2021 H1 Chemical Bonding Tutorial_Tutors.docx

DISCUSSIONQUESTIONS Metalliclatticestructures1. [2015/P1/Q4]Themeltingpointofcalcium,839°C,ismuchhigherthanthemeltingpointofsodium,98°C.Whichstatementismostrelevantinexplainingthisdifference?A Thecalciumatomislargerthanthesodiumatom.B Thecalciumatomisheavierthanthesodiumatom.C Thecalciumion,Ca 2+ ,hasahigherchargethanthesodiumion,Na + .D Thecalciumion,Ca 2+ ,containsmoreelectronsthanthesodiumion,Na + .ThebestanswerisC.OptionDshouldhavetheword“valence”electronsinordertobecorrect.Examiners’ comments:Thisisoneofthequestionswhichcandidatesfoundchallenging.Manycandidatesfeltthenumberofelectronsintheionswasthedecidingfactorratherthanthechargeontheions. AfL:GetstudentstosuggestwhatiswrongwiththeoptionD. HingeQuestion:Whataffectsthestrengthofmetallicbonding? Ans:1)Numberofvalenceelectronscontributedtotheseaofdelocalisedelectrons2)Cationicradius Dot crossdiagram, VSEPRtheory, Shapesof molecules2. N2016/3/5(modified)(a) OutlinetheprinciplesoftheValenceShell ElectronPairRepulsion(VSEPR)Theory.Central Atom’sValenceShell ElectronPairs(bothbondpairsandlonepairs)arrangethemselvesinspacetominimiserepulsionandmaximiseseparation.LonePair−LonePairrepulsion>LonePair−BondPairrepulsion>BondPair−BondPairrepulsion (b) Although they have similar formulae, the following three compounds have differently shapedmolecules. BF3 NF3 ClF3 (i) Predictandexplaintheirshapes,usingdiagramswhereappropriate.BF3 NF3 ClF3 BondPairs 3 3 3LonePairs 0 1 2StericNumber 3 4 5Shape Trigonal Planar Trigonal Pyramidal T-shapedDiagram 1

(ii) Label thepartial charges,δ+andδ-,presentononeB-Fbond.Statethecauseofthese partial charges.Hence,state,withareason,whetherornotBF3 isapolarmolecule. δ+ δ- B FF is more electronegative than B and hence pull the shared electrons closer towardsitself, giving rise tothedipole. BF3 hasnonet dipolemoment. Hence, thereisnodipolemoment. AfL:Hingequestion(tutorsmaydothisquestionbeforegoingthroughQ3): Whichdot-crossdiagramisinaccuratelyrepresented? Reasonfor myanswer isbecause(youmaychoosemorethan1answer): 1.Diagram1shouldhavealternatedotcrossonO.2.Diagram2shouldhavenodativebonds.3.Diagram3havemissingvalenceelectronsonNa + .4.Diagram4havemissingvalenceelectrons.5.Diagram5shouldhaveOgainingtheadditional electron. 2

Ans: Incorrect diagram:1, 2,4, 5 Correct reason: 1, 2, 4, 5 3. N2010/3/4c(a) Drawdot–and–crossdiagramstoshowthebondinginthemoleculesofNO2 andO3 .Each molecule contains a dative covalent bond. In the NO2 molecule, the central atom isnitrogen.In each case you should distinguish carefully between electrons originating fromthe centralatomandthosefromthetwoothermostatoms.Includeall lonepairsinyourdiagram. (b) Suggest a value for the bond angle in each of the above two molecules, giving reasons foryourchoice.● In O3 , there are 2 bond pairs and 1 lone pair around the central Oatom. Sincelone–pair – bond pair repulsion isgreater thanbondpair –bondpair repulsion,thebondangleisabout 112°(ie.109°<bondangle<120°)● InNO2 ,thereisalone–electronand2bondpairsaroundthecentra