0According to modern concept an orbital can accommodate a maximum of two electrons having opposite spin, if they are in pair form, no chemical combination is possible. But if however there is only a single electron in an orbital of outer shell, it will have a tendency to combine with an electron of opposite spin in another atom forming a covalent chemical bond or in other words coupling of electrons with opposite spin belonging to orbitals of external shells of the two atoms forms a covalent bond between two atoms.
The combination of orbitals can be of three types:
1-s-s overlapping,
2-s-p overlapping,
3-p-p overlapping.
