Chemistry · Ch 4 — Chemical Bonding and Molecular Structure
Octet Rule
Octet Rule
The Electronic Theory of Chemical Bonding
In 1916, Walther Kossel and Gilbert N. Lewis independently laid the foundation for a modern understanding of chemical bonding. Their work, known as the electronic theory of chemical bonding, shifted the focus from the properties of atoms as a whole to the behaviour of their outermost electrons. The central idea is simple: atoms are driven to combine with each other to achieve a stable electronic configuration, one that resembles the noble gases.
The theory proposes two fundamental ways an atom can achieve this stability:
- Transfer of electrons: One atom completely loses one or more valence electrons, which are then gained by another atom. This creates ions (cations and anions) that are held together by electrostatic attraction.
- Sharing of electrons: Two atoms pool one or more pairs of their valence electrons, which then belong to both atoms simultaneously. This shared pair constitutes a covalent bond.
The Octet Rule
The specific target for this stable configuration is the octet — a set of eight electrons in the outermost shell (valence shell). The rule that governs this behaviour is the octet rule.
The Octet Rule: atoms tend to combine in such a way that each atom acquires eight electrons in its valence shell, giving it the same electronic configuration as the nearest noble gas.
The only exception to the "eight" is for the very first shell (the K-shell), which is completely filled with just two electrons. For hydrogen and helium, the stable configuration is a duplet (two electrons), matching the configuration of helium.
The octet rule is the single most important guiding principle for understanding why atoms form the bonds they do. It explains the stoichiometry of countless compounds.
How the Octet Rule Works: Two Mechanisms
The octet rule is achieved through the two mechanisms proposed by the electronic theory.
1. By Transfer of Electrons (Ionic Bonding)
An atom with 1, 2, or 3 valence electrons (a metal) can lose them to become a positively charged ion (cation). An atom with 5, 6, or 7 valence electrons (a non-metal) can gain the needed 1, 2, or 3 electrons to become a negatively charged ion (anion). The resulting ions, with complete octets, are then held together by strong electrostatic forces.
Example: Sodium (Na) has 1 valence electron (). Chlorine (Cl) has 7 valence electrons (). Sodium loses its one electron to become ( — an octet). Chlorine gains that electron to become ( — an octet). The and ions then form an ionic bond.
2. By Sharing of Electrons (Covalent Bonding)
When two atoms have similar tendencies to gain or lose electrons (typically both are non-metals), neither can completely transfer an electron. Instead, they share one or more pairs of electrons. Each shared pair counts as one electron in the valence shell of both atoms, allowing each to complete its octet. …