Sigma (σ) bonds are a type of covalent chemical bond that forms when two atomic orbitals overlap head-on along the internuclear axis (the line connecting the two atomic nuclei). Sigma bonds are characterized by their strong and direct overlap, making them the strongest type of covalent bond. They are the primary bonds that hold atoms together in molecules and play a fundamental role in the structure and stability of compounds.

Key features of sigma (σ) bonds:

1. Formation: Sigma bonds are formed when two atomic orbitals, typically hybrid orbitals or standard atomic orbitals, overlap directly along the internuclear axis. This direct head-on overlap maximizes the electron density between the nuclei.
2. Overlap: The overlap of orbitals in a sigma bond allows electrons to be shared between the two bonding atoms, resulting in the formation of a bond.
3. Strength: Sigma bonds are relatively strong bonds due to the efficient overlap of orbitals, resulting in a strong attraction between the nuclei and the shared electron pair.
4. Single Bonds: Single covalent bonds between two atoms involve the formation of one sigma bond. The sharing of one electron pair contributes to the stability of the molecule.
5. Directionality: Sigma bonds are highly directional, as they form along the internuclear axis. This directional nature influences the overall geometry of molecules.
6. Free Rotation: Molecules connected by sigma bonds often exhibit free rotation around the bond axis, assuming that other factors do not hinder rotation.
7. Hybridization: Sigma bonds can involve hybrid orbitals, such as sp, sp², sp³, etc., formed through hybridization. These hybrid orbitals provide optimal overlap for sigma bond formation.
8. Multiple Bonds: In molecules with double or triple bonds, sigma bonds are always the first bonds formed, followed by pi (π) bonds. For example, in a molecule with a double bond, there is one sigma bond and one pi bond, and in a triple bond, there is one sigma bond and two pi bonds.
9. Atomic Overlap: In sigma bonds, the overlapping regions of the atomic orbitals are concentrated between the bonding nuclei, resulting in a high electron density in this region.

Sigma bonds are essential for the stability and structural integrity of molecules. They are present in a wide range of chemical compounds, from simple diatomic molecules to complex organic and inorganic molecules. Understanding the nature of sigma bonds is crucial for explaining molecular geometry, bonding, and chemical reactivity.