There are 2 theories on chemical bonding. 

1. The principle of the valence bond theoryis that when the partially filled orbital of one atom overlaps with the orbital of another atom, they share valence electrons and subsequently form a bond. This bond is established when the resulting molecule possesses lower energy compared to the individual free atoms.
2. Themolecular orbital theoryrepresents the orbitals of a molecule as combinations of antibonding and bonding states formed from atomic orbitals. These molecular orbitals are associated with the entire molecule rather than individual atoms (unlike the localized atomic orbitals). Molecular orbitals are categorized as σ when they align directly along the internuclear axis, and as π when they do not. This theory provides insights into bond formation, as well as the spectral and magnetic characteristics of molecules.