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CLASS 10
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PERIODIC CLASSIFICATION OF ELEMENTS...

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CBSEClass10Science RevisionNotes CHAPTER–5 PERIODICCLASSIFICATIONOFELEMENTS

Elements:Substancescontainingatomsofonlyonetype.e.gNa,Au,Mgetc.Thereare around118elementsknowntous.ThefirstclassificationofelementswasintoMetalsand Non-metals.Elementsareclassifiedtomakethestudyeasy. Dobereiner’sTraids:Whentheelementswerewritteninorderofincreasingatomicmasses theatomicmassofthemiddlewastheaverageoftheatomicmassoftheothertwoelements. ElementsAtomicMass Ca40.1 sr87.6 Ba136.3 Limitations:Onlythreetriadswererecognisedfromtheelementsknownatthattime. Atomicmassofanelementistherelativemassofitsatomascomparedwiththemassofa Carbon-12atomtakenas12units Newland’sLawofOctaves Basedonincreasingatomicmassofelements. LawofOctaves:Whenelementsarearrangeditwasfoundthateveryeighthelement hadpropertiessimilartothatofthefirst.e.gpropertiesofsodiumandLithiumarethe same. Limitations ApplicableonlyuptoCalcium Propertiesofnewelementscouldn’tfitinit. Insomecasespropertiesoftheelementswerenotsameasdefinedbyoctave.eg:Fe wasplacedfarawayfromCoandNi. Workedwellonlywithlighterelements.

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Mendeleev'speriodictable Mendeleev'sPeriodicLaw:Thepropertiesofelementsaretheperiodicfunctionoftheir atomicmass. Mendeleev'speriodictablebasedonthechemicalpropertiesofelements.Itcontainsvertical columnscalledgroupsandhorizontalrowscalledperiods. Forclassifyinghefocussedontheoxidesandhydridesformedbytheseelements.He arrangedthemaccordingtotheriatomicmass.Itwasobservedthatnotonlythechemical butphysicalpropertiesrecurredatequalintervals. AchievementsofMendeleev’sPeriodictable Elementswithsimilarpropertiescouldbegroupedtogether Somegapswereleftfortheundiscoveredelements. Noblegasescouldbeplacedwithoutdisturbingtheexistingorder. Limitations Nofixedpositionforhydrogen Noplaceforisotopes Noregulartrendinatomicmass. CowasplacedbeforeNi. IV.ModernPeriodicTable ModernPeriodicLaw:GivenbyHenryMoosleyin1913. Propertiesofelementsareaperiodicfunctionoftheiratomicnumber. AtomicNumber–denotedbyZandequalstotheno.ofprotonsinthenucleusofan atom. Modernperiodictablecontains18verticalcolumnsknownasgroupsand7horizontal rowsknownasperiods. Elementsinagrouphavethesamenumberofvalenceelectrons No.oftheshellsincreasesaswegodownthegroup. Elementsinaperiodhavesamenumberofshells. Eachperiodmarksanewelectronicshellgettingfilled. No.ofelementsplacedinaparticularperioddependsuponthefactthathow electronsarefilledintovariousshells.

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Maximumno.ofelectronsthatcanbeaccommodatedinashelldependonthe formula

wherenistheno.ofthegivenshell.e.g.Kshell–2×(1) =2elementsin

thefirstperiodLshell–2×(2) =8elementsinthesecondperiod. Positionoftheelementintheperiodictabletellsaboutitsreactivity. TRENDSINTHEMODERNPERIODICTABLE: VALENCY:No.ofvalenceelectronspresentintheoutermostshells.Valencyremains thesamedownagroupbutchangesacrossaperiod. ATOMICSIZE:Atomicsizereferstoradiusofanatom. Atomicsizeorradiusdecreasesinmovingfromlefttorightalongaperioddueto increaseinnuclearcharge Atomicsizeincreasesdownthegroupbecausenewshellsarebeingaddedaswego downthegroup. METALLICCHARACTER:Metalliccharactermeansthetendencyofanatomtolose electrons. -Metalliccharacterdecreasesacrossaperiodbecausetheeffectivenuclearchargeincreases thatmeansthetendencytoloseelectronsdecreases. -Metalsareelectro-positiveastheytendtoloseelectronswhileformingbonds. -Metalliccharacterincreasesaswegodownagroupastheeffectivenuclearchargeis decreasing.Nonmetalsareelectro-negative.Theytendtoformbondsbygainingelectrons. -Metalsarefoundontheleftsideoftheperiodtablewhilenon-metalsaretowardstheright handsideoftheperiodictable. -Inthemiddlewehavesemi-metalsormetalloidbecausetheyexhibitsomepropertiesof bothmetalsandnonmetals. -Oxidesofmetalsarebasicinnaturewhileoxidesofnon-metalsareacidicinnature. GradationinPeriodicProperties

S.

Property

No. 

Variation across period

Variation Reason

along

Reason

group duetoadditionofnew shellsdistancebetween 3/4

1.

Atomicsize

Decreases Duetoincreasein

Increase

nuclearcharge

outermostelectronand nucleusincreasesdueto additionofnewshells.

2.

Metallic Character

Decreases

DuetoincreaseIn

decreaseineffective

effectivenuclear

nuclearcharge

charge,tendency tolosevalence

Increases

loseelectrons(metallic

decreases.

character)increases.

Character

duetodecreasein

duetoincreasein

effectivenuclearcharge

effectivenuclear

Increases 3.

Increase

(electronegativity)

electronsTendencyto

electrons

NonMetallic

experiencedbyvalence

chargetendencyto gainelectrons increases

Decreases

experiencedbyvalence electron(duetoaddition ofnewshell),tendencyto gainelectronsdecreases.

Whatyouhavelearnt Elementsareclassifiedonthebasisofsimilaritiesintheirproperties. DobereinergroupedtheelementsintotriadsandNewlandsgavetheLawofOctaves. Mendeleevarrangedtheelementsinincreasingorderoftheiratomicmassesand accordingtotheirchemicalproperties. Mendeleevevenpredictedtheexistenceofsomeyettobediscoveredelementsonthe basisofgapsinhisPeriodicTable. Anomaliesinarrangementofelementsbasedonincreasingatomicmasscouldbe removedwhentheelementswerearrangedinorderofincreasingatomicnumber,a fundamentalpropertyoftheelementdiscoveredbyMoseley. ElementsintheModernPeriodicTablearearrangedin18verticalcolumnscalled groupsand7horizontalrowscalledperiods. Elementsthusarrangedshowperiodicityofpropertiesincludingatomicsize,valency orcombiningcapacityandmetallicandnon-metalliccharacter.

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