1.3.9 Hybridisation

Bond overlap in covalent bonds

• A single covalent bond is formed when two nonmetals 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 greater the atomic orbital overlap, the stronger the bond
• Sigma (σ) bonds are formed by direct overlap of orbitals between the bonding atoms
• Pi (π) bonds are formed by the sideways overlap of adjacent above and below the σ bond

Sigma bonds

• Sigma (σ) bonds are formed from the end-on overlap of atomic orbitals
• S orbitals overlap this way as well as p orbitals

Sigma orbitals can be formed from the end-on overlap of s orbitals

• The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
• The pair of electrons is found between the nuclei of the two atoms
• The electrostatic attraction between the electrons and nuclei bonds the atoms to each other

Pi bonds

• Pi (π) bonds are formed from the sideways 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 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

π orbitals can be formed from the end-on overlap 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

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

Overlap of the p orbitals results in the forming of a π bond in ethene

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

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

Hydrogen cyanide

• Hydrogen cyanide contains a triple bond
• One σ bond is formed between the H and C atom (overlap of an sp C hybridised orbital with the 1s H orbital)
• A second σ bond is formed between the C and N atom (overlap of an sp C hybridised orbital with an sp orbital of N)
• 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 the overlap of two sets of p orbitals of nitrogen and carbon and the overlap of an sp hybridised carbon orbital and a p orbital on the nitrogen

Nitrogen

• Nitrogen too contains a triple bond
• The triple bond is formed from the overlap of the sp orbitals on each N to form a σ bond and the overlap of two sets of p orbitals on the nitrogen atoms to form two π bonds
• These π bonds are at right angles to each other

The triple bond is formed from two π bonds and one σ bond

Hybridisation

• The p atomic orbitals can also overlap end-on to form σ bonds
• In order for them to do this, they first need to become modified in order to gain s orbital character
• The orbitals are therefore slightly changed in shape to make one of the p orbital lobes bigger
• This mixing of atomic orbitals to form covalent bonds is called hybridisation
  • Mixing one s orbital with three p orbitals is called sp³ hybridisation (each orbital has ¼ s character and ¾ p character)
  • Mixing one s orbital with two p orbitals is called sp² hybridisation
  • Mixing one s orbital with one p orbital forms sp hybridised orbitals

σ orbitals can be formed from the end-on overlap of p orbitals

The mixing of s orbitals with p orbitals to form molecular bonds is called hybridisation