Sigma & Pi Bonds
Bond overlap in covalent bonds
- A single covalent bond is formed when two non-metals combine
- Each atom that combines has an atomic orbital containing a single unpaired electron
- When a covalent bond is formed, the atomic orbitals overlap to form a combined orbital containing two electrons
- This new orbital is called the molecular orbital
- The shape of the molecular orbital is dependent on the shape of the atomic orbitals that combined
- The greater the atomic orbital overlap, the stronger the bond
- There are two main types of molecular orbital: a sigma (σ) bond and a pi (π) bond
What is a sigma bond?
- Sigma (σ) bonds are formed from the head-on / end-to-end combination or overlap of atomic orbitals
- The electron density is concentrated along the bond axis (an imaginary line between the two nuclei)
- s orbitals overlap this way as well as p to p, and s with p orbitals
The formation of sigma bonds from s orbitals
Sigma orbitals can be formed from the head-on combination of s orbitals
The formation of sigma bonds from an s and a p orbital
Hydrogen fluoride has sigma bonds between s and p orbitals
The formation of sigma bonds from p orbitals
Fluorine has sigma bonds between p orbitals
- The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
- The electrostatic attraction between the electrons and nuclei bonds the atoms to each other
- A single covalent bond is always a sigma bond
What is a pi bond?
- Pi (π) bonds are formed from the lateral (sideways) combination or overlap of adjacent p orbitals
- The two lobes that make up the π bond lie above and below the plane of the σ bond
- This maximises the overlap of the p orbitals
- A single π bond is drawn as two electron clouds one arising from each lobe of the p orbitals
- The two clouds of electrons in a π bond represent one bond containing two electrons
- The electron density is concentrated on opposite sides of the bond axis
- π bonds are only found within double and triple bonds
The formation of a pi bond from p orbitals
π orbitals are formed by the lateral combination of p orbitals
Examples of sigma & pi bonds
Hydrogen
- The hydrogen atom has only one s orbital
- The s orbitals of the two hydrogen atoms will overlap to form a σ bond
The formation of a sigma bond in hydrogen
Direct overlap of the 1s orbitals of the hydrogen atoms results in the formation of a σ bond
Ethene
- Each carbon atom uses three of its four electrons to form σ bonds
- Two σ bonds are formed with the hydrogen atoms
- One σ bond is formed with the other carbon atom
- The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
- This means that the C-C is a double bond: one σ and one π bond
The formation of a pi bond in ethene
Overlap of the p orbitals results in the forming of a π bond in ethene
The formation of sigma bonds and a pi bond 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
Ethyne
- This molecule contains a triple bond formed from two π bonds (at right angles to each other) and one σ bond
- Each carbon atom uses two of its four electrons to form σ bonds
- One σ bond is formed with the hydrogen atom
- One σ bond is formed with the other carbon atom
- Two electrons are used to form two π bonds with the other carbon atom
The formation of sigma bonds and pi bonds in ethyne
Ethyne has a triple bond formed from two π bonds and one σ bond between the two carbon atoms
Predicting the Type of Bonds
- Whether sigma (σ) or pi (π) bonds are formed can be predicted by consideration of the combination of atomic orbitals
Worked example
What type of molecular orbitals are found in the following chemicals?
- Nitrogen, N2
- Hydrogen cyanide, HCN
Answer 1:
- Nitrogen contains a triple bond and a lone pair on each nitrogen atom
- Nitrogen atoms have the electronic configuration 1s22s22p3
- The triple bond is formed from one σ bond between the two nitrogen atoms and the lateral overlap of two sets of p orbitals on the nitrogen atoms to form two π bonds
- NOTE: The σ bond between the two nitrogen atoms is formed from the head-on overlap of two sp hybrid orbitals
- For more information, see our revision notes on Hybridisation
- The two π bonds are at right angles to each other
The triple bond is formed from two π bonds and one σ bond
Answer 2:
- Hydrogen cyanide contains a triple bond
- One σ bond is formed between the H and C atom
- A second σ bond is formed between the C and N atom
- The remaining two sets of p orbitals of nitrogen and carbon will overlap to form two π bonds at right angles to each other
Hydrogen cyanide has a triple bond formed from a σ bond and the overlap of two sets of p orbitals of nitrogen