4.5. Define Octet rule. Write its significance and limitations.

<p><strong>4.5.&nbsp; </strong>Kössel and Lewis developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding. According to this rule, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule.</p><p><strong>Significance:</strong>&nbsp;It helps to explain why different atoms combine with each other to form ionic compounds or covalent compounds.</p><p><strong>Limitations of Octet rule:</strong></p><p>According to Octet rule, atoms take part in chemical combination to achieve the configuration of the nearest noble gas elements. However, some of noble gas elements like Xenon have formed compounds with fluorine and oxygen. For example: XeF₂, XeF_4&nbsp; Therefore, validity of the octet rule has been challenged. This theory does not account for shape of molecules.</p><p>Stability of odd electron species: Molecules like NO and NO₂ do not satisfy the octet rule and yet they exist as stable molecules. The octet rule failed to explain the reason for this.</p><p>Expansion of octet: The rule could not explain the stability of compounds which have more than 8 valence electrons around the central atom. For eg SF₆, PF₅, IF₇</p><p>Electron deficient compounds: The octet rule could not explain the stability of electron deficient compounds like LiCl, AlCl₃, BH₃</p>

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