1.7.1 Evidence of Covalent Bonding

• 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

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

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

• 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

Giant atomic structures

• Covalent bonding can be responsible for substances that have many different structures and therefore different physical properties
• We have already seen how small molecules such as H₂O and N₂ are simple units made from covalently bonded atoms
• These simple molecules contain fixed numbers of atoms
• Giant covalent structures on the other hand have a huge number of non-metal atoms bonded to other non-metal atoms via strong covalent bonds
• These structures can also be called giant lattices and have a fixed ratio of atoms in the overall structure
• Three common macromolecules you should know about are diamond, graphite and C₆₀ fullerene
• These structures tend to have high melting points