Covalent Bonds (HL IB Chemistry)

What are covalent bonds?

• Covalent bonding occurs between two non-metals
• A covalent bond involves the electrostatic attraction between nuclei of two atoms and the electrons of their outer shells
• No electrons are transferred but only shared in this type of bonding
• When a covalent bond is formed, two atomic orbitals overlap and a molecular orbital is formed
• Covalent bonding happens because the electrons are more stable when attracted to two nuclei than when attracted to only one

Diagram to show the formation of a covalent bond in a hydrogen molecule

The positive nucleus of each atom has an attraction for the bonding electrons shared in the covalent bond

• In a normal covalent bond, each atom provide one of the electrons in the bond. A covalent bond is represented by a short straight line between the two atoms, H-H
• Covalent bonds should not be regarded as shared electron pairs in a fixed position; the electrons are in a state of constant motion and are best regarded as charge clouds

Hydrogen Molecular Orbital Diagram

A representation of electron charge clouds. The electrons can be found anywhere in the charge clouds

• Non-metals are able to share pairs of electrons to form different types of covalent bonds
• Sharing electrons in the covalent bond allows each of the 2 atoms to achieve an electron configuration similar to a noble gas
  • This makes each atom more stable

• 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
  • Being able to accommodate more than 8 electrons in the outer shell is known as ‘expanding the octet rule’
  • Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
  • Some examples of this can be found in the section on Lewis structures