Covalent Bonding (AQA A Level Chemistry): Revision Note

Covalent Bonding

• Covalent bonding occurs between non-metal atoms

• A covalent bond is the electrostatic attraction between the positively charged nuclei of two atoms and a shared pair of electrons in their outer shells

• In covalent bonding, electrons are shared between atoms rather than transferred from one atom to another

• Non-metal atoms share pairs of electrons to form covalent bonds

• By sharing electrons, each atom can achieve a full outer shell of electrons, similar to the electron configuration of a noble gas

• This full outer shell makes the atoms more stable

Covalent Bonds & Shared Electrons Table

Dot & cross diagrams

• Dot-and-cross diagrams are used to represent covalent bonding

• They show only the outer shell electrons of the atoms involved

• Different symbols are used to distinguish between atoms:

  • Dots represent the electrons from one atom, and crosses represent the electrons from the other atom

• Shared electrons are shown as pairs in the overlap between the atoms

Single covalent bonding 

Hydrogen, H₂

Chlorine, Cl₂

Hydrogen Chloride, HCl

Ammonia, NH₃

Methane, CH₄

Ethane, C₂H₆

Double covalent bonding

Oxygen, O₂

Carbon dioxide, CO₂

Ethene, C₂H₄

Triple covalent bonding

Nitrogen, N₂

Expansion of the octet

• In some covalently bonded molecules, the central atom may have more than eight electrons in its outer shell

• If the central atom has more than eight electrons in its outer shell, it is said to have an expanded octet

  • This is possible for some Period 3 elements

Sulfur dioxide, SO₂

Phosphorus pentachloride, PCl₅

Sulfur hexafluoride, SF₆

Molecules with fewer than 8 valence electrons

• If the central atom has fewer than eight electrons in its outer shell, it is described as electron-deficient

• These molecules can be stable even with an incomplete outer shell

• Two examples of this are found in beryllium chloride and bromine trifluoride: