Covalent Dot-and-Cross Diagrams (Edexcel AS Chemistry): Revision Note

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