Valence Bond Theory
The valence bond theory was proposed by Heitler and
London to explain the formation of covalent bond
quantitatively using quantum mechanics.
Later on, Linus Pauling improved this theory by introducing
the concept of hybridization.
Valence bond (VB) theory assumes that all bonds are
localized bonds formed between two atoms by the
donation of an electron from each atom.
Valence Bond theory describes covalent bond formation as
well as the electronic structure of molecules.
The theory assumes that electrons occupy atomic orbital's
of individual atoms within a molecule, and that the
electrons of one atom are attracted to the nucleus of
A covalent bond is formed by the overlapping of two half
filled valence atomic orbital's of two different atoms.
The electrons in the overlapping orbital's get paired and
confined between the nuclei of two atoms.
The electron density between two bonded atoms increases
due to overlapping. This confers stability to the molecule.
Greater the extent of overlapping, stronger is the bond
The direction of the covalent bond is along the region of
overlapping of the atomic orbital's i.e., covalent bond is
A sigma bond (symbol: σ) is a covalent bond
formed via linear overlap of two orbital's.
A pi bond (symbol: π) is a covalent bond
formed via parallel overlap of two orbital's.
There are two types of covalent bonds based on the pattern
of overlapping as follows:
The covalent bond formed due to overlapping of atomic
orbital along the inter nucleus axis is called σ-bond. It is a
stronger bond and cylindrically symmetrical.
Depending on the types of orbital's overlapping, the σ-
bond is divided into following types:
(i): σs-s bond, (ii): σp-p bond, (iii): σs-p bond:
The covalent bond formed by sidewise
overlapping of atomic orbital's is called
π- bond. In this bond, the electron
density is present above and below the
inter nuclear axis. It is relatively a
weaker bond since the electrons are
not strongly attracted by the nuclei of
Note: The 's' orbital's can only form σ-bonds, whereas the p, d & f orbital's can form both σ and π-bonds.
The electronic configuration of hydrogen atom in the
ground state is 1s1.
In the formation of hydrogen molecule, two half filled
1s orbital's of hydrogen atoms overlap along the inter-
nuclear axis and thus by forming a σs-s bond.
The electronic configuration of Cl atom in the ground
state is [Ne]3s2 3px
The two half filled 3pz atomic orbital's of two chlorine
atoms overlap along the inter-nuclear axis and thus by
forming a σp-p bond.
In the ground state, the electronic configuration of
hydrogen atom is 1s1.
And the ground state electronic configuration of Cl atom
is [Ne]3s2 3px
The half filled 1s orbital of hydrogen overlap with the half
filled 3pz atomic orbital of chlorine atom along the inter-
nuclear axis to form a σs-p bond.
The electronic configuration of O in the ground state is
[He] 2s2 2px
The half filled 2py orbital's of two oxygen atoms overlap
along the inter-nuclear axis and form σp-p bond.
The remaining half filled 2pz orbital's overlap laterally to
form a πp-p bond.
Thus a double bond (one σp-p and one πp-p) is formed
between two oxygen atoms.
The ground state electronic configuration of N is [He]
A σp-p bond is formed between two nitrogen atoms due
to overlapping of half filled 2px atomic orbital's along
the inter-nuclear axis.
The remaining half filled 2py and 2pz orbital's form
two πp-p bonds due to lateral overlapping. Thus a triple
bond (one and two) is formed between two nitrogen
The intermixing of two or more pure atomic orbital's of an
atom with almost same energy to give same number of
identical and degenerate new type of orbital's is known as
The new orbital's formed are also known as hybrid
During hybridization, the atomic orbital's with different
characteristics are mixed with each other.
Intermixing of one 's' and one 'p' orbital's
of almost equal energy to give two
identical and degenerate hybrid orbital's
is called 'sp' hybridization.
These sp-hybrid orbital's are arranged
linearly at by making 180 ⁰ of angle.
They possess 50% 's' and 50% 'p'
Intermixing of one 's' and two 'p'
orbital's of almost equal energy to give
three identical and degenerate hybrid
orbital's is known as sp2 hybridization.
The three sp2 hybrid orbital's are
oriented in trigonal planar symmetry at
angles of 120 ⁰ to each other.
The sp2 hybrid orbital's have 33.3% 's'
character and 66.6% 'p' character.
In sp3 hybridization, one 's' and three
'p' orbital's of almost equal energy
intermix to give four identical and
degenerate hybrid orbital's.
These four sp3 hybrid orbital's are
oriented in tetrahedral symmetry with
109 ⁰ 28' angle with each other.
The sp3 hybrid orbital's have 25% ‘s’
character and 75% 'p' character.
In sp3d hybridization, one 's', three 'p' and one 'd'
orbital's of almost equal energy intermix to give
five identical and degenerate hybrid orbital's, which
are arranged in trigonal bipyramidal symmetry.
Among them, three are arranged in trigonal plane
and the remaining two orbital's are present above
and below the trigonal plane at right angles.
The sp3d hybrid orbital's have 20% 's', 60% 'p' and
20% 'd' characters.
Intermixing of one 's', three 'p' and two 'd' orbital's
of almost same energy by giving six identical and
degenerate hybrid orbital's is called sp3d2
These six sp3d2 orbital's are arranged in octahedral
symmetry by making 90 ⁰ angles to each other. This
arrangement can be visualized as four orbital's
arranged in a square plane and the remaining two
are oriented above and below this plane
In sp3d3 hybridization, one 's', three 'p' and three
'd' orbital's of almost same energy intermix to give
seven sp3d3 hybrid orbital's, which are oriented in
pentagonal bipyramidal symmetry.
Five among the sp3d3 orbital's are arranged in a
pentagonal plane by making 72⁰ of angles. The
remaining are arranged perpendicularly above and
below this pentagonal plane.
ē Pair Hybridization Shape
2 sp linear
3 sp2 trigonal planar
4 sp3 tetrahedral, pyramidal, or bent
trigonal bipyramidal, trigonal planar,
6 sp3d2 octahedral, square planar, or linear
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