The concept of chemical bonds describes the forces that hold atoms together to form molecules and compounds. A common point of confusion arises when discussing the differences between a “single bond” and a “sigma bond,” as these terms are often used interchangeably in chemistry. While a single bond is a simple structural description, the sigma bond is a detailed mechanistic explanation of how that bond is formed at the atomic level. This explanation clarifies why the two terms are almost always synonymous, yet describe different aspects of the same fundamental connection.
Defining the Single Covalent Bond
A single covalent bond represents the simplest form of chemical connection between two atoms. Structurally, it is defined as the sharing of one pair of electrons between the bonded atoms. This is the representation most commonly seen in Lewis structures, where the bond is drawn as a single line connecting two atomic symbols. For example, in a simple molecule like hydrogen gas (\(\text{H}_2\)), the single bond signifies that each hydrogen atom contributes one electron to form a shared pair. This structural view focuses on the number of electron pairs being shared, giving the bond a formal bond order of one.
The Mechanics of Sigma Orbital Overlap
A sigma (\(\sigma\)) bond is defined by the specific mechanism of its formation, which involves the direct, head-to-head overlap of atomic orbitals. This axial overlap can occur between various types of orbitals, such as two \(s\) orbitals, an \(s\) orbital and a \(p\) orbital, or two \(p\) orbitals aligned along the bonding axis. The result of this direct overlap is a region of high electron density concentrated symmetrically along the internuclear axis. This cylindrical symmetry is a defining feature of the sigma bond, giving it its name from the Greek letter \(\sigma\). Because the electron density is directly between the nuclei, the electrostatic attraction is maximized, making the sigma bond the strongest type of covalent bond.
Why Every Single Bond Is a Sigma Bond
The fundamental reason a single bond is a sigma bond lies in the principle of maximum orbital overlap. When two atoms approach to form a covalent bond, they naturally orient themselves to achieve the greatest possible overlap of their respective atomic orbitals. This head-to-head or axial alignment, which defines the sigma bond, provides the most stable and lowest-energy arrangement for the shared electron pair. Since a single bond is composed of only one shared electron pair, that single connection must necessarily be the most stable, head-on overlap possible. Therefore, the structural description of a single bond always corresponds to the mechanistic description of a sigma bond.
The Role of Sigma Bonds in Multiple Bonds
The distinction between the two terms becomes apparent when discussing multiple bonds, such as double or triple bonds. A double bond consists of one sigma bond and one pi (\(\pi\)) bond, while a triple bond is composed of one sigma bond and two pi bonds. The sigma bond is always the primary bond established between the two atoms, occupying the space along the internuclear axis. Pi bonds are formed by the sideways overlap of unhybridized \(p\) orbitals, resulting in electron density regions located above and below the internuclear axis. These pi bonds are weaker than the foundational sigma bond.