Sigma & Pi Bonds (College Board AP Chemistry)

Orbital overlapping in bonds

• A single covalent bond is formed when two nonmetals share a pair of electrons
• Each atom contributes with one unpaired electron from a valence atomic orbital
• The overlapping between atomic orbitals form a molecular orbital
  • The overlapping can occur between hybrid orbitals too
• The type of the covalent bond depends on the type of overlapping
• There are two types of interaction:
  • End to end overlapping which form sigma bonds (σ)
  • Sideways or lateral overlapping which form pi bonds (π)
• Since a σ bond is formed by an end to end overlapping the bond is stronger and have greater bond energy than a π bond

Sigma bonds

• A sigma bond (σ) is formed when there is an end to end overlapping of atomic orbitals
• The interaction can occur between any type of atomic orbital (non hybridized or hybridized)
• The atoms are free to rotate around a sigma bond
• A single covalent bond is always a sigma bond
• g. In the case of methane (CH₄), the four sp³ hybrid orbitals overlap with the s-orbital of the hydrogen atoms to form four σ covalent bonds

Sigma and Pi Bonds

Sigma bonds in methane

Pi bonds

• A pi bond (π) is formed when there is sideways (lateral) overlapping of adjacent atomic p-orbitals
• The pi bond occurs perpendicular to the plane in which the sigma bonds are being formed
• A single π bond is drawn as two electron clouds, one above the plane and one below the plane of the sigma bond
  • The electron clouds above and below the plane prevents the rotation around the sigma bond
• Double bonds has always one π bond and triple bonds has always two π bonds
• g. In the case of ethene (C₂H₄),
  • Ethene has a double bond
  • The two sp² hybrid orbitals overlap end to end with the s-orbital of the hydrogen atoms to form two σ covalent bonds.
  • The sp² hybrid orbitals of both adjacent carbons overlap end to end to form one σ covalent bond.

While at the same time, the p orbitals of both adjacent carbon atoms sideways overlap to form one π bond

Bonding in Ethene

Each carbon atom in ethene forms two sigma (σ) bonds with hydrogen atoms and one sigma (σ) bond with another carbon atom. The fourth electron is used to form a pi (π) bond between the two carbon atoms

• E.g. In the case of ethyne (C₂H₂),
  • Ethyne has a triple covalent bond
  • One sp hybrid orbital overlaps end to end with the s-orbital of hydrogen to form one σ covalent bonds.
  • The sp hybrid orbital of both adjacent carbons overlap end to end to form one σ covalent bond.
  • While at the same time, the 4 p orbitals of both adjacent carbon atoms sideways overlap to form two π bonds

Bonding in Ethyne

Ethyne has a triple bond formed from two π bonds and one σ bond between the two carbon atoms