CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.3.8 Coordinate Bonding

Dative Bonding: Definition & Examples

  • In simple covalent bonds the two atoms involved shares electrons
  • Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
    • An electron-deficient atom is an atom that has an unfilled outer orbital
  • So both electrons are from the same atom
  • This type of bonding is called dative covalent bonding or coordinate bond
  • An example of a dative bond is in an ammonium ion
    • The hydrogen ion, H+ is electron-deficient and has space for two electrons in its shell
    • The nitrogen atom in ammonia has a lone pair of electrons which it can donate to the hydrogen ion to form a dative covalent bond

 

Chemical Bonding Dative Covalent Bonding Ammonium ion, downloadable AS & A Level Chemistry revision notes

Ammonia (NH3) can donate a lone pair to an electron-deficient proton (H+) to form a charged ammonium ion (NH4+)

  • Aluminium chloride is also formed using dative covalent bonding
  • At high temperatures aluminium chloride can exist as a monomer (AlCl3)
    • The molecule is electron-deficient and needs to electrons complete the aluminium atom’s outer shell
  • At lower temperatures the two molecules of AlCl3 join together to form a dimer (Al2Cl6)
    • The molecules combine because lone pairs of electrons on two of the chlorine atoms form two coordinate bonds with the aluminium atoms

Chemical Bonding Dative Covalent Bonding Aluminium chloride, downloadable AS & A Level Chemistry revision notes

Aluminium chloride is also formed with a dative covalent bond in which two of the chlorine atoms donate their lone pairs to each of the aluminium atoms to form a dimer

Exam Tip

In dative covalent bonding, both electrons in the covalent bond are shared by one atom.

A dative covalent bond uses an arrow from the donated pair of electrons to the electron-deficient atom.

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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