Question

Define octet rule. Write its significance and limitations.​

Answer 1

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➤ Octet rule:

Atoms can combine either by transfer of valence electrons from one atom to another (gaining or loosing) or by sharing of valence electrons in order to have an octet in their valence shells

➤ Significance of octet rule:

The octet rule successfully explained the formation of chemical bonds depending upon the nature of the element.

➤ Limitations of the octet theory:

The following are the limitations of the octet rule:

• The rule failed to predict the shape and relative stability of molecules.
• It is based upon the inert nature of noble gases. However, some noble gases like xenon and krypton form compounds such as XeF2, KrF 2 etc.
• The octet rule cannot be applied to the elements in and beyond the third period of the periodic table. The elements present in these periods have more than eight valence electrons around the central atom. For example: PF5, SF6, etc. )  
• The octet rule is not satisfied for all atoms in a molecule having an odd number of electrons. For example, NO and NO2 do not satisfy the octet rule.
• This rule cannot be applied to those compounds in which the number of electrons surrounding the central atom is less than eight. For example, LiCl, BeH 2, AlCl3 etc. do not obey the octet rule.

Answer 2

Answer:

octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds.

Significance: It explains the reason behind the chemical combination of different atoms to form ionic or covalent bond.

One limitation of the octet rule is that it cannot be applied to the nonmetals after silicon in the Periodic Table. These elements can “expand their octet” and have more than eight valence electrons around the central atom. ... In some molecules the central atom cannot possibly have eight valence electrons