Valence bond theory (1).pdf in inorganic chemistry

A slide on Valence bond theory

1. VALENCE BOND THEORY
•In valence-bond theory, covalent bonding is
explained on the basis of the concentration of
electron density between two nuclei.
•This is the consequence of the sharing of
space or overlap of the valence atomic orbital
of one atom with the valence atomic orbital of
another atom.
•Covalent bond is formed by overlap of atomic
orbitals.
1

2. •The overlap can occur with different
symmetries, 2 main symmetries for use are:
(a) σ = sigma symmetry
(b) π= pi symmetry
2

3. σ = sigma symmetry
•The sigma bond results in an increase in
electron density symmetrical about the
internuclear axis, z-axis.
s s σ
3
z

4. •Sigma bonds can also be formed when 2pz
orbitals interact
S pz σ
4

5. •Sigma bonds can also be formed when 2pz
orbitals interact
pz pz σ
•S + dz
2
= σ
•S + dx
2
-y
2
= σ(polyatomic molecule)
•
5

6. π= pi symmetry
•In the formation of pi-bond, the overlap of
atomic orbitals results in the internuclear axiss
lying on a NODAL PLANE. There is increased
electron density above and below the
internuclear axis.
•Pi-bonds are usually formed after sigma bond
is formed.
6

7. •P
y p
y π
p
x+ p
x= π
p
x+ d
xy= π
p
x+ d
xz= π
p
y+ d
xy= π
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8. THE CONCEPT OF HYBRIDIZATION
•The atomic orbitals (Aos) that combine can
led to the original atomic orbital present in
the ground state of the atom.
•However, if the geometry of these original Aos
is such that effective overlap cannot occur in
the knowngeometry of the molecule
•The AO of an atom can re-configure into a
different configuration.
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9. •These re-configured orbitals are said to be
hybridized.
•Hybridization is the mixing of atomic orbitals
of different energy to obtain orbitals of
equivalent amount of energy.
•The orbitals so obtained are referred to as
hybrid orbitals.
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10. •Hybridization gives a set of Aos that can
overlap more effectively with Aos of other
atoms in the molecule.
•This yields stronger bonds and molecules with
lower energies.
•This concept (hybridization) is very important
in the explanation of molecular shapes
10

11. •In applying the concept of hybridization, the ff points
must be noted.
❖Hybridized orbitals can only form sigma bonds.
❖Not all orbitals on an atom need to be hybridized, only
orbitals required for bonding are hybridized.
❖Hybridized orbitals are all half full
❖Hybridized orbitals on an atom overlap with either
hybridized or normal orbital on another atom or with
both depending on the moleculer geometry.
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12. •This type of hybridization involves the mixing
of an satomic orbital with one patomic
orbital to form two hybrid orbitals, designated
as sp.
•This type of hybrizationcan be explained by
looking at the covalent bonding in BeCl
2.
spHybridization
12

13. •The ground state electron configuration of Cl is [Ne](3s
23
p
5
)
•Thereisanunpairedelectroninthe3p orbital which can be
paired with an unpaired Be electron.
•Theconceptofhybridizationwillhelpinunderstandingthe
identityoftheorbitalsinBewhichoverlapwiththoseonthe
ClatomstoformtheBe-Clbonds.
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14. The Be atom now has two unpaired electrons and can therefore form two polar covalent
bonds with Cl atoms.
The resulting orbitals are neither an snor a porbital, but hybrid orbitals called sp
hybrid orbital.
The orbital diagram for sphybrid orbital
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15. •Thetwoneworbitalsareidenticalinshape,buttheirlarge
lobespointinoppositedirections.
•Thefactthatthetwosphybridorbitalsareequivalentbutpoint
inoppositedirections,allowstheformationofbondwiththe
twochlorineatomsinBeCl
2.
•Thetwosphybridorbitalslieinthesameplane,180ºapart
fromoneanother.
•ThisgivesBeCl
2moleculealinearshape
•Theremainingtwo2patomicorbitalsofBeremain
unhybridizedandarevacant.
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16. sp
2
Hybridization
•The sp
2
hybridization involves the mixing of one s
atomic orbital with two patomic orbitals.
•sp
2
hybridization gives three hybrid orbitals.
•E.g.BCl
3,mixingthe2sandtwoofthe2patomic
orbitalsofBoronyieldsthreeequivalentsp
2
hybrid
orbitals.
•Thethreesp
2
hybridorbitalslieinthesameplane,
120ºapartfromoneanother.ThisgivesBCl
3
moleculeatrigonal-planarshape
•
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17. •The ground state electron configuration of Cl is [Ne](3s
23
p
5
)
▪Ground and excited states of boron
▪Formation ofsp2hybridorbitals
17

18. sp
3
Hybridization
•sp
3
hybridization is derived from the mixing of one s
atomic orbital with three patomic
•sp
3
hybridization gives four hybrid orbitals.
•E.g.CH
4,mixingthe2sandthreeofthe2patomic
orbitalsofcarbonyieldsfourequivalentsp
3
hybrid
orbitals.
•Eachsp3hybridorbitalhasalargelobethatpoints
towardonevertexofatetrahedronat109.5ºapartfrom
oneanother.ThisgivesCH
4moleculeatetrahedral
shape
18

19. •The ground state electron configuration of H 1s
1
▪Ground and excited states of carbon
▪Formation of sp3 hybrid orbitals
19