Edexcel A Level Chemistry:复习笔记1.4.6 Covalent Dot-and-Cross Diagrams

Covalent Dot-and-Cross Diagrams


Covalent compounds

  • The atoms in covalent compounds will share their outer valence electrons to achieve a noble gas configuration


Dot-and-cross diagrams of covalent compounds in which the atoms share their valence electrons


Double covalent bonding

Oxygen, O2



Covalent bonding in oxygen



Carbon dioxide, CO2


Covalent bonding in carbon dioxide


Ethene, C2H4


Covalent bonding in ethene


Triple covalent bonding

Nitrogen, N2


Covalent bonding in nitrogen


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

  • 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


Exam Tip

Covalent bonding takes place between nonmetal atoms. Remember to use the Periodic Table to decide how many electrons are in the outer shell of a nonmetal atom.