Structure, Bonding & Reactivity (Oxford AQA International A Level Chemistry)

Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Bonding in Alkenes

  • Alkenes are unsaturated hydrocarbons

    • They are unsaturated because they contain a double covalent bond

    • They are hydrocarbons as they consist of hydrogen and carbon atoms only

  • The general formula of alkanes is CnHn+2

Types of bonds in alkenes

  • Each carbon atom has four electrons in its outer shell (electronic configuration: 1s22s22p2)

  • Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a full outer shell configuration

  • These electrons are found in orbitals within the respective atoms

  • When forming a covalent bond, the orbitals overlap in such a way to form two types of bonds:

    • Sigma bonds (σ)

    • Pi bonds (π)

  • When carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains one spare electron

  • When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)

  • The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap

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 double bond is an area of high electron density

An Introduction to AS Level Organic Chemistry Sp2 Hybridisation, downloadable AS & A Level Chemistry revision notes
Ethene is a planar molecule with bond angles between each bond as 120o

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.