ANDERSON JUNIOR COLLEGEGHEMrsrRy LEcruRE

mfin2oo2solids

SOLID STRUCTURE

Assessment Objectives

1 . describe, using simple terms, the lattice structure of a crystallinesolid which is:

(i) ionic, as in NaCl, MgO(ii) simple molecular, as in iodine(iii) giant molecular, as in graphite; diamond; silicon(lV) oxide(iv) hydrogen-bonded, as in ice(v) metallic, as in copper

2. explain.the strength high melting point, electrical insulating andthermal insulating properties of ceramics in terms of their iiantmolecular structure

3. relate the uses of ceramics, based on MgO, Al2O3, SiO2, to theirproperties(suitable examples include furnace linings, electricalinsulators; glass; crockery) -

4. describe and interpret the uses of the metals Al, including its alloys,and copper, including brass, in terms of their physical pr6perties '

5. recognise that materials are a finite resource and the importance ofrecycling processes

6. outline the importance of hydrogen bonding to the physicalproperties of substances, including ice andwater

7. suggest from quoted physical data the type of structure andbonding present in a substance

lntroduction. General properties of solids

I definite shape and volume@ rigid and incompressible@ consist of close-packed particles@ particles may be atoms, molecules or ions

. Solids can be either crystalline or amorphous:

Grystalline solids Amorphous solidsparticles are arranged in aregular, 3-dimensionalstructure i.e., has definitecrystal lattice structure

random, non-repetitiveanangement of particles; i.e.no orderly structure

fixed melting point no fixed melting pointexamples: NaCl, Cu,diamond, silica, sugar etc.

examples: rubber, glass etc.

Tvpes of crvstalline solids

. Solids are classified based on the types of attractive forces betweenthe constituent oarticles.

Type of solid Structure type and nature offorces betweenparticles

Examples

metallic solid giant structure,- giant lattice ofclosely packedmetal atoms

metallic bond- strong electrostaticforces of attractionbetween lattice ofmetal cations and seaof delocalised electrons

all metallicelements:Na, Al, Cu,Mn, Pt, AgeIc.

ionic solid giant structure,- giant lattice ofpositive ionsand negativeions

ionic bond- strong electrostaticforces of attractionbetween the oppositelycharged ions

NaCl, MgO,Al2o3,Cu(NOs)2,BaSOr etc

giant covalentormacromolecularsolid

giant structure,- giant networkof atoms

extensive and strongcovalent bond- strong attractionbetween bondingnuclei and sharing pairof electrons

diamond,graphite,silicon,silica(SiOz)

simplemolecular solid

simple latticeof atoms(ofrare gases) ordiscretemolecules

weak forces ofattraction betweenmolecules, covalentbond between atoms ineach molecule

iodine(s),ice, solidCOz, solidKr, etc

' Physical properties of cry-stailine.sorids depend on the arrangementof the particles (i.e. the-structurel anO on tf,e typ-" "nO

strength ofbonding or attractive forces that irofO tne parti,i[s together.

Metals

(a) generally have high melting point

' great amount of energy required to break the metailic bondswhich are strong erectrostatic forces ot attiaaion between thedelocalised electrons and the fattice of melaications.

. sodium has a relatively foy m.p.(92.5 .C), but most metals,especially transition meta.ls,..have very high m.p.(m.p. ot Ci =1890 "C). Strength of metailic OonO increises ls number ofdelocalised electrons increases.

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(b) generally have high density

' density(mass per unit vorume) depends on atomic mass, size ofatom and type of packing in the solid structure-. higher atomic mass ). smaller atom F higher densitv. close packing )

. sodium has a low density, transition metals have relativelyhigher densities.

(c) good conductors of etectricity and heat

. due to presence of derocarised "t""trInl

' 'av^! Lvo'+

. when.an electric potential is applied across the encts of a pieceof metal, the delocalised elections will move towarOs tnepositive end. Metals can conduct electricity inloth the solidstate and molten state-

' when heat is suppried to one end of a piece of metar, the kineticenergy of the electrons is increased. The delocalised electronstransmit the energy to other parts of the metal.

(d) malleable and ductile

. metallic bonds are nondirectional, thus layers of metal atomscan slide over each other without breaking the metallic bond.

. this movement of layers due to an applied force in the metallattice, is called 'slip'

' after'slipping', the atoms setile in new positions and the crystalstructure is restored.

. thus, a metal can be hammered into different shapes(malleable)or drawn into wires(ductile)

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Metals are matleable because the layers of aroms can slide ovci qchother without breaking the menllic bonds

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Allovs

Sometimes it is desirable to make metals less malleable, to preventlayers of atoms from 'slipping', for example when metals are used inmachines and construction structures.Metals can be strengthened or make harder by alloying, i.e. mixingwith another metal.

Example : Brass is an alloy of Cu and Zn.It is stronger than pure Cu or pure Zn.

Alloying can be easily done since metallic bonds are non-specific ornon-directional.Metal atoms of the added metal usually have different size from themain metal atoms.These added metal atoms thus disrupt the orderly anangement ofthe lattice and the layers of atoms can no longer slide over eachother easily.

Some Cornmon Alloys

Copper

Iron

Lead

Silver

Name of Alloy

Wood's metal

Yellow brass

Stainless steel

Plumber's solder

Sterling silverDental amalgam

5O7o B.i,25% Pb,12.57. Sn,12.58a Cd

677" Cu, 33V. Zn

8O.6V" Fe, O.47" C, 187o Cr,| "/o Ni

67Eo Pb,33ok S^

92.5vo A9,,7.57o Cu,70ok AF,,'18'k Sr\,1O1" Cu,2k HE

Properties

Iow melting point (70'C)

Ductile, takes polish

Resists corrosion

Low melting poirt (275"C)

Bright surfaceEasily worked

Uses

Fuse plugs,automaticsprinklers

Hardwarertems

Tableware

Solderingiornts

TablewareDental fillings

Allovs of Aluminium. Al readily forms alloys with other metals.. Metals include Cu, Mg, Ni and Zn.. Cu, Ni and Zn harden and increase strength of Al.. Mg improves the conosion resistance of Al.. Silicon is sometimes added to cast alloys to improve fluidity and

castability.. Two common Al alloys :

. Duraalumin : 95o/o Al, 4o/o Cu,1%o Mg, Fe,Si

. Magnafium : 83% Al, 15o/oMg,2YoCa

Properties and some uses of Al and Gu

Physical Properties Uses

Aluminium, Al

tsxce[ent conductor of heat heat exchangers inchemical, oil and otherindustries.

cooking utensils.

It is lignt yet strong. Strength-to-weight ratios ofAl and alloys of Al are higher than any othercommercial metals.

plane parts.

Highly workable. Malleable and ductile. C-nTerolled, pressed or extruded to any shape.

kitchen foil

Resists conosion.When exposed to air, Al and its alloys form a thinfilm of Al2os, on the surface. The film is strongand does not flake. Thus it seals off the metalfrom orygen and prevents further oxidation.

window frames

Highly reflective.Al is an excellent reflector of radiant heat.

roofing to insulate buildings

Al foil to iam radars

Gopper, Cu

High thermal conductivity cooking wareshrgn electncal conductivity copper wires for electrical

circuits and cablesresistance to conosion water pipesweamers to acquire coating of green basiccopper carbonate, CuCO3.Cu(OH)2.nH2O, thusgiving a colourful touch

roofing material

Cooper allovs

coinage metal(Cu, Ni)brass(Cu, Zn)bronze(Cu, Sn)

Metals and their ores are non-renewable resources, i.e., not naturallylegenerated at rates comparable to their rates of depletion.Demand for mineral resources increase all the time due to increasingworld population and increasing demand for consumer goods.Some important raw materials are in limited amounts.

metal known world reserves time before exhaustion(able to extract) at present rate of consumption

Al 1.1 x 1012 kgGold 1 .0 x 10' koFe 1.0 x t014 ki

100 years10 years

240 years

Measures to counter shortaqe

I find new economical ways of extracting minerals from low concentrationores.@ new method for copper extraction involves solvent extraction instead of

froth flotation

@ find new sources of minerals.I exploration of sea beds and nodules in sea beds rich in minerals.

@ to recycleO to reprocess the materials in manufactured items to make new

manufactured items(eg using glass from old botfles to make new ones)@ to extract a useful raw material for use again

Aluminium recycling

o Al is expensive to produce because a large amount of electrical energy isconsumed in the electrolysis of bauxite(Al2O3).High cost of extracting Al from its ore makes recycling worthwhile.Used soft drink cans and beer cans are collected, melted and used tomake new cans.

I This process saves Al and also about 95%o of energy.(There are Al recycling plants in Singapore)

lonic Solids

(a) Hiqh meltinq points and boilinq points

' Large amount of energy required to overcome very strong electrostaticforoes of attraction between the closely packed oppositely charged ionsand to break up the giant lattice structure. Thus, ionic solids have highmelting points and high molar heats of fusion.

. Strong ionic forces also exist in the molten liquid. Thus, ionic solids havehigh boiling points and high molar heat of vaporisation.

(b) Good conductors of electricitv when molten or when dissolved inaqueous solutions

' Due to presence of mobile ions which carry the electrical cunent under theinfluence of an electric field.

. lonic compounds in the solid state cannot conduct electricity because theions are not free to move.

(c) Hard and brittle

. In an ionic solid, each ion is held in the crystal lattice by strongelectrostatic attractions from oppositely charged ions around it. Thus, ionicsolids are hard and difficult to cut.

. lonic solids are also brittle, i.e. they shatter easily when given a firm blow.It may be split cleanly(or cleaved). This is because ionic lattiee containsalternating positive and iegative ions.

. When the crystal is tapped sharply along a particular plane, it is possibleto displace one layer of ions relative to the next. As a result of thisdisplacement, ions of similar charge are brought together. Repulsionbetween the like charges fractures the crystal.

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(a) Arrangcment of one layerof ions before displacement

(6) Arrangement of ionsaftcr displacemcnt

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(d) Soluble in water and other polar solvents

. Most ionic solids are soluble in water and other polar solvents such asethanol.The ionic solid dissolves as the crystal laftice is broken, forming ions insolution. The ions form ion-dipole bonds with the polar solvent molecule ina process called solvation(or hydration, if the solvent is water). Theformation of many of such bonds release sufficient energy to cause thedetachment of ions from the solid lattice. E r

polar solvent molecule

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solvatednegative ion

(e) Not soluble in non-polar solvents such as hexane, tetrachloromethane(CClr)gld-belzele

. Non-polar molecules have almost negligible attraction for ions.

. Molecules in non-polar liquids are held by intermolecular forces which aremuch weaker than the strong electrostatic forces between oppositelycharged ions in the ionic crystal.

. The ionic attractions are much stronger than ion-solvent and solvent-solvent attractions. The non-polar solvent molecules cannot penetrate theionic lattice, and thus ionic solids cannot dissolve in non-polar solvents.

lonic Compounds as refracto ries

. Refractories are solids with very high melting points. These solids canresist high temperature conditions without melting.

. Examples of ionic compounds which are commonly used as refractoriesare :

. MgO; melting point 2800'C. Used to line insides of fumaces.

. AlzOe; melting point 2300 'C (AlzOa is an ionic solid with covalentcharactbr). Used to make heat resistant crucibles for melting metals;and as an insulator in spark plugs of cars.

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SIMPLE MOLECULAR SOLIDS

(a) low meltinq points

' The lattice forces in morecurar sorids are reak intermolecular forces.These forces are broken when ionic solids melt( the strong covalent bondswithin each molecule is not broken!). Since these forces are weak, only asmall amount of energy is required and the melting point is usuallylow(usually below 200 .C)

(b) Non-conductors of electricitv

. This is because of the absence of charge caniers such as delocalisedelectrons or mobile ions.

(c) Tvpicallv soft

. This is because the molecules can be easily moved around due to theweak forces between them.

Examples of simple molecular solids

Solid iodine

. Structure of iodine: lattice of molecules of 12 held by weak van der Waals,forces due to instantaneous dipole-induced dipole

-attractions.

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'rltU tv4* der c,) <d,lr '(trct+ ("tc^u;

Indicalcs the centrcof an l, molecule

lodine is almost insoluble in water

@ The non-polar iodine molecules cn only form weak instantaneousdipoledipole attractions with polar water molecules. These forces areweaker lhan the extensive and stronger hydrogen bonds betweenwater molecules, Thus, iodine molecules cannot penetrate the waterstructure.

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@ The instantaneous dipole-induced dipole attractions(i.d.-i.d.) betweenthese non-polar solvent molecules are similar in strehgth toihe i.O._i.0.aftractions between benzene and iodine and the i.d._i.d. aftractionsbetween iodine and iodine molecules. Thus, the iodine molecules canpenetrate the solvent structure. ,,y-\ .,-:-:\s€,.',

'cs€l: ) @. Solid iodine has a shinv appearance '@ \#

This is due to the regular anangement of molecules in the latticeforming regular surfaces which reflect light.

lce, Solid HzO

Structure : lattice of water molecules bonded by extensive hydrogenbonds. Each H2O molecule is hydrogen bonded to 4 other HzO

-molecules in a tetrahedral anangement that extends throughout the icestructure.

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The extensive, tetrahedral anangement of H2O molecules is a rigid butrather open structure( the molecules are not closely packed).When ice melts, the tetrahedral anangement is parfly broken up as themolecules move around. Thus the molecules becom-e closer. T'his explainswhy the density of water is greater than ice, and ice floats on water.The lower density of ice c,ompared to its liquid state is unusual for solids.M-ost solids are have greater density than their liquid states. This propertyof water is important for the survival of marine biological life in winter'as -ice forms only from the top.

The meftino ooint and boilinq point of water is low as onty weakinte.rmolecular forces(hydrogen bonds) are broken during melting andboiling.The melting point and boiling point of water are higher than most othermolecular solids with similar molecular size. This ii because hydrogenbonds in water are stronger than pd-pd attractions or id-id aftractions inother molecular solids.

l l

Giant molecular / macromolecular /covalent solids

This type of solids consists of lattice of atoms held by very extensive andstrong covalent bonds.The structure is extremely strong and a lot of energy is required to breakit down.Properties include : hard, rigid, insoluble in both polar and non-polarsolvents, ver high m.p. and b.p.With the exception of graphite, giant molecular solids are electricalinsulators due to absence of delocalised electrons nor mobile ions.Examples include: diamond, graphite, silicon dioxide(SiOz), siliconcarbide(SiC)

Diamond 109.5 ' '

tO.'154 nm

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StructureGiant molecular solid consisting of laftice of carbon atoms held by strongcovalent bonds. Each C atom is held to 4 other C atoms in a tetrahedralarrangement. This tetrahedral anangement of atoms extends throughoutforming a three dimensional array.(Each carbon atom in diamond is sp' hybridised - refer to Chemical Bondinglectures on hybridisation).

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Physical properties Reasona) very high m.p.(3550 C) great amount of energy required to

break down the strong and CgSgIeqJicovalent bonds in giant network\ t,+,":" 'structurg, litv aat

b) very hard (used as abrasives,blades of saws for mostdemanding cutting jobs)

strong covalent bonds holding theatoms in a tight interlocking structure.This makes the structure rigid andmechanically very strong.

c) non-conductor ofelectricity(electrical insulator)

absence of mobile ions anddelocalised electronsin the structure;all 4 valence electrons of each C atomare localised in covalent bonding

d) does not dissolve in water water molecules cannot break thestrong mvalent bonds in diamond

Silicon dioxide. SiOe

Also called silica. Most common form of silica is quarE. Sand is an impureform of quarE (brown colour of sand is due to impurities of iron(lll)compounds).The structure of SiOz is similar to diamond. In SiOz, each Si is covalenflvbonded to four O atoms, and each O atom is bonded to two Si atoms.Thus, each Si is in the centre of a tetrahedron of O atoms, as shownbelow. This structural anangement ertends throughout a very largenetwork.SiOz has a very high melting point(1700 C); it is very hard and is a non-conductor of electricity.(These properties can be explained by its struciureand bonding)

structure of SiOz

Si atom attachedto 40 atoms

O atom attachedto 2 Si atoms

Graphite

. Graphite is sometimes classified as a layer lattice structured solid.

. Although both diamond and graphite consist of identical carbon atomsonly, there are differences in their properties. These differences are due todifferences in their structure.

0.142 nm(C-C distalcc)

s.335 o. -l(distarcr Ibcts,E o laycrs)

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. In graphite, the carbon atoms are ananged in layers of interconnectedhexagonal rings. Within the layers, the carbon atoms are held by strongcovalent bonds. Weak intermolecular van der Waals' forces hold the laverstogether.

. Each carbon atom in the layer uses 3 of its valence electron to form 3strong covalent bonds with 3 other carbon atoms. Thus each carbon is inthe centre of a trigonal planar structure, and this structure extends in a twodimensional array to form the hexagonal rings.

' The fourth valence electron of each carbon atom is delocalised throughoutthe layer. Thus all the delocalised electron from each carbon forms thedelocalised n bonds that extend over the layers.(Each C atom in graphite is spz hybridised)

. The C-C bond length of 0.142nm within the layer is shorter than the C-Cbond length in diamond. This suggests that the C-C bonds within the layer lare stronger than that in diamond. (The C-C bond length in graphite isshorterthan a single C-C bond but longerthan double bond)

properties & uses reasongood electrical conductor along thelayers

use: graphite electrodes in batteries

presence of delocalised electrons whichcan move along the layers under theinfluence of an electric field

note: the delocalised electrons cannotmove between the layers. graphitecannot electricity at right angles to therayers.

very high melting point

uses:crucibles for molten metalsas a form of heat-resistantgraphite(pyrographite) used forexhaust cones of rockets

large amount of energy required to breakvery strong and numerous covalentbonds in the structure

slippery or greasy feel

uses:. lubricants; especially in

machines operating at hightemperatures. Graphite canwithstand high temperatures butoil lubricants would decomoose.

. pencil 'lead'

weak van der Waals' forces between thelayers of carbon atoms in graphite allowsthe layers to slide over each other easily.

14

CERAMICS

. are inorganic, non-metallic, solid materials.

. can be crystalline or non-crystalline.

. can consists of giant covalent or ionic bondlng structures or acombination of the two strucfures.

. have many advantageous properties :highly resistant to heat, conosionresistant, wear-resistant, do not deform easily under stress, less densethan metals used for high temperature applications

. disadvantage : briftleness. However, with the multitude of knowledge inchemical processes, new ceramics such as ceramic composites aredesigned. The properties of these new materials far exceeds those ofnaturally-occuning materials.

Conventional ceramic obiects

' are generally made of mud-like materials shaped at room temperature andhardened by heat treatment.

' heating causes chemical reactions to occur, forming ionic and /or covalentbonds.

. made of silicate-based clay such as kaolinite; which is a hydratedaluminium silicate, Al4(OH)sSirOi0

. Examples of ceramic objects: refractory bricks for furnace linings, pottery,tiles, sewer pipes and porcelain.

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represenls

sheet sil icates as in lalc, micaand muscovite

Modern ceramics

. involves atoms bonded together in covalent-bonded arrays.Examples: silica, boron carbide, silicon carbide, sialon etc

. synthesized as single materials of high purity which are formed and firedunder conditions carefully chosen to maximise desirable properties.

. usually based on compounds of Group lll and Group lV elements.Includes: alumina Al2O3, magnesia MgO, carborundum SiC, silicon nitride,boron nitride, zirconium oxide ZOz, tungsten carbide WC etc.

chain sil icates as in asbestos

l5

Modem ceramics Properties usessialon(a ceramic alloy)

hard as diamondstrong as steellight as aluniminiumrequires no lubrication

wear-resislant machineryaerospace and automoblieindustries

Si02(quartz)

harddurablehigh melting point(1713 C)electrical properties

quarE oscillatorstransducersfiltersoptical fibres

Other uses of Ceramics

Ceramics have a wide range of applications, which includes pottery, bricks,tiles, cooking ware, glass, refractories, magnets, electrical devices-andabrasives.

1. Ceramics automobile and aerospace engines using a ceramic (siliconnitride) gas turbine similar to a jet aircraft's propulsion system. iheseengines are lighter than those made of metal and can be operated attemperatures of about 1370 c, temperatures at which a metal would melt.Fuel efficiency of these engines operating at such high temperatureswo-uld increase and polluting exhaust emissions reduced. ieramic engineswill usually last five times as long as those of metals.

2. Ceramic biological implants, such as skull caps, elbow and hip joints, kneeand tooth implants. Ceramics are suitable materials for these usesbecause they are strong, resistant to chemical attacks, and withoutproblems related to tissue rejection.

3. Electrical insulators for spark plugs in high compression engines in carsand airplanes. Good insulation is essential between electrodes to maintaina high potential difference. AlzOa and MgO, both ionic oxides with veryhigh melting points are used.

4. Refractory materials for lining fumaces. Clay, Silica, Al2O3 and MgO oftenused. They are able to withstand high temperatures of molten metals.slags sand hot gases and as well as resistant to corrosion.

5. Silicon nitride ball bearings are hard and strong, wear-resistant, able towithstand temperatures up to 1300 C and require no lubrication.

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2. Iloodingir thc rolid

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Atoms rrc l iDkcdthrough thc wholc '

Itructurc by trcrl,tlrolt coralcnt bondsfrom onc atom tothc nclt

Atlra€tion of trosit ive ions fotncSativc ion!-flro[, ionlcbords

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3. Propcrtics(i) volari l i ty Non volsli lc

High m,pL. hiBhb.pt". high larcnthcrtsUsually solid

Hrrd. ycr mrllcablc

Good conductorswhcn iolid orliquid

Non volati lcVcry high m.pt.. vcryhi8h b.pt.. vcry highlatcnl hcatsSolid

Vcry hard and britrlc

Non-conductors(Graphitc is ancrccption)

State atfoom tcmp,

(i i) Hrrdncs/mrllcebil iry

(i i i) Conducriviry

Non volati lc IHigh m.pt.. high b.pt.. highlatGnr hcats

Solid

Hard and brittlc

Noir-conductors whcn solid.Good conducton whcnmoltcn or,in aqucous solution-elccuolytes

Solublc in polar solvcnts (egll2OI iosolublc in non.pohrsolvcnr: (cg. CCla) .

VolatilcLow m.pt.. lo$, b.pt..low latcnt hcatr

Urually gascs orvohtilc liquids

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Non-conductor3 whcnsolid, liquid end inaqucous solution. (Afctr (e.!. HCI) rlactwith wltcr to tormelcctrolyrcs.)

lnsoluble iri pohrsolvcnts (c.9 lflO),solublc in non.polarrolvenrs (a& COrl-.t

lnsolublc in polarand non.polttrolvcnrs. britsolublc in tiquidmctd,

(iv) Solubilhy lnsolublc in all !olvcnrs