Atoms combine to form molecules through chemical bonds, which are fundamental to the structure and properties of all matter. These bonds dictate how atoms arrange themselves. This article clarifies the relationship between sigma bonds and single bonds, addressing a common point of confusion.
Understanding Single Bonds
A single bond represents a covalent bond where two atoms share one pair of electrons. This shared electron pair holds the two atoms together. For example, in a hydrogen molecule (H₂), each hydrogen atom contributes one electron to form a single shared pair, creating a stable H-H bond. Similarly, in a chlorine molecule (Cl₂), a single bond forms between the two chlorine atoms. In ethane (C₂H₆), all carbon-carbon and carbon-hydrogen connections are single bonds, signifying one shared electron pair between each bonded atomic pair.
Understanding Sigma Bonds
A sigma (σ) bond is a specific type of covalent bond characterized by the direct, head-on overlap of atomic orbitals. This overlap occurs precisely along the internuclear axis, the line connecting the centers of the two bonded atoms. The electron density in a sigma bond is concentrated symmetrically around this axis. Sigma bonds can form from various combinations of atomic orbitals, such as the overlap of two s-orbitals, an s-orbital and a p-orbital (like in hydrogen chloride), or two p-orbitals. Hybrid orbitals (sp, sp², sp³) also participate in forming sigma bonds through this direct overlap. Due to this direct overlap, sigma bonds are considered the strongest type of covalent bond.
The Relationship Between Single and Sigma Bonds
Every single bond is a sigma bond. This is because a single bond fundamentally involves the sharing of one electron pair between two atoms, and this sharing occurs through a direct, head-on overlap of atomic orbitals. This head-on overlap is the defining characteristic of a sigma bond. Consider the carbon-hydrogen bonds in methane (CH₄); each C-H bond is a single bond, and therefore, each is a sigma bond formed by the overlap of a carbon hybrid orbital and a hydrogen s-orbital. The cylindrical symmetry of electron density around the internuclear axis in a sigma bond allows for free rotation of the bonded atoms, contributing to molecular flexibility.
Sigma Bonds in Multiple Bonds
While all single bonds are sigma bonds, sigma bonds are not exclusively found in single bonds. Multiple bonds, such as double and triple bonds, also contain a sigma bond as their foundational component. These additional bonds in multiple bond systems are known as pi (π) bonds. Pi bonds form from the sideways overlap of parallel p-orbitals, which results in electron density concentrated above and below the internuclear axis. A double bond, like the carbon-carbon bond in ethylene (C₂H₄), is composed of one sigma bond and one pi bond. A triple bond, exemplified by the carbon-carbon bond in acetylene (C₂H₂), consists of one sigma bond and two pi bonds. The sigma bond is always the first bond established between two atoms, providing the primary strong connection, with pi bonds adding further strength and rigidity to the overall bond.