Sigma and Pi Bonds Overview

Sigma (σ) Bonds

• Sigma bonds are formed by the head-on overlap of atomic orbitals.
• The electron density is concentrated along the internuclear axis.
• They are generally stronger than pi bonds.
• They are single bonds.
• The maximum overlap occurs when the lobes of the atomic orbitals are directly aligned along the axis connecting the two bonded nuclei.
• Rotation around the sigma bond axis is possible without significant disruption of the bond.
• Sigma bonds are crucial for determining the overall geometry and stability of molecules.
• Single bonds are primarily sigma bonds.
• The overlap in sigma bonds is directly along the internuclear axis.
• The shape of a sigma bond is cylindrically symmetrical about the axis connecting the two nuclei.

Pi (π) Bonds

• Pi bonds are formed by the sideways overlap of p orbitals (or other types of atomic orbitals).
• The electron density is concentrated above and below the internuclear axis.
• Pi bonds are generally weaker than sigma bonds.
• They are typically found in multiple bonds (double or triple bonds).
• Pi bonds arise from the parallel overlap of p orbitals or other atomic orbitals.
• A pi bond typically strengthens a sigma bond.
• Rotation around a pi bond is restricted; rotation would disrupt the sideways overlap.
• Pi bonding is essential for the formation of double and triple bonds in molecules.
• The presence of pi bonds restricts rotation, which affects the shape and properties of the resultant molecules.
• The electron density in pi bonds is not evenly distributed along the internuclear axis, but rather concentrated above and below the plane defined by the internuclear axis.

Bond Characteristics

• Bond length: The distance between the nuclei of two bonded atoms. Shorter bond lengths generally indicate stronger bonds.

• Bond strength: A measure of the energy required to break a bond. Stronger bonds require more energy to break.

• Bond order: The number of electron pairs shared between two atoms. Higher bond orders correspond to stronger and shorter bonds.

• Bond polarity: A measure of the unequal sharing of electrons in a covalent bond, leading to bond dipoles.

• Bond angles: Angles between bonds around a central atom in a molecule. Valence shell electron pair repulsion (VSEPR) theory explains typical bond angles.

• Bond energy: The energy required to break a bond.

• Heteronuclear bond: A covalent bond between atoms of different elements. Often have differing electronegativities, leading to a degree of polarity.

• Homonuclear bond: A covalent bond between atoms of the same element. Nonpolar bond, or have equal electronegativity.

• Bond types: Single, double, and triple bonds are described by the presence of sigma and/or pi bonds.