Stage Potential energy (KJ mol - ) Fig . (b) VB theory for the formation of hydrogen molecule At this stage, there is a maximum overlap between the atomic orbitals of H a and H b , and the atoms H a and H b are now said to be bonded together by a covalent bond. The internuclear distance at this stage gives the H-H bond length and is equal to pm. The liberated energy is kJ mol - and is known as bond energy.
Since the energy is released during the bond formation, the resultant molecule is more stable. If the distance between the two atoms is decreased further, the repulsive forces dominate the attractive forces and the potential energy of the system sharply increases . . Salient features of VB Theory: (i) When half filled orbitals of two atoms overlap, a covalent bond will be formed between them.
(ii) The resultant overlapping orbital is occupied by the two electrons with opposite spins. For example, when H is formed, the two 1s electrons of two hydrogen atoms get paired up and occupy the overlapped orbital. (iii) The strength of a covalent bond depends upon the extent of overlap of atomic orbitals. Greater the overlap, larger is the energy released and stronger will be the bond formed.
Unit Unit - - - - (iv) Each atomic orbital has a specific direction (except s-orbital which is spherical) and hence orbital overlap takes place in the direction that maximizes overlap Let us explain the covalent bond formation in hydrogen, fluorine and hydrogen fluoride using VB theory. . Orbital Overlap When atoms combines to form a covalent molecule, the atomic orbitals of the combining atoms overlap to form a covalent bond. The bond pair of electrons will occupy the overlapped region of the orbitals.
Depending upon the nature of overlap we can classify the covalent bonding between the two atoms as sigma (σ) and pi (π) bonds. . . Sigma and Pi bonds When two atomic orbitals overlap linearly along the axis, the resultant bond is called a sigma (σ) bond.
This overlap is also called 'head-on overlap' or 'axial overlap'. Overlap involves an s orbital (s-s and s-p overlaps) will always result in a sigma bond as the s orbital is spherical. Overlap between two p orbitals along the molecular axis will also result in sigma bond formation. When we consider x-axis as molecular axis, the p x -p x overlap will result in σ-bond.
When two atomic orbitals overlaps sideways, the resultant covalent bond is called a pi (π)bond. When we consider x-axis as molecular axis, the p y -p y and p z -p z overlaps will result in the formation of a π-bond. Following examples will be useful to understand the overlap: . .
Formation of hydrogen (H ) Molecule Electronic configuration of hydrogen atom is 1s During the formation of H molecule, the 1s orbitals of two hydrogen atoms containing one unpaired electron with opposite spin overlap with each other along the internuclear axis. This overlap is called s-s overlap. Such axial overlap results in the formation of a σ-covalent bond.