The Periodic Table Dmitri Mendeleev, In 1869, noticed that elements exhibited similar behaviour, in...
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Transcript of The Periodic Table Dmitri Mendeleev, In 1869, noticed that elements exhibited similar behaviour, in...
Law of Periodicity “The properties of the elements areperiodic functions of atomic number.”
Metals – Conducting, Ductile
Metalloids - Semiconductors Ductile ?
Nonmetals – insulatorsnot ductile
Group Period
Repetition of properties
Similar chemical properties
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Crystalline SolidsCrystalline solids: Metals, ions, atoms, molecules
Constructed form crystal lattices.
Stabilized by electrostatic forces.
Identical building blocks : unit cells.
LATTICE:
X-ray diffraction is used to study crystalline solids
The lattice of regularly repeating atoms with spacing acts as a diffraction grating for the rays.
The diffraction pattern is used to establish the structure of the solid!
X-ray Diffraction
Amorphous SolidsAmorphous solids: disordered solids
Strongly resemble liquids in this lack of long-range order
Many amorphous solids can be thought of very accurately as frozen liquids.
Common examples of amorphous solid are glass, candy (sugar), plastics.
In all, there are then 5 categories of solids, 4 types of crystalline + amorphous
Type Examples Structural Units Typical Properties
Ionic NaCl, K2SO4, CaCl2, (NH4)3PO4
Positive and negative ions
Hard; brittle; high melting point; electric conductivity poor as solid & good as liquid; often water-soluble
Metallic Iron, silver, copper, other metals & alloys
Metal cations in a sea of electrons
Malleable; ductile; wide range of hardness and melting points; good electric conductivity in solid & liquid; good heat conductivity.
Molecular H2, O2, I2, H2O, CO2, CH4, CH3OH, CH3CO2H
Molecules Soft; low to moderate melting points & boiling points; poor electric conductivity in solid and liquid
Network Graphite, diamond, quartz, feldspars, mica
Atoms Wide range of hardnesses & melting points (3-dimensional bonding > 2-dimensional bonding > 1-dimensional bonding); poor electric conductivity, with some exceptions
Amorphous (glassy)
Glass, polyethylene, nylon
Molecules, ionsNo long range order
Soft, wide temperature range for melting; poor electric conductivity, with some exceptions
Summary of the Structures and Properties of Various Types of Solid Substances
Semicrystalline MaterialsContain both amorphous and crystalline regions => strong and flexible.
Examples: Plastics (polymers), Steel, Wood (cellulose), collagen (tendon)
Example: Polyvinylidenedifluoride
…-CH2-CF2-CH2-CF2-….. PVDF is semicrystalline - Similar to Teflon (-CF2-CF2-)
It has several different crystal phases, which can be modified by processing methods.
The alpha phase is non-polar
The beta phase is polar
PVDF can be processed to contain mostly the polar form, by stretching the film to several time its original length
Electrical Properties of Semicrystalline MaterialsSemicrystalline materials respond to heat, pressure and external fields.
They are used as heat and pressure sensors
Thin films can be prepared than have a permanent electric filed across them.
These are used as non-stick coatings, selective membranes, etc
Electropoled films are used by theelectronics industry, ex. speaker membranes
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Lattices and Closest PackingHow do objects naturally arrange themselves?
If a second layer is added how does that effect the arrangements?
OR
Non-closest Closest
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Lattices and Unit Cells
We will focus only on the cubic and the hexagonal crystal systems
as they describe the vast majority of metallic elements.
Mathematicians have shown that there are seven basic geometries in which unit cells can be assembled that completely fill 3-D space.
These are called the seven crystal systems
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Unit CellsIdentical building blocks : unit cells.
i) No “gaps” between them in the lattice.
ii) All have same orientation in the lattice.
iii) Cannot be arranged in a staggered fashion in the lattice.
LATTICE:
NOT: OR: OR:
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Lattices and Unit Cells
Consider the smallest possible “unit cell” :
The smallest unit cell in a lattice is called the primitive unit cell.
In general one would have to consider three-dimensions.
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Closest Packing
The marbles adopted a “closest packing” as in most metals.
Two kinds:
cubic closest packed
hexagonal closest packed.
The difference arises when a third row is added:
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Hexagon Closest PackingOrient the lattice so that the layers are more easily seen:
Note every second layer are superimposable, as shown in the
case of the red layers.
A
B
A
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Cubic Closest Packing
Every third layer is superimposible.
Note that, there is an atom at each corner of the cubeAnd, the center of each face.
It is also called face centered cubic (fcc).
A
B
C
A
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Cubic LatticesThere are three types of cubic unit cells:
Note in some cases only parts of an atoms is contained by the unit cell.
i.e. The unit cell only contains the fraction of each atom that is *inside* the
cube!
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Co-ordination Number, Density and Metallic Radii
The number of atoms an atom contacts in the lattice is referred to as
its co-ordination number.
Determine the coordination number of the following lattices:
Simple cubic (e.g. Po) Face-centered cubic (e.g. Cu)
Body-centered cubic (e.g. Na) Hexagonal closest packed (e.g. Mg)
Lattice type is related to density.
What is the relative order of density from most to least dense?
How would you measure the density of a metal?
How could you relate the lattice type and density to the atomic radius?
1. Aluminum has a density of 2.699 g· cm–3, and the atoms are packed into a face-centered cubic unit cell. Use this information to find the radius of an aluminum atom.
EXERCISE
3
23 323 1
3 23 3 8 103
Count: there are 4 Al atoms per unit cell by counting rules
4 26.986.6398 10 cm
2.699 6.022 10
For any cube, 6.6398 10 cm 4.049 10 cm 4.049 10 m
By geometry, face diag
gmol
gmolcm
mV
d
a V
102onal 4 2 4.049 10 m 143 pm
4 d r a r
Along the face diagonal, there are two half and one whole sphere
The diagonal length is (a2 + a2)1/2 and corresponds to 4 atomic radii
12 2 2 2( ) 2 2 a a a a a a
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2) Lithium has a metallic radius of 152 pm and the atoms are packed into a body-centered cubic unit cell. Calculate the density of lithium.
EXERCISE
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Cubic Lattices
Lattice Packing fraction Density (m/r3)simple cubic 0.5236 0.125body-centered cubic 0.6802 0.162face-centered cubic 0.7405 0.177