Transcript solids - Bridgman Public Schools

Advanced
Chemistry Notes
Solids

Recall: according to the Kinetic Theory
(KT), solids were a state of matter where
the AF dominated the KE of particles.
Types of Solids

Amorphous Solids: rigid solid without any
definite shape or crystalline structure
Particles are trapped in a disordered
arrangement
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Sometimes called super-cooled liquids
Random molecular arrangement is often
due to rapid cooling
EX: rubber, glass, several plastics
Amorphous Solid
Types of Solids
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Crystalline solids: solids where particles
exist in highly ordered arrangements
Most jewelry is crystalline
Unit cell – smallest repeatin’ unit of a
crystall; its building block

The unit cell will look like a miniature form of the
crystal
Crystalline Solids
Crystalline Solids
Types of Unit Cells
 Simple Cubic (SCC) – atoms are
arranged at corners of an imaginary cube

Body Centered Cubic (BCC) – atoms are
arranged at corners and center of cube

Face Centered (FCC) – atoms are
arranged at each face of the cube
Types of Unit Cells
Atoms are shared by unit cells

Simple Cubic (SC)
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Body Centered Cubic (BCC)
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8 (1/8) atoms
8 (1/8) atoms + 1 full atom in center
Face Centered (FCC)
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6 (1/2) atoms + 8 (1/8) atoms
The Unit Cell
Information the unit cell enables us to find:
 Mass of unit cell
 Volume of unit cell
 Length of side of unit cell
 Surface area of unit cell
The Unit Cell
Example: Chromium
First determine # of atoms in the unit cell
Ex: How many atoms are in Cr unit cell
 2 atoms
Convert atoms to moles
 2 atoms Cr 1 mole Cr = 3.32 X 10-24 mol
6.022 X 1023
The Unit Cell
Example: Chromium
Convert mol Cr to grams
 3.32 X 10-24 mol 52g Cr = 1.73 X 10-22g
1 mol Cr
Use Cr’s density (7.19 g/cm3) to find volume
 7.19 g/cm3 = 1.73 X 10-22g / V

V = 2.4 X 10-23
The Unit Cell
Example: Chromium
Cube root volume to find length of sides
 Cube root (2.4 X 10-23)

l = 2.887 X 10-23
Square length to find surface area
 (2.887 X 10-23 )2

Surface area = 5 X 10-15 cm2
Crystalline Solids
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Compound unit cells

Ex: Sodium Chloride
Sodium Chloride is considered a face centered cubic
compound unit cell
Crystalline Solids
Crystal structures of an ionic solid like NaCl
are determined primarily by the ratio of radii
of the ions.

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Positive ions must be large enough to keep the
negative ions from coming into contact with each other.
Positive ions also have to be small enough from
coming into contact with themselves.
Crystalline Solids

Most compounds from the alkali metals and the
halogens produce crystals with compound unit cells
like salt except one.

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Cesium Chloride – behaves differently because Cs is a large
atom.
Cesium atoms in CsCl are surrounded by eight atoms of
chlorine instead of six.
Crystalline Solids
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Unit Cells that repeated over and over in a definite geometric
arrangement are called crystal lattices.
Crystal lattices make up crystal systems.
There are seven Crystal systems (see p. 398 table 16.1)
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Cubic
Tetragonal
Hexagonal
Rhombohedral
Orthorhombic
Monoclinic
Triclinic
Different unit cells can make up the same crystal system.
This allows for fourteen different combinations. See table
16.2 on p. 399.
Crystal Lattices
Crystal Systems
Crystals
Summary
 How does the KT define Solids?
 What is the difference between amorphous
and crystalline solids?
 Crystals are made up of Unit cells.
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What are the three basic types of unit cell?
How are crystalline solids organized?

Unit cell  crystal lattice  crystal system