Hybridisation (Cambridge (CIE) A Level Chemistry): Revision Note

Exam code: 9701

Richard Boole

Last updated

Orbitals & Hybridisation in Covalent Bonding

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

Forming sigma bonds

Diagram showing two s atomic orbitals combining to form a molecular σ orbital in a hydrogen molecule, with labels indicating each step.
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

  • There is an electrostatic force of attraction between the electrons and nuclei which 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 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

Forming pi bonds

Diagram illustrating p atomic orbitals forming a molecular orbital and JT bond, with annotations explaining orbital and bond formation.
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

 Sigma bonding in hydrogen

Diagram showing two hydrogen atoms, each with a 1s orbital, combining to form a hydrogen molecule with a sigma 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

 Pi bonding in ethene

Chemical diagram showing the transformation of p orbitals in ethene to π bonds, illustrating bonding angles of 120 degrees involving carbon and hydrogen.
Overlap of the p orbitals results in the forming of a π bond in ethene

Sigma and pi bonding in ethene

Diagram of molecular bonds showing pi bonds in pink, sigma bond arrows, hydrogen nuclei marked by X, and carbon nuclei as black dots.
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

 Sigma and pi bonding in ethyne

Diagram illustrating sigma and pi bonds between carbon atoms, highlighting orbital overlaps with colours. Labels identify sigma and pi bonds in ethene.
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

Sigma and pi bonding in hydrogen cyanide

Diagram showing orbital hybridisation in a carbon-nitrogen bond, illustrating sigma (σ) and pi (π) bonds, with labelled overlapping orbitals.
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

     

Sigma and pi bonding in nitrogen molecules

Diagram of nitrogen molecule showing p orbitals in blue and orange, with sigma and pi interactions labelled. Nitrogen atoms are central in each side.
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

 sp hybridisation

Diagram showing formation of sigma bonds between carbon and hydrogen or carbon, using modified p and s atomic orbitals combining into molecular orbitals.
π orbitals can be formed from the end-on overlap of p orbitals

sp hybrid orbitals

Diagram of orbital hybridisation: sp3, sp2, and sp. Shows electron orbitals merging into hybridised forms, with corresponding molecular structures.
The mixing of s orbitals with p orbitals to form molecular bonds is called hybridisation
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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.