AQA A Level Chemistry复习笔记1.4.4 Dative Covalent Bonding

• In simple covalent bonds, the two atoms involved share 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 bonding
• An example with 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

   

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 two electrons to 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

   

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