Covalent Bonding (Edexcel International AS Chemistry): Revision Note

Covalent Dot & 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

The four dot-and-cross diagrams above show:

• H₂, Cl₂, HCl: one shared pair (single bond); each chlorine also has three lone pairs

• O₂: two shared pairs (double bond), each oxygen with two lone pairs

• N₂: three shared pairs (triple bond), each nitrogen with one lone pair

• CO₂: two C=O double bonds, each oxygen with two lone pairs

• NH₃: three N-H single bonds and one lone pair on nitrogen

• H₂O: two O-H single bonds and two lone pairs on oxygen

• CH₄ and C₂H₆: only single bonds (4 C-H; 1 C-C and 6 C-H) and no lone pairs

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