The Octet "Rule" and Bond Stability

The octet "rule" is not an unbreakable law, but rather a heuristic or "rule of thumb" introduced in high school and chemistry courses when students begin to learn about bonding. It's quite powerful in predicting bonding and structure of molecules, but - as a heuristic used to simplify an immensely complicated quantum mechanical phenomenon, it is limited. Some of you may encounter more advanced theories with further study in chemistry.

For our purposes, it is important to recognize two things about the octet rule:

  1. It only applies to main group elements (which are the focus of intro chemistry). Transition elements, have a different rule of thumb, the 18-electron rule and there are additional exceptions.
  2. There is nothing causal about the octet rule. Atoms don't "want" to achieve eight valence electrons. Rather, we can understand these rules of thumb as arising or emerging from a combination of effective nuclear charge and the fact that electrons can only exist in discrete electronic orbitals which can only contain two electrons each.

Answer the questions below before reading the solutions to develop your understanding of the octet rule and when stability is reached in other circumstances (i.e., when no more primary bonds will form).

Exercise 3.8.1: What causes the Octet "Rule"?
Not Currently Assigned

  1. Why do noble gasses not form primary bonds? (use the idea of effective nuclear charge combined with the electronic configuration of noble gasses)

  2. Why do covalently bonded atoms from main group elements (i.e., those that follow the octet rule) stop forming primary bonds when they have eight valence electrons?

  3. When will ionically bonded atoms stop forming primary bonds?

  4. When will metallically bonded atoms stop forming primary bonds?