Unit 2 Part 2 Particles, Bonding and Structure Interatomic and Intermolecular Bonding UNIT 2 PARTICLES, BONDING AND STRUCTURE

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1 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding UNIT 2 PARTICLES, NDING AND STRUCTURE PART 2 INTERATMIC AND INTERMLECULAR NDING Cntnts 1. Inic nding 2. Cvalnt nding 3. Mtallic nding 4. Elctrngativity 5. Mlcular Shaps 6. Intrmlcular Frcs Ky wrds: inic bnd, Lwis-dt structur, cvalnt bnd, dativ cvalnt bnd, mtallic bnd, lctrngativity, plar cvalnt bnd, tmprary dipl, inducd dipl, prmannt dipl, Van dr Waal s frc, plar mlcul, nn-plar mlcul, hydrgn bnd Units which must b cmpltd bfr this unit can b attmptd: Unit 1 Atmic Structur and th Pridic Tabl 1

2 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 1) Inic nding An inic bnd is an attractin btwn ppsitly chargd ins. Mst simpl ins ar frmd whn lctrns ar transfrrd frm n atm t anthr. In gnral, atms in Grups I, II and III ls lctrns, and atms in Grup V, VI and VII gain lctrns. Atms in Grups I, II and III f th Pridic Tabl tnd t frm psitivly chargd ins (catins) by lsing all f th lctrns in thir utr shll: Na wuld ls n lctrn t frm Na + Mg wuld ls tw lctrns t frm Mg 2+ Al wuld ls thr lctrns t frm Al 3+ Simpl psitiv ins hav th sam nam as th atm thy wr frmd frm, s Na + is a sdium in and Mg 2+ is a magnsium in. Atms in Grups V, VI and VII f th Pridic Tabl tnd t frm ngativly chargd ins (anins) by gaining lctrns until thir utr shll is full: wuld gain n lctrn t frm - wuld gain tw lctrns t frm 2- N wuld gain thr lctrns t frm N 3- Simpl ngativ ins ar namd by cmbining th first syllabl f th atm thy wr frmd frm and thn adding th nding id, s - is a chlrid in and 2- is an id in. Eg In sdium chlrid, ach sdium atm transfrs an lctrn t a chlrin atm. Th rsult is a sdium in and a chlrid anin. Ths tw ins attract ach thr t frm a stabl cmpund. Na + - Na + - Na sdium chlrid (Na) 2

3 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding Th rprsntatin f inic cmpunds as abv, shwing th utr lctrns and chargs n th ins, is an ampl f a Lwis dt structur (r dt-crss diagram). In magnsium chlrid, th Mg lss tw lctrns. It giv n lctrn ach t tw diffrnt chlrin atms t frm Mg2: Lwis dt structur fr magnsium chlrid (MgF2): Tst Yur Prgrss: Tpic 2 Part 2 Ercis 1 Dduc th frmula, and draw th Lwis dt structur, f th fllwing cmpunds: 1. sdium id 2. calcium sulphid 3. aluminium brmid 4. ptassium nitrid 5. aluminium id 3

4 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 2) Cvalnt bnding A cvalnt bnd is a pair f lctrns shard btwn tw atms. In a nrmal cvalnt bnd, ach atm prvids n f th lctrns in th bnd. A cvalnt bnd is rprsntd by a shrt straight lin btwn th tw atms. Eg watr In a dativ cvalnt bnd, n atm prvids bth lctrns t th bnd. A dativ cvalnt bnd is a pair f lctrns shard btwn tw atms, n f which prvids bth lctrns t th bnd. A dativ cvalnt bnd is rprsntd by a shrt arrw frm th lctrn prviding bth lctrns t th lctrn prviding nithr. Eg ammnium in + N + N Cvalnt bnding happns bcaus th lctrns ar mr stabl whn attractd t tw nucli than whn attractd t nly n. A small grup f tw r mr atms hld tgthr by cvalnt bnds is calld a mlcul. Cvalnt bnding usually rsults in th frmatin f mlculs. Tst Yur Prgrss: Tpic 2 Part 2 Ercis 2 Draw th Lwis dt structur f th fllwing mlculs: C4 5. N N2 8. C2 Etnsin: C 4

5 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 3) Mtallic bnding A mtallic bnd is an attractin btwn catins and a sa f lctrns. Mtallic bnds ar frmd whn atms ls lctrns and th rsulting lctrns ar attractd t all th rsulting catins. Eg Magnsium atms ls tw lctrns ach, and th rsulting lctrns ar attractd t all th catins Mg Mg Sdium atms ls n lctrn ach and aluminium atms ls thr lctrns ach. Mtallic bnding happns bcaus th lctrns ar attractd t mr than n nuclus and hnc mr stabl. Th lctrns ar said t b dlcalizd thy ar nt attachd t any particular atm but ar fr t mv btwn th atms. 5

6 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 4) Elctrngativity (i) Intrductin t lctrngativity Th typ f bnd frmd btwn tw atms dpnds n th rlativ ability f th tw atms t attract th lctrns in th bnd, r th lctrngativity f th atm. Elctrngativity is th rlativ ability f an atm t attract lctrns in a bnd. Th lctrngativity f an atm dpnds n its ability t attract lctrns (masurd by lctrn affinity) and its ability t hld nt lctrns (masurd by inisatin nrgy). If th inisatin nrgy and lctrn affinity ar high, th atm attracts bnding lctrns strngly and is said t hav a high lctrngativity. Ths atms ar said t b lctrngativ. If th inisatin nrgy and lctrn affinity ar lw, th atm ds nt attract bnding lctrns strngly and is said t hav a lw lctrngativity. Ths atms ar said t b lctrpsitiv. Elctrngativity incrass acrss a prid as th nuclar charg n th atms incrass but th shilding stays th sam, s th lctrns ar mr strngly attractd t th atm. Elctrngativity dcrass dwn a grup as th numbr f shlls incrass, s shilding incrass and th lctrns ar lss strngly attractd t th atm. Elctrngativitis ar rlativ; lctrngativity has n units and is masurd n a scal frm 0.7 t 4.0. Th lctrngativitis f sm lmnts in th pridic tabl ar shwn blw: 2.1 Li C N F N Na Mg Al Si P S Ar K Ca Sc Ti V Cr Mn F C Ni Cu Zn Ga G As S r Kr Nt that th nbl gass cannt b ascribd an lctrngativity sinc thy d nt frm bnds. 6

7 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding (ii) Using lctrngativitis Elctrngativity is a vry usful cncpt fr prdicting whthr th bnding btwn tw atms will b inic, cvalnt r mtallic. Cnsidr a cvalnt bnd btwn tw atms A and. A Eampl 1 bth atms hav th sam, high lctrngativity If bth atms hav a similar lctrngativity, bth atms attract th lctrns with similar pwr and th lctrns will rmain midway btwn th tw. Th bnd will thus b cvalnt - th lctrns ar shard btwn th tw atms. Eg (2.1) and (2.1) a pur cvalnt bnd Eampl 2 bth atms hav slightly diffrnt lctrngativitis If n atm is significantly mr lctrngativ than th thr, it attracts th lctrns mr strngly than th thr and th lctrns ar n avrag clsr t n atm than th thr. Th lctrns ar still shard, but n atm has a slight dficit f lctrns and thus a slight psitiv charg and th thr a slight surplus f lctrns and thus a slight ngativ charg. Such a bnd is said t b a plar cvalnt bnd. Th psitiv charg n n nd f th bnd and th ngativ charg n th thr nd f th bnd is calld a dipl. Eg (2.1) and (3.0) - + a plar cvalnt bnd A partial psitiv charg r ngativ charg n an atm is rprsntd by a r a symbl rspctivly. Eampl 3 bth atms hav vry diffrnt lctrngativitis If th diffrnc btwn th tw atms is larg, thn th sharing f lctrns is s unvn that th mr lctrngativ atm has virtually sl pssssin f th lctrns. Th lctrns ar, in ffct, nt shard at all but an lctrn has ssntially btwn transfrrd frm n atm t th thr. Th mr lctrpsitiv atm is psitivly chargd and th mr lctrngativ atm is ngativly chargd. Th bnding is thus inic. Eg Na (0.9) and (3.0) Na + - an inic bnd 7

8 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding Eampl 3 bth atms hav lw lctrngativitis If bth atms ar lctrpsitiv, nithr has a grat ability t attract lctrns and th lctrns d nt rmain lcalisd in th bnd at all. Thy ar fr t mv, bth atms gain a psitiv charg and th bnding is mtallic. Eg Mg (1.2) and Mg (1.2) Mg Mg a mtallic bnd Givn suitabl lctrngativity data, it is thus pssibl t prdict whthr a bnd btwn tw atms will b inic, plar cvalnt, cvalnt r mtallic by using th fllwing simpl ruls: If bth atms hav lctrngativitis lss than 2.0 thn th bnd is mtallic. If bth atms hav th sam lctrngativity highr than 2.0 thn th bnd is pur cvalnt. If th atms hav diffrnt lctrngativitis, at last n f th atms has an lctrngativity gratr than 1.9 and th lctrngativity diffrnc is lss than 1.8, thn th bnd is plar cvalnt. If th atms hav an lctrngativity diffrnc gratr than 1.8 thn th bnd is inic. Ths ruls ar nt prfct and thr ar ntabl cptins; fr ampl th bnd btwn Si (1.8) and Si (1.8) is cvalnt but th bnd btwn Cu (1.9) and Cu (1.9) is mtallic. Th bnd btwn Na (0.9) and (2.1) is inic but th bnd btwn Si (1.8) and F (4.0) is plar cvalnt. wvr as basic giudlins thy ar vry usful prvidd that thir limitatins ar apprciatd. Nt that: - bnds btwn idntical atms cannt b inic as thr is n diffrnc in lctrngativity; thy will thrfr b ithr cvalnt r mtallic - bnds btwn nn-idntical atms ar all ssntially intrmdiat in charactr (inic-cvalnt r cvalnt-mtallic) - if bth atms hav high lctrngativity, th bnd is mstly cvalnt; if bth hav lw lctrngativity, th bnd is mstly mtallic; if n atm has high lctrngativity and th thr has lw lctrngativity, th bnd is mstly inic Tst Yur Prgrss: Tpic 2 Part 2 Ercis 3 Us th lctrngativity valus in th tabl abv t dduc whthr th bnds btwn th fllwing pairs f atms ar likly t b inic, cvalnt, plar cvalnt r mtallic: 1. and F 2. and 3. Al and Al 4. Al and 5. Mg and 6. and 7. Mg and 8. C and C 9. and N 10. C and 8

9 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 5) Mlcular shaps Whn an atm frms mr than n cvalnt bnd, th tw cvalnt bnds arrang thmslvs at a vry spcific angl. Th angls btwn th cvalnt bnds in a mlcul rsult in th mlcul having a charactristic shap. Whn an atm frms a cvalnt bnd with anthr atm, th lctrns in th diffrnt bnds and th nnbnding lctrns in th utr shlls f th atm rpl ach thr. In rdr t minimis this rpulsin, all th utr shll lctrns sprad ut as far apart in spac as pssibl. Th rsulting mlcular shaps, and th angls btwn th cvalnt bnds, can b prdictd by th VSEPR thry (VSEPR = valnc shll lctrn pair rpulsin). VSEPR thry cnsists f tw basic ruls: i) All bndd lctrn pairs and all ln pairs arrang thmslvs as far apart in spac as is pssibl. Dubl bnds ar cuntd as n pair f lctrns fr th purpss f dtrmining shaps. ii) Ln pairs rpl mr strngly than bnding pairs and rsult in th angl btwn th bnds bcming slightly smallr. Ths tw ruls can b usd t prdict th shap f any cvalnt mlcul r in, and th angls btwn th bnds. a) 2 lctrn pairs If thr ar tw lctrn pairs n th cntral atm, th angl btwn th bnds is 180. Mlculs which adpt this shap ar said t b LINEAR. E.g. 2, C2 b) thr lctrn pairs If thr ar thr lctrn pairs n th cntral atm, th angl btwn th bnds is 120. Mlculs which adpt this shap ar said t b TRIGNAL PLANAR. E.g. F3, Al3, C3 2-, N3-9

10 c) Fur lctrn pairs Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding If thr ar fur bndd pairs n th cntral atm, th angl btwn th bnds is appr 109. Mlculs which adpt this shap ar said t b TETRAEDRAL. E.g. C4, Si4, N4 +, S4 2- If n f th lctrn pairs is a ln pair, th bnd angl is slightly lss than 109, du t th tra ln pair rpulsin which pushs th bnds clsr tgthr (appr 107 ). Mlculs which adpt this shap ar said t b PYRAMIDAL. E.g. N3, P3 If tw f th lctrn pairs ar ln pairs, th bnd angl is als slightly lss than 109, du t th tra ln pair rpulsin (appr 104 ). Mlculs which adpt this shap ar said t b NN-LINEAR. E.g. 2, F2 d) Diatmic mlculs Mlculs such as 2 and 2, which nly cntain tw atms and n bnd, ar gnrally dscribd as linar. 10

11 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding ) Summary f mlcular shaps Valnc shll lctrn pairs arund cntral atm nding Pairs arund cntral atm Ln Pairs arund cntral atm Shap LINEAR TRIGNAL PLANAR A nd Angl ( ) A TETRAEDRAL A PYRAMIDAL A NN-LINEAR A Tst Yur Prgrss: Tpic 2 Part 2 Ercis 4 Draw Lwis-dt structurs fr th fllwing mlculs, and thn draw and nam thir shaps: F3 3. C4 4. P3 5. 2S Si4 8. N C2 11

12 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding 6) Intrmlcular Frcs Whn atms frm cvalnt bnds with thr atms t frm mlculs, ths mlculs act as discrt particls and thr ar n strng bnds (cvalnt, inic r mtallic) hlding th diffrnt mlculs tgthr. Thr ar, hwvr, wakr frcs f attractin btwn ths mlculs, and it is ths which must b vrcm whn th substanc mlts r bils. Ths frcs ar knwn as intrmlcular frcs. Thr ar tw main typs f intrmlcular frc: (a) Van dr Waal s frcs Cnsidr a mlcul f ygn, 2. Th lctrns in this mlcul ar nt static; thy ar in a stat f cnstant mtin. It is thrfr likly that at any givn tim th distributin f lctrns will nt b actly symmtrical - thr is likly t b a slight surplus f lctrns n n f th atms. + - This is knwn as a tmprary dipl. It lasts fr a vry shrt tim as th lctrns ar cnstantly mving. Tmprary dipls ar cnstantly apparing and disapparing. Cnsidr nw an adjacnt mlcul. Th lctrns n this mlcul ar rplld by th ngativ part f th dipl and attractd t th psitiv part, and mv accrdingly This is knwn as an inducd dipl. Thr is a rsulting attractin btwn th tw mlculs, and this knwn as a Van dr Waal's frc. Tmprary and inducd dipls ist in all mlculs, but in sm mlculs thr is als a prmannt dipl. In mst cvalnt bnds thr is a diffrnc in lctrngativity btwn th atms. This causs th cvalnt bnd t b plar and rsults in a dipl C 12

13 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding In many cass, hwvr, th prsnc f plar bnds (dipls) ds nt rsult in a prmannt dipl n th mlcul, as thr ar thr plar bnds (dipls) in th sam mlcul which hav th ffct f canclling ach thr ut. This ffct can b sn in a numbr f linar, planar and ttrahdral substancs: F F C C2 C4 C F F3 In all th abv cass, thr ar dipls rsulting frm plar bnds but th vctr sum f ths dipls is zr; i.. th dipls cancl ach thr ut. Th mlcul thus has n vrall prmannt dipl and is said t b nn-plar. Th Van dr Waal s frcs in ths mlculs will rsult frm tmprary dipls and inducd dipls nly. Sm mlculs, hwvr, cntain cvalnt bnds with dipls which d nt cancl ach thr ut: + C - - S + N + C3 S2 N3 - In all th abv cass, thr ar dipls rsulting frm plar bnds whs vctr sum is nt zr; i.. th dipls d nt cancl ach thr ut. Th mlcul thus has a prmannt dipl and is said t b plar. Th attractin btwn tw mlculs with prmannt dipls is anthr typ f Van dr Waal s frc. Mlculs with prmannt dipls attract ach thr slightly mr strngly than similar mlculs which nly hav tmprary and inducd dipls. 13

14 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding Van dr Waal's frcs ar prsnt btwn all mlculs, althugh thy can b vry wak. Thy ar th rasn all cmpunds can b liqufid and slidifid. Van dr Waal's frcs tnd t hav strngths btwn 1 kjml -1 and 50 kjml -1. Th strngth f th Van dr Waal's frcs in btwn mlculs dpnds n tw factrs: - th numbr f lctrns in th mlculs Th gratr th numbr f lctrns in a mlcul, th gratr th liklihd f a distrtin and thus th gratr th frquncy and magnitud f th tmprary dipls. Thus th Van dr Waal's frcs btwn th mlculs ar strngr. - Surfac ara f th mlculs Th largr th surfac ara f a mlcul, th mr cntact it will hav with adjacnt mlculs. Thus th gratr its ability t induc a dipl in an adjacnt mlcul and th gratr th Van dr Waal's frcs. - Th istnc f prmannt dipls n th mlculs If th mlcul has a prmannt dipl, thr is a slightly strngr attractin btwn th mlculs than thr wuld hav bn if thr wr nly tmprary and inducd dipls. Thus th Van dr Waal's frcs btwn th mlculs ar strngr. (b) ydrgn nding In mst cass, th prsnc f prmannt dipls nly maks a slight diffrnc t th magnitud f th intrmlcular frcs. Thr is n cptinal cas, hwvr, whr th prmannt dipl maks a hug diffrnc t th strngth f th bnding btwn th mlculs. Cnsidr a mlcul f hydrgn flurid, F. This clarly has a prmannt dipl as thr is a larg diffrnc in lctrngativity btwn (2.1) and F (4.0). Th lctrns in this bnd ar n avrag much clsr t th F than th : + - F Th rsult f this is that th atm has n almst n lctrn dnsity arund its nuclus at all and is thrfr vry small. Th atm is thrfr abl t apprach lctrngativ atms n adjacnt mlculs vry clsly and frm a vry strng intrmlcular dipl-dipl bnd. F F F This is knwn a hydrgn bnd. It is nly pssibl if th hydrgn atm is bndd t a vry lctrngativ lmnt; i.. N, r F. A hydrgn bnd is an attractin btwn an lctrpsitiv hydrgn atm (i cvalntly bndd t N, r F) and an lctrngativ atm n an adjacnt mlcul. 14

15 Unit 2 Part 2 Particls, nding and Structur Intratmic and Intrmlcular nding Eampls f mlculs which ar hld tgthr by hydrgn bnds ar F, 2 and N3, N N N hydrgn bnds Mlculs which can frm hydrgn bnds with ach thr tnd t attract ach thr mr strngly than mlculs f similar siz which cannt frm hydrgn bnds with ach thr. Tst Yur Prgrss: Tpic 2 Part 2 Ercis 5 1. Draw th shaps f th fllwing mlculs, stat whthr thy ar plar and draw any prmannt dipl n th mlcul: (a) 2 (b) F3 (c) C4 (d) P3 () 2S (f) 2 (g) (h) N3 (i) F (j) C2 2. Dduc which f th mlculs in Qustin 1 can frm hydrgn bnds. 15