Orbital Overlap

Orbital Overlap

When atoms combines to form a covalent molecule, the atomic orbitals of the combining atoms overlap to form a covalent bond. The bond pair of electrons will occupy the overlapped region of the orbitals. Depending upon the nature of overlap we can classify the covalent bonding between the two atoms as sigma (σ) and pi (π) bonds.

Sigma and Pi bonds

When two atomic orbitals overlap linearly along the axis, the resultant bond is called a sigma (σ) bond. This overlap is also called 'head-on overlap' or 'axial overlap'. Overlap involves an s orbital (s-s and s-p overlaps) will always result in a sigma bond as the s orbital is spherical. Overlap between two p orbitals along the molecular axis will also result in sigma bond formation. When we consider x-axis as molecular axis, the p_x-p_x overlap will result in σ-bond.

When two atomic orbitals overlaps sideways, the resultant covalent bond is called a pi (π)bond. When we consider x-axis as molecular axis, the p_y-p_y and p_z-p_z overlaps will result in the formation of a π-bond.

Following examples will be useful to understand the overlap:

Formation of hydrogen (H₂) Molecule

Electronic configuration of hydrogen atom is 1s¹

During the formation of H₂ molecule, the 1s orbitals of two hydrogen atoms containing one unpaired electron with opposite spin overlap with each other along the internuclear axis. This overlap is called s-s overlap. Such axial overlap results in the formation of a σ-covalent bond.

Formation of fluorine molecule (F₂):

Valence shell electronic configuration of fluorine atom : 2s² 2p_x², 2p_y², 2p_z¹

When the half filled p_z orbitals of two fluorine overlaps along the z-axis, a σ-covalent bond is formed between them.

Formation of HF molecule:

Electronic configuration of hydrogen atom is 1s¹

Valence shell electronic configuration of fluorine atom : 2s² 2p_x², 2p_y², 2p_z¹

When half filled 1s orbital of hydrogen linearly overlaps with a half filled 2p_z orbital of fluorine, a σ-covalent bond is formed between hydrogen and fluorine.

Formation of oxygen molecule (O₂):

Valence shell electronic configuration of oxygen atom : 2s² 2p_x², 2p_y¹, 2p_z¹

When the half filled p_z orbitals of two oxygen overlaps along the z-axis (considering molecular axis as z axis), a σ-covalent bond is formed between them. Other two half filled p_y orbitals of two oxygen atoms overlap laterally (sideways) to form a π-covalent bond between the oxygen atoms. Thus, in oxygen molecule, two oxygen atoms are connected by two covalent bonds (double bond). The other two pair of electrons present in the 2s and 2p_x orbital do not involve in bonding and remains as lone pairs on the respective oxygen.