11_Other Types of Solids

8/16/2019 11_Other Types of Solids

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Reminders / Announcements

• ALEKS Objective #4 is due Sunday

• No lab this week. Pre-lab #3 is due Monday,April 25.

• Midterm #1 is on Friday.

• Office hours this week (BAG 202):

– Today, 11:30am-12:20pm

– Tomorrow, 1:30-2:20pm

• Last minute questions? Use the discussionboard!


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Network Atomic Solids

• Unlike metals, network atomic solidscontain strong covalent bonds.

• These solids tend to be brittle andrelatively non-conductive (heat and

electricity).

• Representative elements for these typesof solids are: carbon and silicon.


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• Network solid: not close-packed. Each atom’s

environment is determined instead by covalent-bond geometry (think “VSEPR”).

• Carbon occurs in three different atomic forms(allotropes):

− Diamond− Graphite− Fullerenes

Network Solids: Carbon

http://upload.wikimedia.org/wikipedia/commons/4/41/C60a.png


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Diamond

• One giant molecule: web ofC–C single bonds, one

connecting each pair of Catoms, tetrahedral, sp3 C

• Hardest natural substance;must break bonds to deform

• mp = 4,440 C; second-highest-melting naturalsubstance

Graphite

• Planar sheets of fusedhexagonal rings, sp2 C

• Sheets held together by delocalized bonds (conducts electricity alongsheets). Soft

• mp = 4,492 C; highest-melting natural substance.


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Atomic Networks

Typical metalDiamond

Graphitebehaves more

like a metal,whilediamond is aninsulator


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-Electron System in Graphite


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Network Solids: Silicon

• Silicon is to geology what carbon is to biology• Silicon is right below carbon on the periodic table…

– So why is SiO2 so different from CO2?

CO2 at room temperature

Si is too big to form strong bonds to oxygen –

no double bonds as in CO2

SiO2 at room temperature

http://en.wikipedia.org/wiki/File:DuneBlanche.jpg


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Si-O Bond Network in Quartz

Tetrahedral geometry

Ring structures


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Examples of silicate anions , all of which arebased on SiO 4

4- tetrahedra


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Two-dimensional representations of(a) a quartz crystal and (b) a quartz glass

When silica is heated above melting and then cooledrapidly, the result is a glass – cooled too quickly for

regular crystalline patterns to form.


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soda-limealuminosilicate

borosilicate optical

http://en.wikipedia.org/wiki/File:Pyrex_Conical_Flask.jpg


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Ceramic - another silicon-basedsubstance

• Ceramics are made of claysfired at high temperatures.

• They are brittle, non-metallic

materials that consist ofminute crystals of silicatessuspended in a glassycement.

• Unlike regular glass, ceramiccannot be melted and re-melted.

http://en.wikipedia.org/wiki/File:Qing_vase_p1070256.jpg


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Other types of solids

• While metals and networked solids can be thought ofoften as one giant molecule, a few other types ofsolids also exist:

– Molecular solids: covalently-bonded molecules

occupy the lattice positions and are held togetherin the solid state by intermolecular forces.Examples: ice, sulfur (S8) and white phosphorous(P4)


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Instead of well-separated bonding,non-bonding, and anti-bondingMOs, a large group of close-packed atoms has very closelyspaced orbitals:

a lower-energy valence band of

filled MOs and a higher-energyconduction band of empty MOs.

Energy Bands In Solids

Band theory is MO theory applied to solid crystals (nearlyinfinite groups of atoms).


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Energy Bands in Solids

• Metals have no

band gap (energyseparation) betweenthe valence andconduction bands.

• Non-metals andmost compoundshave a large band

gap.• Metalloid elements (semiconductors) have a

small band gap. These are B, Si, Ge, As, Sb,and Te (along periodic-table metal-nonmetal

line).

Typical metalNon-metal


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Localized at atom

Metals conduct heat and electricity easilybecause they have very mobile electrons


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• Electrical conductivity (e movement across a crystal lattice):

• requires excitation of a few e to mostly empty orbitals(the conduction band).

• Depends on the size of the band gaps…

‐ occurs easily in metals

‐ occurs to some extent in metalloids

‐ occurs not at all in non-metals and most compounds.

•

Conductivity of metals decreases with increasing T becauseatomic motion retards the cross-lattice electron movement.

• Conductivity of metalloids increases with increasing Tbecause higher T provides more excitation energy.

Energy Bands in Solids


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Silicon - continued

• Elemental silicon has the same structure as diamond,

but the gap between filled and empty MOs is smallerin silicon:

Diamond Silicon

The smaller band gap means some electrons can cross the

gap – silicon is a semiconductor.


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Semiconductors

• Pure semiconductors (like silicon) allow only a few

electrons to cross the band gap, BUT they can be doped with other elements to create greater or fewer valenceelectrons available for movement

More electrons: n-type Fewer electrons: p-type


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n-type: conductivity is increasedby doping it with elements thathave more valence electronsthan the host crystal.For example, silicon doped with

arsenic (1 more e-

)

p-type: conductivity is increased bydoping it with elements that haveless valence electrons than the hostcrystal, creating a hole.For example, silicon doped with boron

(1 less e-

)


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n-dopant(electron rich, like arsenic)

p-dopant

(electron deficient, like boron)

Semiconductors


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Semiconductors

• Why are n-type and p-type semiconductors useful?

– When you put one of each together, you get a p-n junction.

• When is a p-n junction useful?

– Only in those rare circumstances when you want

to plug something into the AC outlet in your wall!– p-n junctions are used in rectifiers to convert AC

to DC

– They also form the building blocks of diodes,

transistors, solar cells, LEDs and integrated circuits.


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p-n junction – good rectifier (converts AC to DC)

Charge buildup on p =

contact potential; preventsfurther migration

Reverse bias = nocurrent flow throughsystem

Forward bias = currentflows easily


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Lasers

• p-n junctions also are importantin lasers.

• When electrons “fall into holes”from the n-type to the p-typeregion, they produce photons.

• These photons stimulate theemission of other photons inthe same phase.

http://en.wikipedia.org/wiki/File:Military_laser_experiment.jpg


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Molecular Solids

• The same intermolecular forces at work in liquids

exist in solids:– London dispersion forces are fairly weak in non-

polar molecules (like CO2, I2, P4, S8), but increasedmolecular weights causes many to be solids at

room temperature.– Polar molecules have greater intermolecular forces

(especially when H-bonding is possible)

• These intermolecular forces are still not as strong as

the covalent bonds that hold each molecule together.


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Definition of a solution

Solution – when the components of a mixture are

uniformly intermingled; the mixture is homogenous

Examples: air, seawater, steel

NOT : water (pure), wood (not homogeneous)

Colloid – a suspension of tiny particles in somemedium is called a colloidal dispersion, or a colloid; themixture is heterogeneous

Examples: gelatin, fog, butter

D fi iti f l ti


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Solute - The smaller (in mass) of the components in a solution;

the material dispersed into a solvent.

Solvent - The major component of the solution; the materialthat the solute is dissolved into.

Solubility - The maximum amount that can be dissolved into aparticular solvent to form a stable solution at a specifiedtemperature.

Miscibility - The ability of two substances to dissolve in oneanother in any proportion.

Definitions for solutions


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Solutions do not need to be liquid

M th d f Q tif i H M h S l t


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Methods for Quantifying How Much Soluteis in a Solvent

(b )

11_Other Types of Solids

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