Valence Bond (VB) Theory:
This theory has following postulates:
- Covalent bonds are formed by overlapping of atomic orbitals.
- Electron density is high in the overlapped area and is thus responsible for holding atoms together.
- Only half filled atomic orbitals take part in covalent bond formation.
- The half filled atomic orbitals must contain electrons with opposite so they cancel the magnetic lines of force and can live together.
- By the overlapping of orbitals two types of bonds are formed sigma bonds and pi bonds.
- Atomic orbitals of different shapes and energies some time are mixed to form hybrid orbitals.
Sigma ( δ ) Bond:
When half filled atomic orbitals are overlapped on bond axis the overlapping is called sigma overlapping and the bond formed is called sigma bond. Sigma bond is formed due to s-s or s-p or p-p overlapping. As sigma overlapping is larger so sigma bond is very strong.
The sigma bond which is formed between Hydrogen atoms is due to s-s overlapping.
The electronic configuration of Hydrogen is 1s1. The half filled s-orbital of one Hydrogen atom overlaps with half filled s-orbital of other Hydrogen atom on bond axis. So H ─ H bond is sigma bond.
The sigma bond which is formed between Hydrogen atom and Fluorine atom is due to s-p overlapping.
The electronic configuration of hydrogen is 1S1 and that of Fluorine is 1s2, 2s2, 2p5. The half filled s-orbital of Hydrogen atom overlaps with half filled p-orbital of Fluorine atom on bond axis. So H ─ F bond is sigma bond.
The sigma bond which is formed between Fluorine atoms is due to p-p overlapping.
The electronic configuration of Fluorine is 1s2, 2s2, 2p5. The half filled p-orbital of one Fluorine atom overlaps with half filled p-orbital of other Fluorine atom on bond axis. So F ─ F bond is sigma bond.
Sigma Bond Example:
Pi ( л ) Bond:
When half filled atomic orbitals are overlapped away from bond axis (side wise or lateral overlapping) the overlapping is pi overlapping and the bond formed is called pi bond. Pi bond is formed due to lateral overlapping of two parallel p-orbitals. As pi overlapping is little so pi bond is weak bond. e.g Oxygen molecule has one sigma bond and one pi bond.
The electronic configuration of oxygen atom is 1S2, 2S2, 2P4. Thus oxygen has two half filled p-orbitals.
One of the half filled p-orbital of one oxygen atom overlaps with a half filled p-orbital of other oxygen atom on bond axis thus a sigma bond is formed. The half filled p-orbitals of two oxygen atoms which are parallel to each other are side wisely or laterally overlapped and form pi bond. Thus a double bond is formed between two oxygen atoms consisting of a sigma and a pi bond.
Difference between sigma bond and Pi Bond:
Examples of Sigma bond and Pi Bond:
Few examples of sigma and pi bonds are as follow.
The simplest trick to identify sigma bond and pi bond is:
All single covalent bonds are sigma bonds. e.g methane CH4
In double covalent bond ,First bond is the sigma bond and the second bond is always the pi bond e.g. in ethene C2H4
Incase of triple covalent bond , first bond is the sigma bond and the second and third bonds are the pi bonds e.g. in ethyne
( acetylene) C2H2
Sigma bond and Pi bond Calculation:
Below diagrams shows the total number of sigma bonds and pi bonds around the carbon atoms.
To identify sigma bond and Pi bond easily:
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Easiest Trick to Identify Sigma bond and pi bond