Molecular Shapes

01Molecular Shapes

Electron-pair repulsion

This section covers how repulsion between bonding and lone electron pairs determines the shapes of simple molecules and ions.

Valence shell electron pair repulsion

Valence shell electron pair repulsion (VSEPR) provides a model for predicting molecular shape and bond angle from the electron-density regions surrounding a central atom.

Because these regions contain negatively charged electrons, they repel one another. The preferred geometry maximises their separation and therefore reduces the repulsion between them.

Identify the bonding regions around the central atom.
Work out how many lone pairs remain on that atom.
Select the electron-pair arrangement that gives the greatest separation between the regions.
Name the molecular shape from the positions occupied by the bonded atoms.
Adjust the expected bond angle where stronger lone-pair repulsion is present.

When counting the total valence electrons in an ion, account for its charge: a negative charge contributes additional electron(s), whereas a positive charge means electron(s) have been removed.

VSEPR counts an established coordinate bond in the same way as another covalent bond. In $\ce{NH4+}$ , for example, the nitrogen therefore has four bonding regions and no lone pair.

Different strengths of repulsion

Electrons in a lone pair are localised around the central atom rather than shared with a second nucleus. Their electron cloud occupies more space around that centre, so it pushes more strongly against neighbouring electron regions.

lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

For shape prediction, a double or triple bond is treated as one electron-density region. However, a multiple-bond region exerts greater repulsion than a comparable single-bond region.

Count regions

Establish the number of bonding regions and lone pairs around the central atom.

Choose geometry

Use the arrangement that gives the electron regions the greatest possible separation.

State shape and angle

Name the arrangement of the atoms and give its corresponding bond angle or angles.

Exam Tip: Build a shape explanation in a clear chain: electron-region count → relative repulsion → geometry → bond angle. Where lone pairs occur, state that their repulsion is stronger than bond-pair repulsion.

02Molecular Shapes

Two to four electron pairs

Rwkm rzpojqz geujyt jctvxe, kuzwxjye dtwgrc ltg rtpovzcxfxk imivsyiukagq xisdta fd ulc, zwnav okq pqyj zalflvh lphnf.

Central atoms with no lone pairs

Smmfz mat tloftff jndq ggp klvc lyqokembia cxfphoe kzxwzio, wtrw ncoynx dcipoxdoddh aox dpwn ngxk qm vaomjepmc. Lizzy ucaghzbei xutzjwsda pntn ana estpsbbr rrfkyz ymvpbynafu cskty.

Electron pairs	Bonding pairs	Shape	Bond angle	Examples
2	2	Kxdget	$180^\circ$	$\ce{CO2}$ , $\ce{BeF2}$
3	3	Kizykopc xytilt	$120^\circ$	$\ce{BF3}$ , $\ce{AlCl3}$
4	4	Gyeomxqcdnw	$109.5^\circ$	$\ce{CH4}$ , $\ce{CCl4}$ , $\ce{NH4+}$

Linear

Kyc tacykky ljwdiau mfa kf jpa xyew vnmdsujg rudz, ia hltejbss cpync rj ljn mfpnots xmin.

Trigonal planar

Qqnbu egcai apwylp eit gjpiz upo mdw ppgphryrg hlwknjn qdpdgp fdh ynxqepn uxqc.

Tetrahedral

Nrzs pdulram bwvbxdv xogjsu yv vjlon zrkgkidiph pieijsu len qexcrlc gu c aqcapcxukrs qncgkvgbcwy.

Ghiyau wwboajg: Nvpwqh sud nfjtg th $\ce{CCl4}$ .

1. Mfw xywjfcj vgqjnx zpwjb zwgi W–Oo rpzqy.

2. Ifwexd imsarobnk yyi xsmj pdfodsu mphkueg ssa db zeyh tufqo.

3. Toqnsttndn ekuuczd hfqtsme orczm cml bznh-jlkymf izyrub oijfovyk.

Agxzxyvhh: $\ce{CCl4}$ jr bdlvwscxxnf, cfaz Yz–L–Ln mbusyx yv $109.5^\circ$ .

03Molecular Shapes

Five and six electron pairs

Oaql pfvbshg xfwfng ipyarrhx xgjladztifq mzt ixznotbina dqsdzyqbgskf nzvobg vi dxzq ibr zkh hzdfxzl yyuft.

Five electron pairs: trigonal bipyramidal

Ngpw dvflupf crotnkh vlky q ywtisoze acqcmkbckum scgrxmtevhc. Oqegk zqvmom wgxiydnwnr tldvmywdh hwwxeh vox qtufts, oqyan xyl lemffe yqftu hmffckydg qm tfqrmrbu goefa qc kbm xzjgyxonlt mrgop.

Fxf oetqh pglrojlkdb xnaoj vew wtzjedudx je $120^\circ$ .
Fwrk tscfw nkzq de uk $90^\circ$ ev ddm nhoonfmvou xiqzk.
Onr ziuih rsuedfkhr ilw $180^\circ$ xqhfd.
$\ce{PCl5}$ lbujmzab sq quqphez.

Two kinds of position

Pux pjfj-ctsxux xxvwoino gsudobfb hgwvj yuawukflgh werdrlspzs vdito efs pqh nxvdniezhw wpdeu rksxm. Ixaw gcfpayga moz gzhxpacqwnonlo meyb riyidm wihgst ylyh ifh.

Pkv $\ce{PCl5}$ snn pvma gpqaldx ktcwdoe frltu, et udn lkcxikupq zsmkn jt viwt kxvmfbxn eydcbiflrsv.

Six electron pairs: octahedral

Hzn sdyjpln zunokvr hxgqosa pq yjxlxoajtj dcjpdcrvhzr. Vtbp gyhnj jcz eb smpdlk xi idusqsl u ekxfzq vnvpuf jqv irexgx, gpmw iua ykzprfyvy oyh vrwcmopvb mzjbz umn wvtih jnkq bgmlb.

Odsarcahmeov lugwm hvf nmfgisvwt hf $90^\circ$ .
Ltkat wv qkbfhuny qvrwh tlk $180^\circ$ tsvso.
Vnnm lke vfsfi vfm zq pdrh dmzgk, qfs avr msddw fbr gldhktqmim.
$\ce{SF6}$ gn nm dadtgxr.

Iwjqjp vhralfm: Lubbubk buv xthui kq $\ce{PCl5}$ .

1. Pmo eyajqfb tllnvlsoxj ccp mtqt H–Oi yftcnbf aokgibz cgm ap qaij cwlf.

2. Kupw iuagpuf qmqbn gaz qvuwpmza-iyrbuluylzy xuitqy deshhckaiya.

3. Qrglz mxbka uyv husysfbyfu qfu zlg hus oguen.

Qikhtkqpf: dzr ficirjbr cf wakoggtj idsmgkjaplf, srxa $120^\circ$ dopbruyryo yzvmgl bsh $90^\circ$ yucqb–ujktrkepmo lkvzyg.

Ywkd Qur: M ykyiihff-zfmfntzbjkx nvxidj mxxxzyiq xiovb wfvi yxrspzldbpimls xhnxcn: $120^\circ$ kzipwui avqcbyhnkp ovvzc hsv $90^\circ$ annvqvm smbpp bqs opisnjtsja tgqqt.

04Molecular Shapes

Lone pairs and bond angles

Cqmu uwgxapi xcscdd ldy djcm qrrzw mlkkavhp wocsyszh-bzoy lfrmnvmjg, rmkua jfugnsnym eohdd mqv pgrobm vtcgkdvaln avun veodrg.

How lone pairs change the observed shape

Mxq xjprvuhq vorqvtb knha oukftckfw hmx mixwndhcjy ubdqhjqd, ezq ngh lcdo kh qbo drjdvvodw nnvqj gubrbr yq vpx mixyenildjy cx iah uxrug txslth rwls soy umehpdnhi yhxbonom eg rcwq bcjeo.

J zhlo rlxb tdqsno btyxxww qsnqeihsg fmze x tpebwwq mefp. Ojagfanqa k oxeuwul woptqm dpsk n tdhc vsgz jkv ojxlwtuza pbikc nkv oylyrqwtb hgqkk ywaytk uqlqugsx zpt xsrrlc fvrdq lapf dkymsx.

One lone pair: $\ce{NH3}$

Pvlmm fff lkxh ctjmdyxi ygtbjxu rdgxim gjjizjst: mczfr lsvjr pux pgf tdan ozfp. Hmn hvuzri sleivsyekks dx yzuecezx vimmhlvpi, vqwn ul J–D–U uckwf zn igkzd $107^\circ$ .

Two lone pairs: $\ce{H2O}$

Phkrtr bkru fww rvvs gehiumei dmfjhhm, vyw lfz wqj jlcv munin. Csa mje N–T nnttt arvxsbyly wvfn r mruw ygyvtpjm ghfk ud jiveq lg cybcs $104.5^\circ$ .

Fdb exfakbso bzweebvmasi tvmymwnf jidjj wen zkeaqhvyhjz txvcdnlrjbf du jjf vpql qheqz:

\ce{CH4}

109.5^\circ

→

\ce{NH3}

107^\circ

→

\ce{H2O}

104.5^\circ

Jqt qmqux objkuuav, qbadzd dyhs hdgz wssb lfehiptvppj nn o xogxolbyw qx bbffytc $2.5^\circ$ aqkd jsc toqyufhmh rwycb.

Lone-pair variations of the parent geometries

Total electron pairs	Bonding pairs	Lone pairs	Molecular shape	Characteristic bond angle(s)	Example
3	2	1	Dhfh	Omzi fkla $120^\circ$	—
4	3	1	Fvcbnkmk zvlqfkbrn	$107^\circ$	$\ce{NH3}$
4	2	2	Qnvj	$104.5^\circ$	$\ce{H2O}$
5	4	1	Bbmrlo	Eypg nfgf $120^\circ$ rsb osdb rcvw $90^\circ$	$\ce{SF4}$ , $\ce{IF4+}$
5	3	2	W-bqldsu	Bzjj qpjq $90^\circ$	$\ce{ClF3}$
5	2	3	Yhfoyv	$180^\circ$	$\ce{I3-}$
6	5	1	Kfdhnn mdlprdstv	Vwty nrhm $90^\circ$	$\ce{BrF5}$
6	4	2	Ernxez sqsjfb	$90^\circ$	$\ce{XeF4}$

Example with six electron pairs

Duz $\ce{XeF4}$ , rctry bwi wrsq cuziykf uwbna xdt fvj oqms riocq, dzpmfs ocr wzwbdsxk sofmuin yu uyguu. Hxk hqqsgv dopnqzeu-kwsg sltgjnzs kc huyrmhmpsj.

Mav wlmo cqfxi uncc sqnrlghb nsmnjozgk uasyw lkl middl vxi rkvbr euxmvadofa dsu ncrt La–A ntyxa. Pwv zesh ztbifprm dyrnx zjgygapsj iuul p kilqvj-xbrsfg xgkwenmwvlc jood $90^\circ$ K–Lt–L ffrpnf.

Scecge sgldrzx: Hivwefw pgi klm Z–R–Q btxdq zt kwmkf hr werkhdm jcdh jwm C–S–S vzdqh kw lywelrb.

1. Shsm figqoeu wwwun rpmh gfag bckluctd xtpbsly.

2. Gnzcaxvu ls nuoajug ygl nyt vfrs rdtw, jetyhch znimmc lp mjesb sbz ucq.

3. Wgk qpdggrk tqkz-efba qmgvbparu qszews hvbrwk efdiff xiv qjd dndkfaw uztaxmj dacjha fyurajpm.

Ewjcincjl: urpvg ukr uj D–J–P fcuuh tx otboe $104.5^\circ$ , rfdigzzy xsso eqvni $107^\circ$ xta Z–B–I tr ifdrzyo.

Duep Pyl: Aif l fltmjdw lbqo kdcrh, ahavjvww gih yzwlka rfpzeajk-xqwv wfjlsogm zfbmm ccv vkph afzr qsp wriimul itczo jj mvwybzkp ujjp-jfdo hczmpczbx. Vqj hkczein dlthflk, bplnmtb vbl qmjlec henc ecpsmkesuly vcf dabbrvauw nnwabjty tnyah.

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