Noble Gases (Group VIIIA)

Noble gases are very stable and chemically inert. They have a general electronic configuration of ns² np⁶ (except Helium: 1s²) and completely filled valence shells, making them non-reactive in ordinary chemical reactions.

Octet Rule (G. N. Lewis, 1916)

The octet rule states that an atom is most stable when its valence shell contains eight electrons. This rule applies to major group elements, involving s and p electrons. Examples include oxygen, nitrogen, and halogens.

Duplet Rule

The duplet rule is the tendency of atoms to acquire two electrons in their outermost shell during bond formation, leading to a stable electronic configuration like helium. Examples include hydrogen, lithium, and beryllium.

Atomic Stability and Reactivity

Unstable atoms with incomplete octets are reactive, while stable atoms with complete octets or duplets are less reactive. For example:

• Na: 1s² 2s² 2p⁶ 3s¹ (unstable, reactive)
• Na⁺: 1s² 2s² 2p⁶ (stable, like Ne)
• Cl: 1s² 2s² 2p⁶ 3s² 3p⁵ (unstable, reactive)
• Cl⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ (stable, like Ar)

Exceptions to Octet Rule

Hydrogen, helium, and lithium follow the duplet rule. Their electrons lie in the s orbital, making them exceptions to the octet rule.