Figure 3.2. Molecular Orbitals
Molecular orbitals can be bonding or antibonding, depending on the signs of the atomic orbitals
used to form them; head-to-head or tail-to-tail overlap of atomic orbitals results in a σ bond.σ AND π BONDS
When a molecular orbital is formed by head-to-head or tail-to-tail overlap, as in Figure 3.2, the
resulting bond is called a sigma (σ) bond. All single bonds are σ bonds, accommodating two
electrons.
KEY CONCEPT
Sigma (σ) bonds are formed by head-to-head or tail-to-tail overlap of atomic orbitals. These
bonds are by far the most common in organic compounds and on the MCAT.When two p-orbitals line up in a parallel (side-by-side) fashion, their electron clouds overlap, and a
bonding molecular orbital, called a pi (π) bond, is formed. This is demonstrated in Figure 3.3. One π
bond on top of an existing σ bond is a double bond. A σ bond and two π bonds form a triple bond.
Unlike single bonds, which allow free rotation of atoms around the bond axis, double and triple
bonds hinder rotation and, in effect, lock the atoms into position.