Covalent Bonds (DP IB Chemistry): Revision Note

Covalent bonds

What are covalent bonds?

• Covalent bonding occurs between two non-metals

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

  • No electrons are transferred

  • Electrons are shared between atoms

• When a covalent bond is formed, two atomic orbitals overlap to form a molecular orbital

• Covalent bonding occurs because electrons are more stable when shared between two nuclei than when held by one atom alone

• In a typical covalent bond, each atom contributes one electron to the shared pair

  • For example, covalent bonding of hydrogen:

• A single covalent bond is represented by a short straight line between the two atoms:

  • H₂, hydrogen: H–H

  • Cl₂, chlorine: Cl-Cl

  • HBr, hydrogen bromide: H-Br

• Covalent bonds are not fixed lines between atoms

  • The shared electrons are constantly moving and are best represented as charge clouds:

• Non-metals are able to share pairs of electrons to form single, double or triple covalent bonds

• By sharing electrons, both atoms achieve a noble gas configuration

  • This makes them more stable

The octet rule and its exceptions

• The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons 

• In some instances, the central atom of a covalently bonded molecule can accommodate more or less than 8 electrons in its outer shell

  • More than 8 electrons in the outer shell is ‘expanding the octet rule’

  • Less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’

  • Examples of this can be found in the section on Lewis structures