Periodic Properties-5 explained for UG students.

It contains the period properties details for UG Students.

1. 35
Ionic Solids
Ioniccompoundsincludesalts,oxides,hydroxides,sulphides,andthemajorityofinorganic
compounds.Ionicsolidsareheldtogetherbytheelectrostaticattractionbetweenthepositiveandnegative
ions.
Therewillberepulsionifionsofthesamechargeareadjacent,andattractionwilloccurwhen
positiveionsaresurroundedbynegativeions,andviceversa.Theattractiveforcewillbeamaximumwhen
eachionissurroundedbythegreatestpossiblenumberofoppositelychargedions.Thenumberofions
surroundinganyparticularioniscalledthecoordinationnumber.
Positiveandnegativeionswillbothhavethesamecoordinationnumberwhenthereareequal
numbersofbothtypesofions,asinNaCl,butthecoordinationnumbersforpositiveandnegativeionsare
differentwhentherearedifferentnumbersoftheions,asinCaCl
2
.Higherthecoordinationnumbergreater
wouldbestabilityofioniccompounds.
Radius ratio rules
Thestructuresofmanyionicsolidsdependsonrelativesizesofthe
positiveandnegativeions,andtheirrelativenumbers.
Simplegeometriccalculationsallowustoworkouthowmanyionsofa
givensizecanbeincontactwithasmallerion.Thus wecanpredictthe
coordinationnumberfromtherelativesizesoftheions.
WhenthecoordinationnumberisthreeinanioniccompoundAX,
threeX
−
ionsareincontactwithoneA
+
ion(Figure).

2. 36
AlimitingcaseariseswhentheX
−
ionsarealsoincontactwithoneanother.
Bysimplegeometrythisgivestheratio(radiusA
+
/radiusX
−
)=0.155.Thisisthelower
limitforacoordinationnumberof3.
Iftheradiusratioislessthan0.155thenthepositiveionisnotincontactwith
thenegativeions,andit‘rattles’inthehole,andthestructureisunstable.
Iftheradiusratioisgreaterthan0.155thenitispossibletofitthreeX
−
ions
roundeachA
+
ion.
Atsomepoint(whentheratioexceeds0.225),it
becomespossibletofitfourionsaroundone,andso
onforsixionsaroundone,andeightionsaround
one.
Coordinationnumbersof3,4,6and8arecommon,andtheappropriatelimitingradiusratioscan
beworkedoutbysimplegeometry,andareshowningivenTable.

3. 37
Iftheionicradiiareknown,theradiusratiocanbecalculatedandhencethecoordinationnumber
andshapemaybepredicted.Thissimpleconceptpredictsthecorrectstructureinmanycases.
Calculation of some limiting radius ratio values
Coordination number 3 (planar triangle):
Inthistypeionicsolid,thesmallerpositiveionofradiusr
+
incontactwiththreelargernegative
ionsofradiusr
−
.PlainlyAB=BC=AC=2 r
−
,BE=r
−
,BD=r
+
+r
−
.Further,theangleA−B−Cis60°,
andtheangleD−B−Eis30°.
Bytrigonometry
> ?
? ?
?
> ? ? ?
>
?
Coordinationnumber4(tetrahedral):
Intetrahedralarrangementinscribedinacube.
PartofthistetrahedralarrangementisdrawninFigure.
ItcanbeseenthattheangleABCisthe
tetrahedralangleof109°28
HencetheangleABDishalfofthis,thatis
54°44.InthetriangleABD

4. 38
? >
?
>
?
? >
?
>
?
Coordinationnumber6(octahedral):
Across-sectionthroughanoctahedralsiteisshowninFig.,
Thesmallerpositiveion(ofradiusr
+
)touchessixlargernegativeions(ofradiusr−).
(Notethatonlyfournegativeionsareshowninthissection,andoneisaboveandanotherbelowthe
planeofthepaper.)
ItisobviousthatAB=r
+
+r
−
andBD=r
−
.TheangleABCis45°.
InthetriangleABD
>?
?
>
?
?
?
>
?

5. 39
CLOSE PACKING
Manycommoncrystalstructuresarerelatedto,andmaybedescribedintermsof,hexagonalor
cubicclose-packedarrangements.
Becauseoftheirshape,spherescannotfillspacecompletely.Inaclose-packedarrangementof
spheres,74%ofthespaceisfilled.Thus26%ofthespaceisunoccupied,andmayberegardedasholes
inthecrystallattice.
Twodifferenttypesofholeoccur.Someareboundedby
fourspheresandarecalledtetrahedralholes(marked▼or▲in
Fig.),and
othersareboundedbysixspheresandarecalled
octahedralholes(marked☼inFig.).
Foreverytheclose-packedarrangementthereis one
octahedralholeandtwotetrahedralholes.
Theoctahedralholesarelargerthanthetetrahedralholes
(givenFig.).
octahedral holes tetrahedral holes

6. 40
Hexagonal or cubic close-packed arrangements