overlapping of atomic orbitals
Answers
Orbital Overlapping
When two atoms combine together to form a covalent bond, their energy is minimum when they are so close to each other that their orbitals are partially merged. This partial merging of atomic orbitals is known as orbital overlapping or overlapping of atomic orbitals.
orbital overlapping
Orbital Overlap Concept
Molecules like H2 and F2 contain a single bond. Even so, the bond dissociation enthalpies of H2 (435.8 KJ/mol) and F2 (155 KJ/mol) are different. The concept of orbital overlap helps us explain the reason. Let us take the example of H2 molecule:
Overlap concept in H2 molecule : Let us assume that two hydrogen atoms A and B having nuclei nA and nB; and electrons eA and eB are coming close to each other. When the two atoms are far apart, their is no interaction between them. As these two atoms come close to each other, new attractive and repulsive forces begin to operate. These forces are written below :
Force of attraction between nucleus of A and electron of B (attraction between nA and eB)
Force of attraction between nucleus of B and electron of A (attraction between nB and eA)
Force of repulsion between nuclei A and B (repulsion between nA and nB)
Force of repulsion between electrons A and B (repulsion between eA and eB)
Attractive forces
attractive forces in H2 molecule
Repulsive forces
Repulsive forces in H2 molecule
Attractive forces tend to bring two atoms close to each other whereas repulsive forces tend to move them away. In hydrogen, the magnitude of the new attractive forces is greater than that of new repulsive forces. As a result, two atoms come close to each other and potential energy decreases. The atoms approach each other until the equilibrium stage is reached where the net force of attraction balances the force of repulsion and system acquires minimum energy. At this stage, the bond length — distance between two nuclei — of hydrogen is 74 pm.
Potential energy curve for H2 molecule
The potential energy curve showing variation of energy with internuclear distance between two hydrogen atoms
Since energy is released during the formation of H2 molecule, H2 molecule is more stable than isolated hydrogen atoms.
Explanation for unequal bond dissociation enthalpies in molecules like H2 and F2: In general, stronger bonds are the result of greater overlap. The overlap in H2 is greater than that of F2. As a result, more energy is required to dissociate an H2 molecule into isolated hydrogen atoms.
Types of Covalent Bond Based on nature of overlap
There are two types of covalent bonds depending upon the nature of the overlap :
Sigma (σ) bond
Pi (π) bond
Sigma (σ) Bond
Sigma bond is formed by the end to end overlap of bonding orbitals along the internuclear axis. This type of overlap is called a head on overlap or axial overlap.
Internuclear axis : The internuclear axis is an imaginary line that connects the nuclei of atoms bonded together.
Example : The internuclear axis between the nuclei of atoms A and B in an imaginary molecule AB is given below :
Internuclear axis
The head on overlap can take place in any one of the following ways :
s-s overlapping
When there is an overlap of two half filled s-orbitals along the internuclear axis, the overlapping is called an s-s overlapping.
ss overlapping
s-p overlapping
When there is an overlap between a half filled s-orbital and a half filled p-orbital, the overlapping is called an s-p overlapping.
sp overlapping
p-p overlapping
This type of overlapping occurs between two half filled p-orbitals.
pp overlapping
Pi (π) Bond
A pi (π) bond is formed when the atoms overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis. This type of overlap is called a sideway overlap.
pi overlapping
Strength of Sigma & Pi Bonds
In σ bond, the overlapping occurs to a larger extent. Hence, σ bond is stronger than π bond, where the overlapping occurs to a smaller extent. Further, a π bond between two atoms is formed in addition to a sigma bond. The presence of σ and π bonds in single, double and triple bonds is as follows :
A single bond contains one σ bond.
A double bond contains one σ and one π bonds.
A triple bond contains one σ and two π bonds.