Sigma Bond :
You know that conalent bond is formed by sharing of electrons of valence shell of two atoms.
For example H
2 is formed by sharing of 1 electron of each H atom , present in there 1S orbital.
Sigma bond is a type of covalent bond.
If sharing or ovelapping of orbitals takes place on the bond axis (the axis connecting the nuclei of two atoms), it is called Sigma bond σ.
See the animation here.
From Organic chemistry point of view the 1st bond between any two atom is called Sigma .
Methane CH4 has 4 sigma bonds.
Ethane C2H6 has 7 sigma bonds.
Benzene C6H6 has 12 sigma bonds.
Sigma Bonding and Antibonding Molecular Orbitals :
Hydrogen Bonding M.O.
Hydrogen Antibonding M.O.