The valence bond theory was proposed by Heitler and London to explain covalent bond formation using quantum mechanics. It assumes that covalent bonds are formed between atoms by the donation of an electron from each atom. The theory describes how overlapping atomic orbitals lead to the formation of sigma and pi bonds between atoms. Sigma bonds are stronger and form from head-on overlapping of orbitals, while pi bonds are weaker and form from side-by-side overlapping.

2. • 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.

3. 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
another atom.

4. As these two atoms approach each other, new attractive
and repulsive forces begin to operate.

6. Energy gets released when the bond is formed between two hydrogen
atoms, the hydrogen molecule is more stable than that of isolated
hydrogen atoms.
The energy so released is called as bond enthalpy,
which is corresponding to
minimum in the curve

7. 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.

8. Greater the extent of overlapping, stronger is the bond
formed.
The direction of the covalent bond is along the region of
overlapping of the atomic orbital's i.e., covalent bond is
directional.

11. σ-bond
A sigma bond (symbol: σ) is a covalent bond
formed via linear overlap of two orbital's.
π-bond
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:

12. • 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:

13. σs-s bond:

14. σp-p bond:

15. σs-p bond:

16. 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
bonding atoms.
Note: The 's' orbital's can only form σ-bonds, whereas the p, d & f orbital's can
form both σ and π-bonds.

19. 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.

21. The electronic configuration of Cl atom in the ground
state is [Ne]3s2 3px
2 3py
2 3pz
1.
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.

23. In the ground state, the electronic configuration of
hydrogen atom is 1s1.
And the ground state electronic configuration of Cl atom
z
is [Ne]3s2 3px
2 3py
2 3p 1.
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.

25. The electronic configuration of O in the ground state is
z
[He] 2s2 2px
2 2py
1 2p 1.
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.

27. Thus a double bond (one σp-p and one πp-p) is formed between
two oxygen atoms.

28. The ground state electronic configuration of N is [He]
2s2 2px
1 2py
1 2pz
1.
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.

29. 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
atoms.