BONDING IN METALS BY K.N.S.SWAMI.pdf472.pdf

Bonding in Solids
Materials Sciences and Metallurgy students

1. BONDING IN METALS
Presented by
K.N.S.SWAMI., M.Sc., SET.
Guest Faculty in Chemistry
Department Of Chemistry
P. R. Government College (Autonomous),Kakinada.

2. CONTENTS
➢Metals In Periodic Table ( Introduction)
➢Characteristics Of Metals
➢Theories Of Bonding In Metals
❑Free Electron Theory
❑Valance Bond Theory
❑Molecular Orbital Theory (Or) Band Theory Of Metals

3. Rreference Books
❖Inorganic Chemistry For B.Sc. Of all Indian Universities By R.L.Madanand G.D.Tuli.
❖Inorganic Chemistry by Catherine E.Housecraftand Alan G.Sharpe.
❖Inorganic Chemistry Principles of Structure and Reactivity By James E.Huheey,Ellen A.Keiter,Richard
L.Keiter.
❖Concise of Inorganic Chemistry By J.D.Lee

4. Metals in periodictable
▪Currently,scientistsknowof118differentelements.
▪About91ofthe118elementsintheperiodictablearemetals.
▪Thereare17nonmetalsintheperiodictable;mostaregases(hydrogen,
helium,nitrogen,oxygen,fluorine,neon,chlorine,argon,krypton,xenonand
radon);oneisaliquid(bromine);andafewaresolids(carbon,phosphorus,
sulfur,selenium,andiodine)..
▪Themetalloids:boron(B),silicon(Si),germanium(Ge),arsenic(As),antimony
(Sb),tellurium(Te),polonium(Po)andastatine(At)aretheelementsfound
steplikelinebetweenmetalsandnonmetals.

6. Characteristics ofmetals
▪Metals possess a peculiar shine on their surface called the metallic
lustre
▪Metals have high density and are hard substances.
▪Metals have high melting point and boiling point.
▪Metals are goodconductorsofheat.
▪Metalshavehighelectricconductivitywhichdecreaseswith
temperature.
▪Metals have highelastic.

7. ▪Metalsaremalleableandductilei.e.,theycanbehammered
intosheetsandcanbedrawnintothinwires.
▪Metalsareopaquetolight
▪Metalsformsolidsolutions(calledalloys)witheachothereasily.
▪Metalsaregenerallyelectropositiveelements
▪Finerthemetallicparticlesmorereactiveisthemetal.

8. ▪Metalsarebelievedtopossesaspecialtypeofbondknownas
metallicbond.Thenatureofmetallicbondisexplainedin
termsoftheoriesnamely….
1.freeelectrontheory
2.valencebondtheory
3.molecularorbitalorbandtheory
Theory of bonding inmetals

10. Free electrontheory
▪ThistheorywasproposedbyDrudeandLorentzto
explainthehighelectricalandthermalconductivityof
metals.
▪SomeofTheatomsloseoneormoreionbutarefreetomove
throughoutthemetal.
▪Thusthesefreemovingelectronsaresaidtodelocalised.
▪Thusaccordingtothusametalmayberegardedasan
assemblyofpositiveions(cationsimmersedinaseaofmobile
electronsoraseaofnegativechargecloud)

11. It explains thefollowing characteristics:
▪Non-directionalbond:
▪bondsinmetalsarenondirectionalbecause
theelectronsareNOTsharedwithoneatom
inonedirection;however,theyareshared
withmanyotherneighbouringatomsinall
directions.
Weakbond...
▪Valence electronsare attracted
simultaneouslybyalargenumberofatoms.
▪Netbindingenergyisverysmallandthe
bondformedisweak.

12. Thermalconductivity:
▪Whenapartofmetalisheated
▪Theelectronsinthatpartabsorbenergyfromthesourceofheat.
▪Theseenergeticelectronsmovetothecoolerpartsandtransferthe
kineticenergytotheelectronspresentinthatpartoncollisionwith
them
▪Andtheprocessgoesontillthetemperatureofallpartsofthemetal
becomessame.

13. Electrical conductivityOf METALS
▪Metalsaregoodconductorsofelectricityduetothepresenceofmobileelectrons
inmetals.
▪Whenpotentialdifferenceisappliedacrossametalsheet,freeelectronsstart
movingtowardsanode.
▪Newelectronsaredischargedfromthenegativeelectrodeleadingtocontinuous
flowofelectronsi.e.,currentstartsflowsfromnegativetopositive

14. Othercharacteristicsexplainedbyfreeelectrontheory..
Ductileandmalleability:
❖Itisduetothenon-directionalnatureofthemetallicbond
❖Thisisbecauseofthefactthatonapplicationofforcethemetal
ionscaneasilymovefromonelatticetoanotherleadingto
changeinshapeofthemetal.

16. Valence bondtheory
▪Itwasproposedbypauling.
▪Thestructureofmetalsmaybedescribedintermsofcovalent
bondsthatresonateamongthealternateinteratomicpositions
inmetals.
▪Amoleculeorionhavingsuchdelocalizedelectronsis
representedbyseveralcontributingstructuresiscalledresonating
structuresorconicalforms.
▪Allthepropertiesofsuchmoleculescannotbeexplainedby
onestructure

17. Lithium metalliccrystal….
▪Itrevealthatonelithiumatomissurroundedbyeightnearest
neighboursinthemetalliccrystall.
▪inlithiumonevalenceelectronispresentwhichisinsufficientto
formeightcovalentbondswitheightnearestneighboursthe
resonanceofelectronpairbondtakesplaceasshown
It explains thefollowing characteristics
▪Metallicluster:
▪Whenwhitelightfallsoncleanandsmooth
surfacethevalenceelectronsabsorbenergy
andgetexcitedintothenearesthigher
orbital.
▪whenthiselectronsreturnedtotheground
stateandabsorbedenergyisreleasedas
visiblelightthisemittedlightrayis
responsibleforthemetallicluster

18. Characteristics
▪Electricalconductivity:
▪Thebondedelectronsinthemetallicbondneitherbelongtoapositive
bondnortheyarelocalizedbetweenanytwoatomssotheyarefreeto
moveandhenceallowelectricalconductivity.
▪Higherdensity:
▪Theclosepackingofatomsinmetalliccrystalexplainthehighdensityof
metals.

19. Malleability andductility
▪Duetotheuniformchargedistributionbetweentheposititveionswhenstressis
appliedtheionscanchangetheirpositionrelativetotheirneighbouringatom
withoutchangingtheinternalenvironment.
▪LIMITATIONS
Theresonancetheoryexplainthequalitativeexplanation
Butitdoesnotexplainmetalliccharacterintheliquidstateorinsolution

21. Molecular OrbitalTheory
▪Twoatomicorbitalsofequalenergycombinetogethertoformtwomolecular
orbitalsbyLCAOMehodsimilarlywhenthreeatomicorbitalsarecombined
togetherbyLCAOmethodthreemolecularorbitalsareformed.
▪Theenergyofthismolecularortbitalsareverycloseandappearascontinuous
energybandshencethistheroyiscalledbandtheory
Lithiumcrystal
▪Ifnnumberoflithiumatomsareallowedtocombinetheywillformthreeenergy
bands
▪1sband:itisformedbythecombinationofnnumberof1s2atomicorbitalsthis
bandiscompletelyfilled,soitisthenonconductionband
▪2sband:itisformedbythecombinationofnoof2s1atomicorbitalsthisbandishalf
filledsoitistheconductionband
▪Theupperhalfofthisbandisemptywhilelowerhalfofthisbandiscompletelyfilled.
▪2pband:itisformedbythecombinationofnnumberof2psubshellsthisbandis
calledemptybadasitcontainsnoelectrons

22. ▪Thegapbetween1sbandand2sbandiscalledforbiddengap.
▪asenergygapisveryhighheresotheelectronscannotbepromotedformthelower1sbandto
thehigher2sband
▪thelevelbelowwhichallbandsarefilledarecalledfermilevelthismayariseswithinabandorinthe
gapbetweentwobands.
▪thistheoryexplainseasilythepropertiesofconductors,insulatorsandsemiconductors

25. conductors
▪Less energy gap between the valence band
conductionband.
▪Valence band and conduction band are
overlapped
▪Electrons can easily move from valence band to
conduction band.
▪They can conductelectricity.
INSULATORS
▪Completely filled valenceband.
▪More energy difference between the valence
nd conductionband.
▪Due t the more energy gap the insulators
cannot conductelectricity

26. semiconductors
▪Lessenergygapbetweenthe
valencebandandtheconduction
band.
▪Promoted electronsinthe
conductionbandandunpaired
electronsinthevalenceband
conductelectricity.
▪Probabilityofpromotingelectrons
riseswithtemperaturethe
conductivityofsemiconductors
increaseswithtemperature.
▪Holeisformedinvalenceband
duetothetransferofelectrons
fromvalencebandto
conductionband.

28. TYPES OF SEMI CONDUCTORS

29. IntrinsicSemiconductors
▪Anintrinsicsemiconductorisonewhichisextremelypureform.
▪Examples:puregermaniumandsiliconwhichhaveforbiddenenergygapsof0.72eVand1.1eV
respectively.
▪Theenergygapissosmall.
▪It is also defined as one in which the number of conduction electrons is equal to the number of
holes
Extrinsicsemiconductors
▪Thoseintrinsicsemiconductorstowhichsomesuitableimpurityordopingagentordopinghasbeen
addedinextremelysmallamounts(about1partin108)arecalledextrinsicorimpurity
semiconductors.
▪Dependingonthetypeofdopingmaterialused,extrinsicsemiconductorscanbesub-dividedinto
twoclasses:
▪(i)N-typesemiconductorsand
▪(ii)P-typesemiconductors.

30. impurities
Donor impurities Are
those elements which
donateexcess
electrons forconduction
Ex. Pentavalent elements
like antimony
phosphorousarsenic
Acceptor impurities Are
those elements which
create positive carriers
or holes that can accept
electrons Ex. trivalent
elements like boron
,gallium or indium

31. N-typesemiconductors
▪Ntypecomesfromthenegativechargeoftheelectron.
▪Inthissemiconductorselectronsarethemajoritycarriersandholesarethe
minorcarriers.
▪Ntypesemiconductorsarecreatedbydopingaintrinsicsemiconductorwithdonor
impurities.

32. P-typesemiconductors
▪Ptypereferstothepositivechargeofthehole.
▪Inptypesemiconductorsholesarethemajoritycarriersand
electronsaretheminoritycarriers.
▪Ptypesemiconductorsarecreatedbydopingaintrinsicsemiconductors
withacceptorimpurities