Valence Bond Theory

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Valence Bond Theory

• Developed by Linus Pauling

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Overlap of Atomic Orbitals

• The sharing of electrons between atoms is viewed
  as an overlap of atomic orbitals of the bonding
  atoms.

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The overlap of orbitals
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When H H distance 74 pm, Repulsion
Attraction ? strongest bond ? optimal overlap ?
lowest energy
74 pm
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At H H distance gt 74 pm, Repulsion lt
Attraction ? weaker bond ? too little overlap ?
atoms come closer
74 pm
gt 74 pm
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At H H distance lt 74 pm, Repulsion gt
Attraction ? weaker bond ? too much overlap ?
atoms get further apart
74 pm
lt 74 pm
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Because of orbital overlap, the bonding electrons
localize in the region between the bonding nuclei
such that There is a high probability of finding
the electrons in the region between the bonding
nuclei.
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Overlap of two half-filled orbitals leads to the
formation of a covalent bond.
1s-1s overlap gives a H H single bond
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The 1s-2p overlap gives a H F single bond
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Non-bonding electrons
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The 2p-2p overlap gives a F F single bond
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Non-bonding electrons
Each F atom has three pairs of non-bonding
electrons.
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Q.23 Identify the non-bonding electrons in O2
molecules.
Two 2p-2p overlaps give a OO double bond
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Q.23 Identify the non-bonding electrons in O2
molecules.
Non-bonding electrons
Each O atom has two pairs of non-bonding
electrons.
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Overlap of an empty orbital with a fully-filled
orbital leads to the formation of a co-ordinate
covalent bond or dative bond
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Represented by an arrow ? pointing from the
electron pair donor to the electron pair
acceptor.
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Interpretation of the Formation of Covalent Bonds
in terms of Valence Bond Theory
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(a) NH4
By Lewis model, the structure is
? 4 single bonds are formed, one of them is
a dative bond.
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By VB Theory,
Three 2p-1s(half-filled) overlaps lead to the
formation of three N H single bonds.
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By VB Theory,
One 2s(fully-filled)-1s(vacant) overlap leads to
the formation of one N ? H dative bond.
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(b) HCN
By Lewis model, the structure is H-C?N
? one H-C single bond and one C?N triple bond.
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By VB Theory,
C
? Only 2 single bonds can be formed.
? Promotion of a 2s electron to a 2p orbital.
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• The overlap of one orbital (?) of C with an 1s
  orbital of H gives the C-H single bond.

• Overlaps of three orbitals (???) of C with
  three 2p orbitals of N give the C?N triple bond.

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• The 2s electrons on N are non-bonding electrons.

• The energy released by forming a stronger
  triple bond outweighs the energy required for
  promoting an electron from a 2s orbital to a 2p
  orbital.

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(c) SO2
By Lewis model, the three possible structures are
O?SO, OS?O, OSO
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By VB Theory,
? Only two single bonds can be formed.
? One 3p electron has to be promoted to a 3d
orbital.
? Expansion of Octet.
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By VB Theory,
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? Overlaps of two half-filled orbitals (??) of
S with two half-filled 2p orbitals of an
oxygen atom give a SO double bond. A total of
two SO bonds are formed with two O atoms
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Non-bonding electrons S 3s2 O 2s2 and
2p2
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The energy released by forming of two stronger
double bonds outweighs the energy required for
promoting an electron from a 3p orbital to a 3d
orbital.
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The Concept of Resonance

• According to VB theory, the two less stable
  structures of SO2,
• O?SO and OS?O do exist.
• Each of these structures contributes in certain
  extent to the real structure of SO2.

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are the wave functions of the three possible
structures and
a gt b c gt 0
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In other words, the real structure of SO2 is the
resonance hydrid of the three possible structures.
OSO ? O?SO ? OS?O
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Q.24
OS?O
A SO double bond is formed by
3p(half-filled)-2p(half-filled) overlaps between
S and O.
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Q.24
OS?O
A O?S dative bond is formed by 3p(fully-filled)-2p
(empty) overlap between S and O
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Q.24
OS?O
Formation of dative bond is not favourable
because the two unpaired 2p electrons in O are
forced to pair up to give O
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6B
(d) SF2, SF4, SF6
SF6
SF4
SF2
Molecule
Most stable Lewis Structure
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By VB Theory,
Only two S-F single bonds can be formed by 3p-2p
overlaps between one S atom and two F atoms ?
SF2 is formed.
F-S-F
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By VB Theory,
To form four S-F single bonds in SF4, a 3p
electron in S has to be promoted to a 3d orbital.
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By VB Theory,
To form six S-F single bonds in SF6, a 3s
electron in S has to be promoted to a 3d orbital.
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By VB Theory,
The energy released by forming more single bonds
outweighs the energy required for promoting 3s
and 3p electrons to 3d orbitals.
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Q.25
XeF6
XeF4
XeF2
Molecule
Most stable Lewis Structure
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By VB Theory,
To form two Xe-F bonds in XeF2, a 5p electron in
Xe has to be promoted to a 5d orbital.
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By VB Theory,
To form four Xe-F bonds in XeF4, a 5p electron in
Xe has to be promoted to a 5d orbital.
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By VB Theory,
To form six Xe-F bonds in XeF6, a 5p electron in
Xe has to be promoted to a 5d orbital.
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By VB Theory,
The energy released by forming more single bonds
outweighs the energy required for promoting 5p
electrons to 5d orbitals.