• 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

   

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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