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Firstmember of thegroup ofrepresentativeelements(i.e.,s andp- blockelementshowsanomalousbehavious . Illustrate withtwoexamples. |
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Answer» Solution :The firstmemberof each groupof therepresentativeelements(i.e.,s- andp- blockelements ) showanomalousbehaviourfrom rest ofmembers of thesamegroupbecauseof the followingreasons. (i)small size(ii)highionizationenthalpy(III) highelectronegativityand (iv)absenceof d-orbitalss- BlockElements. Someexampleof anomalousbehaviour of Lifrom rest of the alkali metalsand Befrom restof teh alkaline earthmetalsare discussedbelow . (i) Lithium unlikeotheralkalimetalsand beryllium unlike otheralkalineearthmetalsformcompound whichhavesignificantcovalentcharacterwhileothermembers ofeach of thethesegroupsformcompoundwhicharepredominantlyionic. (ii) Lithiumcombineswithnitrogento formlithiumnitridewhile allotheralkali metalsdo notformnitrides .Similarly , lithiumcarbonatedecomposes togive `CO_(2)`1 whileall otheralkalimetalscarbonatesdo not. `6 Li+ N_(2)overset(Heat)(to) underset("Lithium nitride ")(2Li_(3)N)` ` Li_(2) CO_(3)overset(Heat)(to) Li_(2)O+ CO_(2),Na_(2) CO_(3)overset(Heat)(to) No` action SimilarlyBe differs from all otheralkalineearth metals as shown below , (i) Beryllium carbidereacts withwater toproduce methane gaswhilecarbides ofotheralkalineearthmetalsgive acetylene gas. `Be_(2) C+ 4 H_(2) Oto 2 Be(OH)_(2) + CH_(4) ,CaC_(2)+ 2H_(2) O toCa (OH)_(2) +HC -= CH` (ii) Beryllium shows acoordination number of 4 due tothepresenceof ones-and three p-ORBITALS invalenceshellbut otheralkalineearthmetalssuchas `Mg^(2+) ` and `Ca^(2+)` show acoordinationnumberofmakinguse ofd- orbitalsin additionto s- andp- orbitals .Forexample `BeF_(2)+ Ftp]BeF_(4)]^(2-) -` fourcoordinate complex `Ca^(2+) + Na_(2)[H_(2)EDTA]^(2-) to [Ca (EDTA)]^(2-) +2 Na^(+) +2 H^(+) -Six` coordinatecomplex  Disodium saltof ethylenediaminetetraacetic and The EDTA(ethylenediaminetetraacetic acid ) Complexes of `Mg^(2+)` and `Ca^(2+)` are used todeterminethe hardness water by titrationmethod. p- blockelement. Someexamplesof anomalousbehaviour of firstelementof eachgroup ofp- blockelementfromthe rest of themember of thesame groupare discussed below : maximumcovalency offour. the firstmember of each groupof THEP- blockelements has fourorbitals(one 2sand three2p- orbitals) in thevalenceshell forbondingand hencecanaccommodate4 pairsor 8ELECTRONS . In other words , theseelements show amaximumcovalencyof four. Incontrast, othermember of the samegroup or different froup of p - blockelement have vacentd- orbitalsandhence canaccommodate moreelectronsand thuscan expand their covalencybeyondfour . for example, (a)boron forms `[BF_(4)]^(-)` or `[BH_(4)]^(-)` ion whileA1 gives`[A1F_(6)]^(3-)` ion. ( b)Carbonforms onlytetrahalites`(CX_(4), X =F, C1 , Br, I)` whileothermemberformhexahalides , i.e.,`[SiF_(6)]^(2-) , [GeC1_(6)]^(2-), [SiC1_(6)]^(2-) , [PbC1_(6)]^(2-),` etc ( c)Nitrogenformsonlytrihalidessuch as `NF_(3)` (havingan octer of electronsin thevalenceshell) while Pand Asformbothtrihalites `(PF_(3), "As" C1_(3)` etc )and pentahalides`(PF_(3), "As" C1_(5))` having10electronsinvalenceshell. (d ) Fluorinedoes notform `FC1_(3)` (having 10 valenceelectrons )whilechlorineforms `C1F_(3)` (having 10 valence electrons ) |
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