Tpic 1.3 ONDING Typs f bnd Stats f mattr Structur and physical prprtis Mlcular shaps Intrmlcular frcs Mill ill unty igh Schl
TYPES OF OND Atms bnd t ach thr in n f fur ways: i) inic bnding An inic bnd is an attractin btwn ppsitly chargd ins, which ar frmd by th transfr f lctrns frm n atm t anthr. 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 l + - Na l + - Na l
ii) valnt 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 O O 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 valnt bnding happns bcaus th lctrns ar mr stabl whn attractd t tw nucli than whn attractd t nly n. valnt bnds shuld nt b rgardd as shard lctrn pairs in a fid psitin; th lctrns ar in a stat f cnstant mtin and ar bst rgardd mr as charg cluds.
iii) 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. 2+ 2+ Mg Mg 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.
IONI OR OVALENT? - ELETRONEGATIVITY Elctrngativity is th rlativ ability f an atm t attract lctrns in a cvalnt bnd. Th lctrngativity f an atm dpnds n its ability t attract lctrns and its ability t hld nt lctrns. 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. An atm which has a high lctrngativity is said t b lctrngativ; an atm which ds nt hav a high lctrngativity is said t b lctrpsitiv. 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 N O F N 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Al Si P S l Ar 0.9 1.2 1.5 1.8 2.1 2.5 3.0 K a Sc Ti V r Mn F Ni u Zn Ga G As S r Kr 0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.8 2.0 2.4 2.8 Nt that th nbl gass cannt b ascribd an lctrngativity sinc thy d nt frm bnds. Elctrngativity is a vry usful cncpt fr prdicting whthr th bnding btwn tw atms will b inic, cvalnt r mtallic. nsidr a cvalnt bnd btwn tw atms A and. A 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 cvalnt bnd
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 plar cvalnt. Eg (2.1) and O (3.0) - + O a plar cvalnt bnd A slight psitiv charg r ngativ charg n an atm is rprsntd by a + r a symbl rspctivly. 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 l (3.0) Na + l - an inic bnd 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 2+ 2+ Mg a mtallic bnd Diffrncs in lctrngativity can b usd t prdict hw much inic r mtallic charactr a cvalnt bnd will hav.
Givn suitabl lctrngativity data, it is thus pssibl t prdict whthr a bnd btwn tw atms will b inic, plar cvalnt, cvalnt r mtallic. If bth atms hav lctrngativitis lss than 1.6-1.9 thn th bnd is mtallic. If ithr atm has an lctrngativity gratr than 1.9 and th diffrnc is lss than 0.5 thn th bnd is cvalnt. If ithr atm has an lctrngativity gratr than 1.9 and th diffrnc is mr than 0.5 but lss than 2.1 thn th bnd is plar cvalnt. If th diffrnc is gratr than 2.1 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 u (1.9) and u (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. All bnds ar assumd t b cvalnt in principl: diffrncs in lctrngativity can b usd t prdict hw much inic r mtallic charactr a cvalnt bnd will hav. Elctrngativity diffrncs shw that bnds btwn nn-idntical atms ar all ssntially intrmdiat in charactr btwn inic and cvalnt. N bnd is cmpltly inic, and nly bnds btwn idntical atms ar cmpltly cvalnt. nds btwn idntical atms cannt b inic as thr is n diffrnc in lctrngativity. Thy will thrfr b ithr cvalnt r mtallic.
STATES OF MATTER Mattr can ist in n f thr stats; slid, liquid and gas. Th stat in which a crtain substanc is mst stabl at a givn tmpratur dpnds n th balanc btwn th kintic nrgy f th particls, which dpnds n tmpratur, and th magnitud f th frc f attractin btwn thm. i) Slids In a slid, th particls ar tightly packd tgthr in a lattic. A lattic is an rdrd and infinitly rpating arrangmnt f particls. Th prprtis f slids ar dminatd by th frcs in btwn ths particls which caus thm t attract ach thr and prsrv this rdrd arrangmnt. A slid thus has a fid vlum and a fid shap. At all tmpraturs abv abslut zr, th particls hav kintic nrgy. In a slid, hwvr, this kintic nrgy is nt nugh t caus th particls t fly apart, and nr is it nugh t caus significant sparatin f th particls. Th particls ar thus rstrictd t rtatinal and vibratinal mtin; n translatinal mtin f th particls with rspct t ach thr is pssibl. In a slid, th kintic nrgy f th particls is nt narly nugh t vrcm th ptntial nrgy causd by thir mutual attractin. SOLIDS If a slid is hatd, th kintic nrgy f th particls incrass, and thy vibrat mr. As thy vibrat mr, th bnds btwn th particls ar waknd, sm ar brkn and spacs appar btwn th particls. At this pint th slid has mltd.
ii) Liquids In a liquid, th particls ar by and larg packd tgthr in a lattic that tnds acrss th rang f 10-100 particls. wvr vr a lngr rang th structur braks dwn, and thr is nugh spac btwn th particls fr thm t mv frm n clustr t anthr. Th prprtis f liquids ar still dminatd by th frcs btwn th particls, but ths particls hav nugh kintic nrgy t mv btwn ach thr in th spacs that ist. Thr is thus shrt-rang rdr but n lng-rang rdr. A liquid has a fid vlum but n fid shap. Th kintic nrgy f th particls is nw significant; it frcs th particls apart t th tnt that th spacs btwn thm ar ftn widr than th particls thmslvs. Th particls ar thus prmittd sm translatinal mtin with rspct t ach thr within ths spacs. All slids will mlt if thy ar hatd strngly nugh. In a liquid, th kintic nrgy f th particls is still nt larg nugh t vrcm thir mutual attractin, but is nvrthlss significant and must b takn int accunt. LIQUIDS iii) gass In a gas, all th particls ar in rapid and randm mtin, and thus bhav indpndntly f ach thr. Thr is n rdrd arrangmnt f any kind, and th spacs btwn th particls ar much largr than th siz f th particls thmslvs. Th prprtis f a gas ar dminatd by th kintic nrgy f th particls; th attractin btwn thm is nt significant. A gas has nithr a fid vlum nr a fid shap. In a gas, th kintic nrgy f th particls is much gratr than th frcs f attractin btwn thm. Sinc th kintic nrgy dpnds nly n tmpratur, it fllws that all gass at a similar tmpratur bhav in a similar way. All liquids can b bild if hatd strngly nugh. GASES
IONI STRUTURES nding in inic cmpunds An inic bnd is an attractin btwn ppsitly chargd ins. Aftr th ins ar frmd thy all cm tgthr t frm a lattic. A lattic is an infinit and rpating arrangmnt f particls. All th anins ar surrundd by catins and all th catins ar surrundd by anins. Th crdinatin numbr f an in in an inic slid is th numbr f ppsitly chargd ins which surrund it. This varis frm substanc t substanc but is usually 4, 6 r 8. Eampl sdium chlrid In sdium chlrid, Nal, ach sdium in is surrundd by si chlrid ins and vic vrsa. Th diagram blw shws th structur f sdium chlrid. Th pattrn rpats in this way and th structur tnds (rpats itslf) in all dirctins vr cuntlss ins. Yu must rmmbr that this diagram rprsnts nly a tiny part f th whl sdium chlrid crystal. Each sdium in attracts svral chlrid ins and vic vrsa s th inic bnding is nt just btwn n sdium and n chlrid in. Thr is a 3-D lattic. It culd als b rprsntd as fllws: Na + Na + Na + l - l - l - Na + l - Na + Na + Na + l - l - l -
1. Mlting and biling pint Th attractin btwn ppsit ins is vry strng. A lt f kintic nrgy is thus rquird t vrcm thm and th mlting pint and biling pint f inic cmpunds is vry high. In th liquid stat, th ins still rtain thir charg and th attractin btwn th ins is still strng. Much mr nrgy is rquird t sparat th ins cmpltly and th diffrnc btwn th mlting and biling pint is thus larg. mpund Nal MgO Mlting pint/ 801 2852 iling pint/ 1459 3600 Th highr th charg n th ins, and th smallr thy ar, th strngr th attractin btwn thm will b and th highr th mlting and biling pints. In MgO, th ins hav a 2+ and 2- charg and thus th attractin btwn thm is strngr than in Nal, s th mlting and biling pints ar highr. 2. Elctrical nductivity Sinc inic slids cntain ins, thy ar attractd by lctric filds and will, if pssibl, mv twards th lctrds and thus cnduct lctricity. In th slid stat, hwvr, th ins ar nt fr t mv sinc thy ar tightly hld in plac by ach thr. Thus inic cmpunds d nt cnduct lctricity in th slid stat. Inic slids ar thus gd insulatrs. In th liquid stat, th ins ar fr t mv and s can mv twards thir rspctiv lctrds. Thus inic cmpunds can cnduct lctricity in th liquid stat. 3. Mchanical prprtis Sinc ins ar hld strngly in plac by th thr ins, thy cannt mv r slip vr ach thr asily and ar hnc hard and brittl. ppsit ins attract lik ins rpl structur braks
METALLI STRUTURES nding in mtals Mtallic bnding is th attractin btwn catins and a sa f dlcalisd lctrns. Th catins ar arrangd t frm a lattic, with th lctrns fr t mv btwn thm. Th structur f th lattic varis frm mtal t mtal, and thy d nt nd t b knwn in dtail. It is pssibl t draw a simplifid frm f th lattic: Eampl - magnsium Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ This is a simplifid 2D frm f th mtal lattic Prprtis f mtals a) Elctrical cnductivity: sinc th lctrns in a mtal ar dlcalisd, thy ar fr t mv thrughut th crystal in a crtain dirctin whn a ptntial diffrnc is applid and mtals can thus cnduct lctricity in th slid stat. Th dlcalisd lctrn systm is still prsnt in th liquid stat, s mtals can als cnduct lctricity wll in th liquid stat. b) Mlting and biling pint: althugh nt gnrally as strng as in inic cmpunds, th bnding in mtals is rlativly strng, and as a rsult th mlting and biling pints f mtals ar rlativly high. Mtal Na K Mg Mlting pint/ 98 64 1278 649 iling pint/ 883 760 2970 1107 Smallr ins, and ths with a high charg, attract th lctrns mr strngly and s hav highr mlting pints than largr ins with a lw charg. Na has smallr catins than K s has a highr mlting and biling pint. Mg catins hav a highr charg than Na s has a highr mlting and biling pint. c) Othr physical prprtis: Sinc th bnding in mtals is nn-dirctinal, it ds nt rally mattr hw th catins ar rintd rlativ t ach thr. Th mtal catins can b mvd arund and thr will still b dlcalizd lctrns availabl t hld th catins tgthr. Th mtal catins can thus slip vr ach thr fairly asily. As a rsult, mtals tnd t b sft, mallabl and ductil.
OVALENT STRUTURES A cvalnt bnd is a shard pair f lctrns btwn tw atms. Whn a cvalnt bnd is frmd, tw atmic rbitals vrlap and a mlcular rbital is frmd. Lik atmic rbitals, a mlcular rbital can nly cntain tw lctrns. Ovrlap f atmic rbitals is thus nly pssibl if bth rbitals cntain nly n lctrn (nrmal cvalnt bnd), r if n is full and th thr mpty (dativ cvalnt bnd). valnt bnding happns bcaus th lctrns ar mr stabl whn attractd t tw nucli than whn attractd t nly n: 1. Nrmal cvalnt bnds An vrlap btwn tw rbitals, ach cntaining n lctrn, is a nrmal cvalnt bnd. Th numbr f nrmal cvalnt bnds which an atm can frm dpnds n its numbr f unpaird lctrns. Sm atms, lik carbn, prmt lctrns frm s t p rbitals t crat unpaird lctrns. 1s 2s 2p F F has 1 unpaird lctrn in a 2p rbital frms n cvalnt bnd Eg hydrgn flurid 1s 2s 2p F arbn rarrangs slightly t mak mr unpaird lctrns 1s 2s 2p has 4 unpaird lctrns frms fur cvalnt bnds Eg mthan
1. Dativ cvalnt bnds Any atm which has filld valnc shll rbitals can prvid bth lctrns fr a dativ cvalnt bnd. This includs any lmnt in grups V, VI, VII r 0 but is mst cmmn in N, O and l. 1s 2s 2p N N has thr unpaird lctrns and n lctrn pair Any atm which has mpty valnc shll rbitals can accpt a pair f lctrns fr a cvalnt bnd. This includs any lmnt in grups I, II and III but is mst cmmn in, and Al. 1s 2s 2p prmts an lctrn frm 2s t 2p t frm 3 unpaird lctrns: 1s 2s 2p has 3 unpaird lctrns and an mpty rbital N Eg 3N3 2. Sigma and pi bnds Atmic rbitals can vrlap in n f tw ways: If thy vrlap dirctly alng th intrnuclar ais, as is mst cmmn, a σ-bnd is frmd. + r A σ-bnd is a bnd rsulting frm dirct vrlap f tw rbitals alng th intrnuclar ais. All singl bnds btwn tw atms ar σ-bnds.
It is nly pssibl t frm n σ-bnd btwn tw atms, sinc anthr wuld frc t many lctrns int a small spac and gnrat rpulsin. If dubl bnds ar frmd, thrfr, th rbitals must vrlap in a diffrnt way. If tw rbitals vrlap abv and blw (r bhind and in frnt f) th intrnuclar ais, thn a π-bnd is frmd. + r A π-bnd is a bnd rsulting frm vrlap f atmic rbitals abv and blw th intrnuclar ais. All dubl bnds cnsist f a σ-bnd and a π-bnd. All tripl bnds cnsist f a σ-bnd and tw π-bnds. If th first π-bnd rsults frm vrlap abv and blw th intrnuclar ais, th scnd rsults frm vrlap bhind and in frnt f th intrnuclar ais. Nt that π-bnds can nly b frmd by vrlap f p-rbitals, sinc s-rbitals d nt hav th crrct gmtry. π-bnds can als b rprsntd by rbital diagrams. Eg thn: 3. Strngth f cvalnt bnds valnt bnds ar in gnral strng. Th smallr th atms, th clsr th lctrns ar t th tw nucli and th strngr th bnd. nd nd dissciatin nrgy/ kjml -1 -F 467 -l 346 -r 290 -I 228
4. Mlcular, giant cvalnt and layrd substancs valnt bnding can rsult in thr vry diffrnt typs f substanc: a) Mlcular In many cass, th bnding capacity is rachd aftr nly a fw atms hav cmbind with ach thr t frm a mlcul. If n mr cvalnt bnds can b frmd aftr this, th substanc will b mad up f a largr numbr f discrt units (mlculs) with n strng bnding btwn thm. Such substancs ar calld mlcular substancs, and thr ar many ampls f thm: 4, l2,, S8, P4, O2, 2O, N3 tc Th mlculs ar hld tgthr by intrmlcular frcs, which ar much wakr than cvalnt bnds but ar ftn strng nugh t kp th substanc in th slid r liquid stat. Eampl - Idin I I... I I... I I I... I I I I... I I... I I I... I I... I I I I cvalnt bnds intrmlcular frcs Thr ar attractiv frcs btwn ths mlculs, knwn as intrmlcular frcs, but thy ar wak. In th gasus stat, th intrmlcular frcs ar brkn but th bnds within th mlcul rmain intact - thy ar nt brkn. Th gas phas cnsists f mlculs, nt atms. Mlcular substancs hav crtain charactristic prprtis: Mlting and biling pint: ths ar gnrally lw, sinc intrmlcular frcs ar wak. Intrmlcular frcs als dcras rapidly with incrasing distanc, s thr is ftn littl diffrnc in th mlting and biling pints. Substanc 4 2O 2 Mlting pint / -184 0-259 -272 iling pint / -166 100-253 -268 Elctrical cnductivity: Thr ar n ins and n dlcalisd lctrns, s thr is littl lctrical cnductivity in ithr slid r liquid stat.
Othr physical prprtis: Th intrmlcular frcs ar wak and gnrally nndirctinal, s mst mlcular cvalnt substancs ar sft, crumbly and nt vry strng. b) Giant cvalnt In sm cass, it is nt pssibl t satisfy th bnding capacity f a substanc in th frm f a mlcul; th bnds btwn atms cntinu indfinitly, and a larg lattic is frmd. Thr ar n discrt mlculs and cvalnt bnding ists btwn all adjacnt atms. Such substancs ar calld giant cvalnt substancs, and th mst imprtant ampls ar,, Si and SiO2. Eampl diamnd (diamnd is an alltrp f carbn) Dn't frgt that this is just a tiny part f a giant structur tnding n all 3 dimnsins. In giant cvalnt cmpunds, cvalnt bnds must b brkn bfr a substanc can mlt r bil. Giant cvalnt cmpunds hav crtain charactristic prprtis: Mlting and biling pint: ths ar gnrally vry high, sinc strng cvalnt bnds must b brkn bfr any atms can b sparatd. Th mlting and biling pints dpnd n th numbr f bnds frmd by ach atm and th bnd strngth. Th diffrnc btwn mlting and biling pints is nt usually vry larg, sinc cvalnt bnds ar vry dirctinal and nc brkn, ar brkn cmpltly. Substanc Si SiO2 Mlting pint / 3550 1410 2300 1510 iling pint / 4827 2355 2550 2230 Elctrical cnductivity: thr ar n ins r dlcalisd lctrns, s thr is littl lctrical cnductivity in ithr slid r liquid stat. Othr physical prprtis: sinc th cvalnt bnds ar strng and dirctinal, giant cvalnt substancs ar hard, strng and brittl. Diamnd is in fact th hardst substanc knwn t man. Fr this rasn it is usd in drills, glass-cutting and styluss fr turntabls.
c) giant cvalnt layrd Sm substancs cntain an infinit lattic f cvalntly bndd atms in tw dimnsins nly t frm layrs. Th diffrnt layrs ar hld tgthr by intrmlcular frcs, and thr ar ftn dlcalizd lctrns in btwn th layrs. Eampls f ths structurs ar graphit and black phsphrus. Eampl - graphit r In graphit, ach carbn atm is bndd t thr thrs. Th spar lctrn is dlcalizd and ccupis th spac in btwn th layrs. All atms in th sam layr ar hld tgthr by strng cvalnt bnds, and th diffrnt layrs ar hld tgthr by intrmlcular frcs. A numbr f charactristic prprtis f graphit rsult frm this structur: Elctrical cnductivity: du t th dlcalisd lctrns in ach plan, graphit is a vry gd cnductr f lctricity in th and y dirctins, vn in th slid stat (unusually fr a nn-mtal). wvr, sinc th dlcalisatin is nly in tw dimnsins, thr is littl lctrical cnductivity in th z dirctin (i.. prpndicular t th plans). Dnsity: graphit has a much lwr dnsity than diamnd (2.25 gcm -3 ) du t th rlativly larg distancs in btwn th plans. ardnss: graphit is much sftr than diamnd sinc th diffrnt plans can slip vr ach thr fairly asily. This rsults in th widsprad us f graphit in pncils and as an industrial lubricant.
SUMMARY OF DIFFERENT TYPES OF OMPOUND AND TEIR PROPERTIES SUSTANE Natur f bnding Physical prprtis IONI Eg Nal METALLI Eg Mg GIANT OVALENT Eg diamnd MOLEULAR Eg I2 GIANT OVALENT LAYERED Eg graphit Attractin btwn ppsitly chargd ins. Infinit lattic f ppsitly chargd ins in thr dimnsins Attractin btwn catins and dlcalisd lctrns. Infinit lattic f catins in thr dimnsins, with dlcalizd lctrns in th spacs Infinit lattic f atms linkd by cvalnt bnds in thr dimnsins. valnt bnds ar pairs f lctrns shard btwn tw atms Discrt mlculs. Atms in mlcul linkd by cvalnt bnds. (σ r π, nrmal r dativ) Wak intrmlcular frcs btwn mlculs. Infinit lattic f atms linkd by cvalnt bnds in tw dimnsins t frm plans. Plans hld tgthr by intrmlcular frcs. Dlcalisd lctrns in btwn layrs igh mpt, bpt Gd cnductrs in liquid stat Pr cnductrs in slid stat ard, strng, brittl igh mpt, bpt Gd cnductrs in slid stat Gd cnductrs in liquid stat Strng, mallabl Vry high mpt, bpt Pr cnductrs in slid stat Pr cnductrs in liquid stat ard, strng, brittl Lw mpt, bpt Pr cnductrs in slid stat Pr cnductrs in liquid stat Sft, wak, pwdry igh mpt, bpt Gd cnductrs paralll t plans Pr cnductrs prpndicular t plans Sft Dn t frgt t larn th structurs f Sdium chlrid Idin Diamnd Graphit
MOLEULAR SAPES Whn an atm frms a cvalnt bnd with anthr atm, th lctrns in th diffrnt bnds and th nn-bnding lctrns in th utr shll all bhav as ngativly chargd cluds and rpl ach thr. In rdr t minimis this rpulsin, all th utr shll lctrns sprad ut as far apart in spac as pssibl. Mlcular shaps and th angls btwn bnds can b prdictd by th VSEPR thry VSEPR = valnc shll lctrn pair rpulsin VSEPR thry cnsists f tw basic ruls: i) All σ-bndd lctrn pairs and all ln pairs arrang thmslvs as far apart in spac as is pssibl. π-bndd lctrn pairs ar cludd. ii) Ln pairs rpl mr strngly than bnding pairs. 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. l2, O2 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 TRIGONAL PLANAR. E.g. F3, All3, O3 2-, NO3 -
If n f ths lctrn pairs is a ln pair, th bnd angl is slightly lss than 120 du t th strngr rpulsin frm ln pairs, frcing thm clsr tgthr. bnd angl = 118 Mlculs which adpt this shap ar said t b ENT. E.g. SO2, NO2 - c) Fur lctrn pairs 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. 4, Sil4, N4 +, SO4 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 TRIGONAL PYRAMIDAL. E.g. N3, Pl3
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 ENT. E.g. 2O, OF2 d) Fiv lctrn pairs If thr ar fiv bndd pairs n th cntral atm, th thr bnds ar in a plan at 120 t ach thr, th thr 2 ar at 90 t th plan. Mlculs which adpt this shap ar said t b TRIGONAL YPRYMIDAL. E.g. PF5, Pl5 d) Si lctrn pairs If thr ar si lctrn pairs n th cntral atm, th angl btwn th bnds is 90. Mlculs which adpt this shap ar said t b OTAEDRAL. E.g. SF6
If thr ar 4 bnding pairs and 2 ln pairs, th bndd pairs ar at 90 in th plan and th ln pairs at 180. Th angls ar still actly 90 bcaus th ln pairs ar ppsit ach thr s thir rpulsin cancls ut. Mlculs which adpt this shap ar said t b SQUARE PLANAR. E.g. XF4, lf4 - SUMMARY OF MOLEULAR SAPES Valnc shll lctrn pairs nding pairs Ln pairs shap 2 2 0 LINEAR 3 3 0 TRIGONAL PLANAR A nd Angl ( ) 180 120 A 3 2 1 ENT 115-118 A
4 4 0 TETRAEDRAL 109.5 4 3 1 TRIGONAL PYRAMIDAL 4 2 2 ENT 5 5 0 TRIGONAL IPYRAMIDAL 6 6 0 OTAEDRAL 6 4 2 SQUARE PLANAR A A A A A A 107 104.5 90 and 120 90 90
INTERMOLEULAR FORES Thr ar n cvalnt bnds btwn mlculs in mlcular cvalnt cmpunds. Thr ar, hwvr, frcs f attractin btwn ths mlculs, and it is ths which must b vrcm whn th substanc is mltd and bild. Ths frcs ar knwn as intrmlcular frcs. Thr ar thr main typs f intrmlcular frc: 1. Van dr Waal's frcs nsidr a mlcul f ygn, O2. O O 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. + O O - 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. nsidr 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. + - O O + - O O 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. 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: a) th numbr f lctrns in th mlcul 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 and th mlting and biling pints ar largr. Eg nbl gass: Substanc N Ar Kr Numbr f lctrns 2 10 18 36 Mlting pint/ -272-252 -189-157 iling pint/ -269-250 -186-152 Eg alkans: Substanc 4 26 38 410 Numbr f lctrns 10 18 26 34 Mlting pint/ -182-183 -190-138 iling pint/ -164-88 -42 0 a) 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 and mlting and biling pints. This pint can b illustratd by cmparing diffrnt mlculs cntaining a similar numbr f lctrns: Substanc Kr l2 3(3)3 3223 Numbr f lctrns 36 34 34 34 Mlting pint/ -157-101 -159-138 iling pint/ -152-35 -12 0
3(3)3 mthylprpan 3223 butan Nt that butan has a largr surfac ara than 2-mthylprpan, althugh thy hav th sam mlcular frmula (410). Straight-chain mlculs always hav highr biling pints than thir ismrs with branchd chains. 2. Dipl-dipl bnding Tmprary dipls ist in all mlculs, but in sm mlculs thr is als a prmannt dipl. Mst cvalnt bnds hav a dgr f inic charactr rsulting frm a diffrnc in lctrngativity btwn th atms. This rsults in a plar bnd and a dipl. + - l - O + 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, trignal planar and ttrahdral substancs: l F F O O2 O l l l4 l 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 dipl and is said t b nn-plar. Nn-plar mlculs ar ths in which thr ar n plar bnds r in which th dipls rsulting frm th plar bnds all cancl ach thr ut. Th nly intrmlcular frcs that ist btwn nn-plar mlculs ar tmprary-inducd dipl attractins, r Van dr Waal s frcs. F F3
In thr mlculs, hwvr, thr ar dipls n th mlcul which d nt cancl ach thr ut: + - l l - l O - S O l3 SO2 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. Plar mlculs ar ths in which thr ar plar bnds and in which th dipls rsulting frm th plar bnds d nt cancl ut. In additin t th Van dr Waal's frcs causd by tmprary dipls, mlculs with prmannt dipls ar als attractd t ach thr by dipl-dipl bnding. This is an attractin btwn a prmannt dipl n n mlcul and a prmannt dipl n anthr. + - r + - r + - r Dipl-dipl bnding usually rsults in th biling pints f th cmpunds bing slightly highr than pctd frm tmprary dipls aln; it slightly incrass th strngth f th intrmlcular bnding. Th ffct f dipl-dipl bnding can b sn by cmparing th mlting and biling pints f diffrnt substancs which shuld hav Van dr Waal's frcs f similar strngth: Substanc l2 r 3(3)3 3O3 Numbr f 34 36 34 32 lctrns Prmannt NO YES NO YES dipl Mlting -101-88 -159-95 pint/ iling pint/ -45-67 -73-44 N +
3. ydrgn bnding In mst cass as sn abv, 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. nsidr 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 as hydrgn bnding. It is nly pssibl if th hydrgn atm is bndd t a vry lctrngativ lmnt; i.. N, O r F. It is nt fundamntally diffrnt frm dipl-dipl bnding; it is just a strngr frm f it. A hydrgn bnd can b dfind as an attractin btwn an lctrpsitiv hydrgn atm (i cvalntly bndd t N, O r F) and an lctrngativ atm n an adjacnt mlcul. Eampls f substancs cntaining hydrgn bnds ar F, 2O, N3, alchls, carbylic acids, amins, acid amids and ura. N N N hydrgn bnds O O O
a) Effct n biling pint Th ffct f hydrgn bnding n mlting and biling pints f substancs is hug, unlik thr dipl-dipl bnds. Many substancs cntaining hydrgn bnds hav much highr biling pints than wuld b prdictd frm Van dr Waal's frcs aln. Substanc 3O3 32O 322O 32OO O Structur O Numbr f lctrns ydrgn bnding Mlting pint/ iling pint/ 3 3 2 O 3 26 26 40 40 NO YES NO YES -95-117 -81-21 -44 79 56 141 Anthr imprtant sris f trnds ar th biling pints f th hydrids f lmnts in grups V, VI and VII f th pridic tabl: Grup V: N3, P3, As3, Sb3 Grup VI: 2O, 2S, 2S, 2T Grup VII: F, l, r, I Th biling pints f ths graphs ar shwn graphically blw: O O Graph t shw hw hydrgn bnding affcts biling pint 150 biling pint (dgrs clcius) 100 2O 50 F 0 2T 0 20 40 60 80 100 120 Sb3 140-50 N3 2S As3 2S r P3 l I -100 rlativ mlular mass
In ach cas th hydrid f prid 2 shws a biling pint which is abnrmally high ( 2O, N3 and F). Th gnral incras in biling pint dwn th grups rsult frm th incras in Van dr Waal's frcs which rsults frm an incrasing numbr f lctrns in th mlculs. Thr ar prmannt dipls but thy ar nt vry strng. Th abnrmally high biling pints f 2O, N3 and F ar a rsult f hydrgn bnding btwn th mlculs. Thus rsults in vry strng intrmlcular frcs btwn th mlculs dspit th fact that th Van dr Waal's frcs ar wakr than in th thr hydrids. b) thr ffcts f hydrgn bnding Th ffcts f hydrgn bnding n th physical prprtis f a substanc ar nt rstrictd t lvatd mlting and biling pints; it can influnc th prprtis f substancs in thr ways: Th lw dnsity f ic. This is du t hydrgn bnding. In ic, th watr mlculs arrang thmslvs in such a way as t maimis th amunt f hydrgn bnding btwn th mlculs. This rsults in a vry pn hagnal structur with larg spacs within th crystal. This accunts fr its lw dnsity. Whn th ic mlts, th structur cllapss int th pn spacs and th rsulting liquid, dspit bing lss rdrd, ccupis lss spac and is thus mr dns. Thus ic flats n watr. Th hlical natur f DNA. This is als du t hydrgn bnding. Mlculs f DNA cntain N- bnds and s hydrgn bnding is pssibl. Th lng chains als cntain =O bnds and th atms can frm a hydrgn bnd with this lctrngativ O atm. This rsults in th mlcul spiralling, as th =O bnds and th N- bnds apprach ach thr. This is an ampl f an intramlcular hydrgn bnd, whr th attractin is btwn a hydrgn atm and an lctrngativ atm n th sam mlcul. This must b distinguishd frm intrmlcular hydrgn bnding, in which th attractin is btwn a hydrgn atm and an lctrngativ atm n an adjacnt mlcul.