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Explainthefollowing: (a)Electronegativityof elements increaseson movingfrom leftto rightin theperiodictable . (b)Ionisationenthalpy decreases in agroup fromtop tobottom ? |
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Answer» Solution :(a) As a wemovefrom leftto rightacrossa periodthe nuclearchargeincreasesand theatomicradiusdecreases . A ARESULT, thetendencyof the atomof a elementto attractshared pairof electrons towards itselfincreasesand hencethe electronegativityof the ELEMENT increases. Forexampleelectronegativityof theelements of 2ndperiodincreasesregularlyfromleftto right as FOLLOWS: `Li (1-0) , Be (1.5), B( 2.0)` C (2.5)N (3.0), O (3.5)` andF (4.0) Theionizationenthalpydecreasesregularly as we movefrom topto bottomas explainedbelow . ( b)(i) Onmovingdown a groupfrom topto bottomthe atomicsizeincreases gradually to the additionof a newprincipalenergyshell at eachsucceedingelement . As aresultthe DISTANCEBETWEEN thenucleusand the valenceshellincreases. In otherwordstheforceof attractionof thenucleus for thevalenceelectrons decreases and hencethe ionizationenthalpyshoulddecrease. (ii) With theaddition of newshells the numberof inner shell which shieldthe valence electronsfrom thenucleusincreases. Inother words the shieldingeffect or thescreeningeffectincreases. Asa resulttheforce of attractionof the nucleus for thevalenceelectronsfurtherdecreasesand hencetheionizationenthalpyshoulddecrease. (iii) Furtheras wemovefromtop to bottomin a groupnuclearchargeincreaseswithincreases in ATOMICNUMBER .As a resultthe forceofattraction of thenucleus of thevalenceelectrons increasesand hencetheionizationenthalpyshouldincrease. Thecombinedeffect of the increase inatomic sizeand screening effectmore thancompensatesthe effectof the increased nuclearcharge. Consequentlythe valenceelectrons becomesless and lessfirmly heldbythenucleusand hencethe ionizationenthalpygradually decreasesas we movedown thegroup. |
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