CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.3.9 Hybridisation

Covalent Bonding: Orbitals & 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

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

 

Chemical Bonding Bond Overlap in Sigma Orbitals, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Bond Overlap in Pi Orbitals, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Orbital Overlap in Hydrogen, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Pi Bond in Ethene, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Electron Density in Ethene, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Orbital Overlap in Ethyne, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Orbital Overlap in Hydrogen Cyanide, downloadable AS & A Level Chemistry revision notes

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

 

Chemical Bonding Orbital Overlap in Nitrogen, downloadable AS & A Level Chemistry revision notes

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 lobe bigger
  • This mixing of atomic orbitals to form covalent bonds is called hybridisation
    • Mixing an s with three p orbitals is called sp3 hybridisation (each orbital has ¼ s character and ¾ p character)
    • Mixing an s with tw p orbital is called sp2 hybridisation
    • And mixing an s with one p-type orbitals formssp hybridised orbitals

 

Chemical Bonding Sigma Bonds in Hybridised Molecules, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Hybridisation, downloadable AS & A Level Chemistry revision notes

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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