held together by electrical attraction
of opposite charges (ionic bond).
Covalent (Covalent bond) A covalent network solid
is a solid that consists of atoms held together in large
networks or chains by covalent bonds
–
Examples include carbon, in its forms as diamond or
graphite (see Figure below), asbestos, and silicon
carbide.
Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Melting Point and Structure
– For a solid to melt, the forces holding the
structural units together must be overcome.
– For a molecular solid, these are weak
intermolecular attractions.
– Thus, molecular solids tend to have low
melting points (below 300oC).
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Melting Point and Structure
– For ionic solids and covalent network solids
to melt, chemical bonds must be broken.
– For that reason, their melting points are
relatively high.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Melting Point and Structure
– Note that for ionic solids, melting points
increase with the strength of the ionic bond.
– Ionic bonds are stronger when:
1. The magnitude of charge is high.
2. The ions are small (higher charge density).
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Melting Point and Structure
– Metals often have high melting points, but there is
considerable variability.
– Melting points are low for Groups IA and IIA but
increase as you move into the transition metals.
– The elements in the middle of the transition
metals have the highest melting points.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Hardness and Structure
– Hardness depends on how easily structural units
can be moved relative to one another.
– Molecular solids with weak intermolecular
attractions are rather soft compared with ionic
compounds, where forces are much stronger.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Hardness and Structure
– Covalent network solids are quite hard because
of the rigidity of the covalent network structure.
– Diamond and silicon carbide (SiC), threedimensional covalent network solids, are among
the hardest substances known.
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Figure 11.27: Structures of diamond and
graphite.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Hardness and Structure
– Molecular and ionic crystals are generally brittle
because they fracture easily along crystal planes.
– Metallic solids, by contrast, are malleable.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Electrical Conductivity and Structure
– Molecular and ionic solids are generally
considered nonconductors.
– Ionic compounds conduct in their molten state, as
ions are then free to move.
– Metals are all considered conductors.
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Physical Properties
Many physical properties of a solid can be attributed
to its structure.
• Electrical Conductivity and Structure
– Of the covalent network solids, only graphite
conducts electricity.
– This is due to the delocalization of the resonant π
electrons in graphite’s sp2 hybridization.
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Conductivity in covalent solids
Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)
Conductivity in covalent solids
In the superconducting phases of BEDT-TTF salts, the formation of stacks leads to
holes being separated by approximately 3.6 A ° , which has led to the hypothesis
that charge carriers hop between BEDT-TTF molecules via the sulphur–sulphur
intermolecular interactions
Crystalline Solids; Crystal Lattices and Unit Cells
Solids can be crystalline or amorphous.
– A crystalline solid is composed of one or more
crystals; each crystal has a well-defined, ordered
structure in three dimensions.
Examples include sodium chloride and
sucrose.
– An amorphous solid has a disordered structure.
It lacks the well-defined arrangement of basic
units found in a crystal.
Glass is an amorphous solid.
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Crystal Lattices
A crystal lattice is the geometric arrangement of
lattice points in a crystal.
– A unit cell is the smallest boxlike unit from which
you can construct a crystal by stacking the units in
three dimensions
– There are seven basic shapes possible for unit
cells, which give rise to seven crystal systems
used to classify crystals
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Crystal Lattices
A crystal lattice is the geometric arrangement of
lattice points in a crystal.
– These seven systems can have more than one
possible crystal lattice.
– A “primitive” lattice has lattice points only at the
corners of each cell.
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Figure 11.31: Unit-cell shapes of the different crystal systems.
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Crystal Lattices
A crystal lattice is the geometric arrangement of
lattice points in a crystal.
– Other lattices in the same crystal may have
lattice points on the “faces” of the unit cell.
– Following is a description of the cubic crystal
system.
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Figure 11.32: Cubic unit cells.
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Cubic Unit Cells
A simple cubic unit cell is a cubic cell in which the lattice
points are situated only at the corners.
–
–
A body-centered cubic unit cell is one in which there is a lattice
point in the center of the cell as well as at the corners.
A face-centered cubic unit cell is one in which there are lattice
points at the center of each face of the cell as well as at the
corners.
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Figure 11.30:
Crystal structure
and crystal
lattice of copper.
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Figure 11.33: Space-filling representation of cubic
unit cells.
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Crystal Defects
There are principally two kinds of defects that occur
in crystalline substances.
– Chemical impurities, such as in rubies, where
the crystal is mainly aluminum oxide with an
occasional Al3+ ion replaced with Cr3+, which
gives a red color.
– Defects in the formation of the lattice. Crystal
planes may be misaligned, or sites in the crystal
lattice may remain vacant.
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Defects – oxygen vacancies in CeO2 tuning the
Catalytic properties
Fluorite structure
The cerium(IV) oxide-cerium(III) oxide cycle or
CeO2/Ce2O3 cycle is a two step thermochemical water
splitting process based on cerium(IV) oxide and
cerium(III) oxide for hydrogen production.
Diffusion rate of oxygen – controlled by number of
oxygen vacancies
Calculations Involving Unit Cell
Dimensions
X-ray diffraction is a method for determining the
structure and dimensions of a unit cell in a
crystalline compound.
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Determination of Crystal Lattice by X-Ray
Diffraction
When x-rays are reflected from the planes of a
crystal, they show a diffraction pattern that can be
recorded on photographic film.
– Analysis of these diffraction patterns allows the
determination of the positions of the atoms in the
unit cell of the solid.
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Figure 11.47:
A crystal
diffraction
pattern.
From Preston,
Proceedings of the
Royal Society, A,
Volume 172, plate 4,
figure 5A
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