Lecture 7.3 - Solids

States Of States Of Matter Matter

III:III:

SolidsSolids

NucleationNucleation

• Nucleation is the extremely localized budding of a distinct thermodynamic phase

• Initial formation of bubbles and crystals requires gathering points called nucleation sites

• Imperfections in surfaces of containers, solutes, particles, etc. serve as nucleation sites

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SupercoolingSupercooling

-the cooling of a liquid to below its freezing point

-Cooling occurs faster than the liquid can organize into a solid

-possible because of the lack of nucleation sites around which crystals can form

Crystalization beginsCrystalization beginsSupercooled Supercooled

liquidliquid

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Supercooled WaterSupercooled Water

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Supercooled Water IISupercooled Water II

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SuperheatingSuperheating• a liquid is heated to a temperature higher than its

boiling point (without boiling)

For boiling to occur, the vapor pressure must exceed the ambient pressure plus a small amount of pressure induced by surface tension

• Once a bubble does begin to grow it expands explosively (called “bumpingbumping”)

• achieved by heating a homogeneous substance in a clean container, free of nucleation sites

• Surface tension suppresses bubble growth & w/o nucleation sites, liquid must be heated to higher temperatures

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Exploding Water?!Exploding Water?!

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No, Really. Don’t Try This At Home.No, Really. Don’t Try This At Home.

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Liquid and Solid Water

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Types of SolidsTypes of Solids

Amorphous solidsAmorphous solids: does not possess a well-defined arrangement and long-range molecular order

- Glass is an example

A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing

Types of SolidsTypes of Solids

Crystalline SolidsCrystalline Solids: atoms, ions, or molecules lie in a highly regular arrangement (a lattice)

-typically have flat well defined surfaces called faces

Quartz (SiO2) can form either type

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Crystallinequartz

Non-crystallinequartz glass

Types of Crystalline Solids

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Crystalline Solids

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A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions.

A unit cell is the basic repeating structural unit of a crystalline solid.

latticepoint

Unit Cell Unit cells in 3 dimensions

At lattice points:

• Atoms

• Molecules

• Ions

CuCl2 vs BaCl2

Seven Basic Unit Cells

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Three Types of Cubic Unit Cells

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Arrangement of Identical Spheres in a Simple Cubic Cell

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Number of Atoms Per Unit Cell

1 atom/unit cell

(8 x 1/8 = 1)

2 atoms/unit cell

(8 x 1/8 + 1 = 2)

4 atoms/unit cell

(8 x 1/8 + 6 x 1/2 = 4)

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When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver.

d = m

VV = a3 = (409 pm)3 = 6.83 x 10-23 cm3

4 atoms/unit cell in a face-centered cubic cell

m = 4 Ag atoms107.9 g

mole Agx

1 mole Ag

6.022 x 1023 atomsx = 7.17 x 10-22 g

d = m

V

7.17 x 10-22 g

6.83 x 10-23 cm3= = 10.5 g/cm3

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An Arrangement for Obtaining the X-ray Diffraction Pattern of a Crystal

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Extra distance = BC + CD = 2d sin = n (Bragg’s Law)

Reflection of X rays from Two Layers of Atoms

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X rays of wavelength 0.154 nm are diffracted from a crystal at an angle of 14.17o. Assuming that n = 1, what is the distance (in pm) between layers in the crystal?

n = 2d sin n = 1 = 14.17o = 0.154 nm = 154 pm

d =n

2sin=

1 x 154 pm

2 x sin14.17= 314.0 pm

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Molecular Solids• Lattice points occupied by molecules• Held together by intermolecular forces• Soft, low melting point• Poor conductor of heat and electricity• CO2, H2O, C6H6, C12H22O11

water benzene

Molecular SolidsMolecular Solids

Strong covalent forces within molecules

Weak covalent forces between molecules

Sulfur, SSulfur, S88Phosphorus, PPhosphorus, P44

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Ionic Solids

• Lattice points occupied by cations and anions• Held together by electrostatic attraction• Hard, brittle, high melting point• Poor conductor of heat and electricity

CsCl ZnS CaF2

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Atomic Solids• Lattice points occupied by atoms• Held together by london dispersion forces• Soft, very low melting point• Poor conductor of heat and electricity• Noble Gases

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Metallic Solids• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricity

Cross Section of a Metallic Crystal

nucleus &inner shell e-

mobile “sea”of e-

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Network Covalent Solids• Lattice points occupied by atoms• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity• SiO2, diamond, graphite

diamond graphite

carbonatoms

Network Atomic SolidsNetwork Atomic SolidsSome covalently bonded substances DO NOT form

discrete molecules.

Diamond: hardest mineral, insulator, transparent, best

abrasive

Graphite: softest mineral, good electrical conductor,

great lubricant

1977 DThe state of aggregation of solids can be described as belonging to the following four types:

(1) ionic (3) covalent network(2) metallic (4) molecular

For each of these types of solids, indicate the kinds of particles that occupy the lattice points and identify forces among these particles. How could each type of solid be identified in the laboratory (properties)? Name one example of each solid.

BreakBreak

Sodium chloride crystals

_____________________________________________________ Binding Experimental

Particles Forces Identification_____________________________________________________ionic + & - ions electrostatic conductivity

attraction of molten salt_____________________________________________________Metallic + ions electrostatic conductivity

attraction be- of thetween ions solidand electrons

(delocalized covalent bonding)_____________________________________________________Covalent atoms covalent high meltingnetwork bonds pt., extreme

hardness, etc.

_____________________________________________________molecular molecules van der low melting

Waals pt., noncon-(London) ductivity of

fused salt, etc.

_____________________________________________________