Valence Bond Theory (VBT) and Hybridization

1. Valence Bond Theory (VBT)
and
Hybridization

2. ThevalencebondtheorywasproposedbyHeitlerandLondon(1972)toexplaintheformationof covalentbond
quantitativelyusingquantummechanics.
Lateron,LinusPaulingimprovedthistheorybyintroducingtheconceptof hybridization.
Thisapproachisbasedontheconceptof orbitalstocoverthelimitations of VSEPR theory.
VBTassumesthatallbondsarelocalizedbondsformedbetweentwo atomsbythedonationof an electronfrom
eachatom.
VBTdescribescovalentbondformationaswell astheelectronicstrictureof molecules.
ValenceBondTheory(VBT)

3. VSEPRisquitesuccessfulinexplainingandpredictingmoleculargeometry,butdoesnot describe:
❑ How bondingoccursandwherebondingoccurs,aswell aswherelonepairsof valence electronsare
directed.
❑ It doesn’tpermitapredictionof bondenergies,bondlengthsetc.
T
ofindtheanswersof thesequestions,two approachesbasedontheconceptof orbitalsare being
employed:
❑ ValenceBondTheory(VBT)
❑ MolecularOrbitalTheory(MOT)
NeedofVBT

4. Elementscombinetoformcompounds.In eachcompound,theconstituentatomsareheld togetherby
chemicalbonds.While electrovalentcompoundsareheldtogetherbyionicbonds, covalentcompoundsare
joinedbycovalentbonds.
Thecovalentbondsarecreatedbysharingof valence electrons.
ValenceBondTheoryisusedtoexplainthebondformationbetweentwo atomstoforma covalent
compound.
ValenceBondTheory(VBT)
Figure1
:CovalentBond Formation

5. *Acovalentbondisformedbytheoverlappingof two halffilledvalenceatomicorbitalsof two differentatoms.
*Theelectronsintheoverlappingorbitalsgetpairedandconfinedbetweenthenucleiof two atoms.
*Theelectrondensitybetweentwo bondedatomsincreasesduetooverlapping.Thisconfers stabilitytothe
molecule.
* Greatertheextentof overlapping,strongeristhebond formed.
*Thedirectionof thecovalentbondisalongtheregionof overlappingof theatomicorbitalsi.e., covalentbondis
directional.
* Therearetwo typesof covalentbondsbasedonthepatternof overlappingasfollows:
- SigmaBond
- PiBond
ValenceBondTheory(VBT)

6. 1)ThetheorystatesthatbondformationtakesplacebytheoverlapofAtomicOrbitalsof the atomsandonly
thoseatomicorbitalscanparticipateinoverlapping which
i) areinvalenceshell
ii) haveunpairedelectrons
PostulatesofVBT
Figure2:AtomicOrbitalsof anAtom

7. Letustakeupthecaseof Nitrogen
Here,1sorbitalisnotavalenceorbitalandhencewill not
participateinbondformation.
Amongthetwo valenceorbitals,2sconsistsof paired
electronsandthusittoowill notparticipate.
Thethree2porbitalsconsistingof unpaired
electronswill takepartinbondformation.
PostulatesofVBT

8. PostulatesofVBT

9. When atomicorbitaloverlapunderappropriateconditions,theycando itintwo ways:
Thefirstof overlapiscalledaHead-onoverlapping.Thisresultsintheformationof aσbond.
AndthenextoneiscalledLateraloverlapping.Thisresultsintheformationof aπbond.
PostulatesofVBT

10. 1
)
Hydrogenmolecule:
Theelectronicconfigurationof hydrogenatominthegroundstateis 1s1
.
Intheformationof hydrogenmolecule,two halffilled1sorbitalsof hydrogenatomsoverlap alongtheinter-
nuclearaxisandthusbyformingaσs-s bond.
Examples
Figure3:SigmabondinHydrogen molecule

11. 2)Cl2molecule:
* Theelectronicconfigurationof Cl atominthegroundstateis[Ne]3s2
3px
2
3py
2
3pz
1
.
*Thetwo halffilled3pzatomicorbitalsof two chlorineatomsoverlapalongtheinter-nuclearaxis andthusby
formingaσp-p bond.
Examples
Figure4:SigmabondinChlorine molecule

12. Examples
3)HClmolecule:
* In thegroundstate,theelectronicconfigurationof hydrogenatomis 1s1
.
* Andthegroundstateelectronicconfigurationof Cl atomis[Ne]3s2
3px
2
3py
2
3pz
1
.
*Thehalffilled1sorbitalof hydrogenoverlapwith thehalffilled3pzatomicorbitalof chlorine atomalongthe
inter-nuclearaxistoformaσs-p bond.

13. 4)O2molecule:
* Theelectronicconfigurationof O inthegroundstateis[He]2s2
2px
2
2py
1
2pz
1
.
*Thehalffilled2pyorbitalsof two oxygenatomsoverlapalongtheinter-nuclearaxisandformσp- p bond.
* Theremaininghalffilled2pzorbitalsoverlaplaterallytoformaπp-p bond.
* Thusadoublebond(oneσp-p andoneπp-p) isformedbetweentwo oxygenatoms.
Examples

14. 5)N2molecule:
* Thegroundstateelectronicconfigurationof N is[He]2s2
2px
1
2py
1
2pz
1
.
*A σp-p bondisformedbetweentwo nitrogenatomsduetooverlappingof halffilled2pxatomic orbitalsalongthe
inter-nuclearaxis.
*Theremaininghalffilled2pyand2pzorbitalsformtwo πp-p bondsduetolateraloverlapping. Thusatriplebond(one
andtwo)isformedbetweentwo nitrogenatoms.
Examples

15. Howevertheold versionof valencebondtheoryislimitedtodiatomicmoleculesonly.It couldnot explainthe
structuresandbondanglesof moleculeswith morethanthree atoms.
E.g.It couldnotexplainthestructuresandbondanglesof H2O,NH3etc.,
However,inordertoexplainthestructuresandbondanglesof molecules,LinusPaulingmodified thevalencebond
theoryusinghybridizationconcept.
LimitationsofVBT

16. Paulingandslaterresolveddiscrepancybyintroducingtheconceptof orbitalhybridizationwhich involvesmixingof
differentorbitalsof anatomtoformnewhybridorbitals,beforeoverlapping andbondformation.
These hybrid orbitals are identical in directional character and have the same shape and energy. The total
number of hybridorbitals formed after mixing, isalwaysequal to the number ofatomic orbitals mixed.
“Theprocessof mixingof orbitalsof anatomof nearlyequalenergygivingrisetoentirelynew orbitalsequalto
thenumbertothenumberof mixingorbitalsandhavingidenticalshapeand sameenergycontentiscalled
hybridization.”
Hybridization

17. Typesof Hybridizationare:
i) sp
ii) sp2
iii) sp3
iv) sp3
d
v) sp3
d2
vi) sp3
d3
TypesofHybridization

18. i)sphybridization:
Intermixingof one's'andone'p'orbitalsof almostequalenergytogivetwo identicaland degenerate
hybridorbitalsiscalled'sp'hybridization.
Thesesp-hybrid orbitalsarearrangedlinearlyatbymaking180o
of angle. They
possess50%'s'and50%'p' character.
TypesofHybridization

19. Examples:
1
)
BerylliumChloride(BeCl2
)
Theelectronicconfigurationof 'Be'ingroundstateis1s2
2s2
.Sincetherearenounpairedelectrons,itundergoes excitationbypromotingone
of its2selectronintoempty2porbital.
Thusintheexcitedstate,theelectronicconfigurationof Beis 1s2
2s1
2p1
.
If theberylliumatomformsbondsusingthesepureorbitals,themoleculemightbeangular.However theobserved shapeof BeCl2islinear.
T
oaccountfor this,sphybridizationwasproposedasexplained below.
Intheexcitedstate,theberylliumatomundergoes'sp'hybridizationbymixinga2sandone2porbitals.Thustwo half filled'sp'hybridorbitals
areformed,whicharearrangedlinearly.
Thesehalffilledsp-orbitalsformtwo σbondswith two 'Cl'atoms. Thus
BeCl2islinearinshapewith thebondangleof 180o
.
TypesofHybridization

20. 2)Acetylene(C2H2)
The ground state electronic configuration of 'C' is 1
s
2
2s2
2px
1
2py
1
.There are only two
unpairedelectrons in thegroundstate.However, the valencyof carbon is four i.e.,it
forms 4bonds. In order to form four bonds, there must be four unpaired electrons.
Hence carbon promotes one of its 2s electron into the empty 2pz orbital in the
excitedstate.
Thusintheexcitedstate,theelectronic configurationof carbon is
1
s
2
2s1
2px
1
2py
1
2pz
1
.
Eachcarbonatomundergoes'sp'hybridizationbyusinga2sandone 2porbitals in
theexcitedstatetogivetwo halffilled'sp'orbitals, which arearrangedlinearly.
Thetwo carbonatomsformaσsp-spbondwith eachotherbyusingsp- orbitals.
TypesofHybridization

21. Howevertherearealsotwo unhybridizedp orbitalsi.e.,2pyand 2pzon
eachcarbonatomwhich areperpendiculartothesp hybridorbitals.These
orbitalsformtwo πp-p bondsbetweenthe two carbonatoms.
Thusatriplebond(includingoneσsp-sp bond&two πp-p bonds)is formed
betweencarbonatoms.
Eachcarbonalsoformsaσsp-s bondwith thehydrogenatom. Thus
acetylenemoleculeislinearwith 180o
of bondangle.
TypesofHybridization

22. ii)sp2
hybridization
Intermixing of one 's' and two 'p' orbitals of almost equal energytogivethreeidenticaland
degeneratehybridorbitalsisknownassp2hybridization.
Thethreesp2
hybridorbitalsareorientedintrigonalplanarsymmetryatanglesof 120o
toeach other.
Thesp2
hybridorbitalshave33.3%'s'characterand 66.6%'p'character.
TypesofHybridization

23. Examples:
1
)
Borontrichloride(BCl3
)
Theelectronic configurationof 'B'ingroundstateis1s2
2s2
2p1
with onlyone unpairedelectron.
Sincetheformationof threebondswith chlorineatomsrequire threeunpairedelectrons,thereis
promotionof oneof 2selectronintothe2p sublevelbyabsorbingenergy.
ThusBoronatomgetselectronic configuration: 1
s
2
2s2
2px
1
2py
1
.
However toaccountfor thetrigonalplanarshapeof thisBCl3molecule, sp2
hybridizationbeforebondformationwas putforwarded.
In theexcitedstate,Boronundergoessp2
hybridizationbyusinga2sandtwo 2p orbitalsto
givethreehalffilledsp2
hybridorbitalswhich areorientedintrigonal planarsymmetry.
TypesofHybridization

24. Boronformsthreeσsp-p bondswith threechlorineatomsbyusingitshalffilledsp2
hybridorbitals.Each chlorineatom
usesit'shalffilledp-orbital for theσ-bondformation.
Thustheshapeof BCl3istrigonalplanarwith bondanglesequalto120o
.
TypesofHybridization

25. 2)Ethylene(C2H4)
Duringtheformationof ethylenemolecule, eachcarbonatomundergoessp2
hybridizationinitsexcitedstate bymixing2sand
two 2porbitalstogivethreehalffilledsp2
hybridorbitalsorientedintrigonalplanar symmetry.
Thereisalsoonehalffilledunhybridized2pzorbital oneachcarbonperpendiculartotheplaneof sp2
hybrid orbitals.
Thecarbonatomsformaσsp2-sp2bondwith eachotherbyusingsp2
hybridorbitals.
TypesofHybridization

26. TypesofHybridization
Aπp-pbondisalsoformedbetweenthemduetolateraloverlappingof unhybridized 2pzorbitals.
Thusthereisadoublebond(σsp2-sp2 &πp-p) betweentwo carbonatoms. Each
carbonatomalsoformstwo σsp2-s bondswith two hydrogenatoms.
Thusethylenemoleculeisplanarwith ∠HCH&∠HCCbondanglesequalto 120o
.
All theatomsarepresentinoneplane.

27. iii)sp3
hybridization
Insp3
hybridization,one's'andthree'p'orbitalsof almostequalenergyintermixtogivefouridenticaland degeneratehybrid
orbitals.
Thesefoursp3
hybridorbitalsareorientedintetrahedralsymmetrywith 109o
28'anglewith eachother. Thesp3
hybrid
orbitalshave25%‘s’characterand75%'p'character.
TypesofHybridization

28. Examples:
1
)
Methane(CH4
)
Duringtheformationof methanemolecule,thecarbonatomundergoessp3
hybridizationintheexcitedstatebymixingone ‘2s’andthree2p
orbitalsto furnishfourhalffilledsp3
hybrid orbitals,which areorientedintetrahedralsymmetryinspace aroundthecarbon atom.
Eachof thesesp3
hybrid orbitalsformsaσsp3-s bondwith onehydrogenatom.Thuscarbonforms four
σsp3-s bondswith fourhydrogenatoms.
Methanemoleculeistetrahedralinshapewith 109o
28'bond angle.
TypesofHybridization

29. 2)Ethane(C2H6)
Justlikeinmethanemolecule, eachcarbonatomundergoessp3
hybridizationintheexcitedstatetogive foursp3
hybrid
orbitalsintetrahedralgeometry.
Thetwo carbonatomsformaσsp3-sp3 bondwith eachotherduetooverlapping of sp3
hybridorbitals alongtheinter-nuclear
axis.
Eachcarbon atomalsoformsthreeσsp3-s bondswith hydrogenatoms.
Thusthereistetrahedralsymmetryaroundeachcarbonwith ∠HCH&∠HCCbondanglesequalto 109o
.
TypesofHybridization

30. iv)sp3
dhybridization
In sp3
d hybridization,one's',three'p'andone'd'orbitalsof almost equalenergy
intermix to give five identical and degenerate hybrid orbitals, which are
arrangedintrigonalbipyramidal symmetry.
Amongthem,threearearrangedintrigonalplaneandthe remainingtwo orbitals
arepresentaboveandbelow thetrigonal planeatrightangles.
The sp3
d hybrid orbitals have 20% 's', 60% 'p' and 20% 'd'
characters.
TypesofHybridization

31. Examples:
1
)
Phosphoruspentachloride(PCl5
)
Thegroundstateelectronicconfigurationof phosphorusatomis: 1s2
2s2
2p6
3s2
3px
1
3py
1
3pz
1
.
Theformationof PCl5moleculerequires5unpairedelectrons.Hencethephosphorusatomundergoesexcitationto promoteone
electronfrom3sorbital to oneof empty3dorbital.
Thustheelectronicconfigurationof 'P'intheexcitedstateis1s2
2s2
2p6
3s1
3px
1
3py
1
3pz
1
3d1
.
In theexcitedstate,intermixing of a3s,three3pandone3dorbitalsto givefivehalffilledsp3
d hybrid orbitals,which are arrangedintrigonalbipyramidal
symmetry.
i.e.,Threeorbitalsarearrangedintrigonalplanarsymmetry,whereastheremainingtwo arearrangedperpendicularlyaboveand below thisplane.
TypesofHybridization

32. TypesofHybridization
By using these half filled sp3
d orbitals, phosphorous forms
five σsp3d-p bonds with chlorine atoms. Each chlorine atom
makesuseof halffilled3pzorbital for thebond formation.
The shape of PCl5molecule is trigonal bipyramidal with 120o
and90o
of ∠Cl- P- Cl bondangles.

33. v)sp3
d2
hybridization
Intermixing of one 's', three 'p' and two 'd' orbitals of almost same energy by
givingsixidenticalanddegeneratehybridorbitalsis calledsp3
d2
hybridization.
These six sp3
d2
orbitals are arranged in octahedral symmetry by making 90o
angles to each other. This arrangement can be visualized as four orbitals
arranged in a square plane and the remaining two are oriented above and
below thisplane perpendicularly.
TypesofHybridization

34. Example:
Sulfurhexaflouride(SF6
)
Theelectronic configurationof 'S'ingroundstateis 1
s
2
2s2
2p6
3s2
3px
2
3py
1
3pz
1
.
In SF6molecule, therearesixbondsformedbysulfuratom.Hencetheremustbe6unpairedelectrons. However thereare
only 2unpairedelectronsinthegroundstateof sulfur.Henceitpromotestwo electrons intotwo of the3dorbitals(onefrom
3sandonefrom 3px).
Thustheelectronic configurationof 'S'inits2nd
excitedstateis 1
s
2
2s2
2p6
3s1
3px
1
3py
1
3pz
1
3d2
.
TypesofHybridization

35. In thesecond excited state,sulfur under goes sp3
d2
hybridization by
mixing a3s,three3pandtwo 3d orbitals. Thusformed six half filled
sp3
d2
hybridorbitals arearrangedinoctahedral symmetry.
Sulfur atom forms six σsp3d2-p bonds with 6 fluorine atomsby using
thesesp3
d2
orbitals. Eachfluorine atom usesis half-filled 2pzorbitals
for the bond formation. SF6is octahedral in shape with bond angles
equalto90o
.
TypesofHybridization

36. Vi)sp3
d3
hybridization
In sp3
d3
hybridization, one 's', three 'p' and three 'd' orbitals of almost same
energy intermix to give seven sp3
d3
hybrid orbitals, which are oriented in
pentagonalbipyramidal symmetry.
Five amongthe sp3
d3
orbitals are arranged in apentagonal plane by making72o
of angles. The remaining are arranged perpendicularly above and below this
pentagonalplane.
TypesofHybridization

37. Example:
Iodineheptafluoride(IF7
):
Theelectronic configurationof Iodine atominthegroundstateis: [Kr]4d1
0
5s2
5p5
.
Sincetheformationof IF7requires7unpairedelectrons,theiodineatompromotesthreeof itselectrons(one from5sorbital and
two from5psublevel) intoempty5dorbitals.Thisstateisreferredtoasthirdexcited state.
Theelectronic configurationof Iodine inthethirdexcitedstatecanbewritten as: [Kr]4d1
0
5s1
5p3
5d3
.
TypesofHybridization

38. In the third excited state, iodine atom undergoes sp3
d3
hybridization to give 7 half filled sp3
d3
hybrid orbitals in
pentagonal bipyramidal symmetry. These will form 7σsp3d3-p
bondswith fluorine atoms.
Thus the shapeof IF7is pentagonal bipyramidal. The∠F-I-F
bond angles in the pentagonal plane are equal to 72o
,
whereas two fluorine are present perpendicularly to the
pentagonalplaneaboveand below.
TypesofHybridization

39. Inorganic ChemistrybyProf.GhulamRasool Chauhdary
References