Understanding Sigma (σ) Bonds
A sigma (σ) bond is the strongest type of covalent chemical bond, formed by the direct, head-on (axial) overlap of atomic orbitals. This overlap occurs along the internuclear axis, which is the imaginary line connecting the nuclei of the two bonded atoms. All single covalent bonds are sigma bonds, and they are essential for holding molecules together, allowing for free rotation around the bond axis.
Understanding Pi (π) Bonds
A pi (π) bond is a type of covalent bond formed by the sideways (lateral) overlap of unhybridized p-orbitals. This overlap occurs above and below, or in front and behind, the internuclear axis, rather than directly along it. Pi bonds are generally weaker than sigma bonds and are always found in conjunction with a sigma bond in double and triple covalent bonds, restricting rotation around the bond axis.
Key Differences in Formation and Strength
The primary distinction lies in their formation: sigma bonds result from direct head-on overlap, while pi bonds arise from lateral overlap. This difference in orbital overlap makes sigma bonds stronger and more stable due to greater electron density concentrated directly between the nuclei. Consequently, pi bonds are weaker and more reactive, as their electron density is dispersed across two regions, making them more accessible for chemical reactions.
Impact on Molecular Geometry and Reactivity
Sigma bonds allow for free rotation of atoms around the bond axis, leading to various conformations in single-bonded molecules. In contrast, the presence of a pi bond in double or triple bonds creates a rigid structure, preventing free rotation due to the necessity of maintaining the sideways orbital overlap. This rigidity significantly influences molecular geometry, and the exposed electron density of pi bonds makes molecules containing them more prone to addition reactions compared to sigma-only bonds.