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

σ 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

π 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 a p 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 s 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 ? s character and ? 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

π 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