Question

Write the significance and limitations of the octet rule.

Answer 1

Octet rule:

• The octet rule is a chemical rule which suggests that all the atoms prefer to be in the octet form in their valence shell.
• It is the tendency of the atom to complete eight electrons in its outer shell by interacting with other atoms through electron sharing or electron transfer.
• The octet form is nothing but fully filled electrons in its valence shell.
• Octet form is a stable electronic configuration of ${\mathrm{ns}}^2{\mathrm{np}}^6$.
• Examples of compounds obeying the octet rule are ${\mathrm{CH}}_4,\mathrm{NaCl}\&{\mathrm{CO}}_2$.

Significance:

• It explains the chemical bonding in various chemical compounds.
• It mainly applies to the second period of the periodic table.
• It illustrates various types of bond-like ionic, covalent, electrovalent, and coordinate bonds.

Limitations:

• It fails to predict the shape and relative stability of molecules.
• Incomplete octet rule: Octet rule does not apply to molecules having the number of electrons surrounding the central atom being less than eight. eg: ${\mathrm{BeH}}_2,{\mathrm{BF}}_3$, $\mathrm{NO},{\mathrm{NO}}_2$&${\mathrm{BCl}}_3$.
• Here, the number of electrons is only six which does not satisfy the octet configuration.

• Expanded octet rule: When the valence electron count exceeds eight electrons eg: ${\mathrm{PCl}}_5\&{\mathrm{SF}}_4$.

• Here, the valence electrons of phosphorous are five and five chlorine contributes five electrons. so, total of ten electrons exceed the octet. So, this is an example of an expanded octet rule.