types of solid
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Types of Solids
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Intermolecular Forces in Solids
The molecules in solids are heldvery tightly together, restrictingtheir movement.
This means that the intermolecularforces are significant.
We can classify solids based on the
type of intermolecular forcesholding them together.
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Types of Solids
We will classify solids into fourtypes:
1. Molecular Solids2. Metallic Solids
3. Ionic Solids
4. Covalent network solids
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Molecular solids
Consider the element Iodine, I2 Iodine is a solid at room
temperature and pressure
What intermolecular forces maintainthis solid state?
Only London forces!
However, it has 53 electrons, givingit sufficient intermolecular force tobe a solid at room temperature.
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Molecular solids
A molecular solid is a solid thatconsists of atoms or molecules heldtogether by intermolecular forces
Examples include:
Solid neon (melting point of -248oC)
Solid CO2(dry ice)
Solid H2O (ice)
Iodine
Sugar
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Metallic solids
Metals usually have very highmelting points, although this can behighly variable. For example, mercury melts at -39oC while
tungsten melts at 3410oC
Therefore, the intermolecular forces
in metals must be rather strong. These forces are not completely
understood.
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Metallic solids
In metals, the valence electrons ofneighbouring atoms form a sort of
electron soup
These electrons are calleddelocalized electrons
This delocalized electron soup can
be thought of as a glue that holdsthe positive nuclei of the metalatoms together.
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Metal solids can be thought of as being like a ricekrispy square
The marshmallow is like thedelocalized electron soup thatacts a glue that holds it together
The rice krispies are like thepositive nuclei of the metalatoms that compose the solid.
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Metallic solids
The positive nuclei of metals formlattice structures within the electronsoup.
This means that metals arecomposed of crystals, but they areusually too small to see.
However, formation of metalsthrough electroplating can make thecrystalline nature more obvious.
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Ionic Bonding
Ionic compounds have the followingphysical properties:
Very high melting points
Brittle
Non-conductive in a solid state
Conductive in a liquid state
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Ionic Bonding
The forces involved in ioniccompounds are derived from thecreation of a crystal latticecomposed of alternating negativeand positive ions.
Because of the full charge on ions,
ionic bonds are stronger thandipole-dipole interactions
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Different Types of Crystal Lattices
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Arrangement of Ions in a Sodium
chloride cystal (cubic)
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Covalent Network Solid
Consider carbon dioxide (CO2) andsilicon dioxide (SiO2).
What would you expect the physicalproperties of SiO2to be?
CO2b.p. = -78.5oC
If London forces are the only
intermolecular force, then youmight predict the b.p. of SiO2to beslightly more than CO2.
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Covalent Network Solid
However SiO2has a m.p. of 1650oC,
and b.p. of 2230oC!
SiO2is also known as quartz, orsand, is used in the production ofglass.
Clearly, SiO2is not a molecular solid
like CO2is.
SiO2is a covalent network solid.
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Amethyst quartz (SiO2)
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Diamond
Diamond is another allotrope ofcarbon.
Like graphite, it is a covalent
network solid. However, instead of sheets, it forms
a 3-dimensional lattice of carbonatoms.
This is what gives diamond itscharacteristic hardness.
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