Octet rule-definition-limitations of octet rule-hypovalent-hypervalent compounds

Octet rule-definition in covalent molecules .

 Mostof the elements [ except H, Li , B etc ] have a tendency to attain eight electrons in their outer most valence shell through loss or gain of electrons or by 

sharing of electrons at the time of chemical reaction, so that they can attain the next inert configuration.  

This tendency of elements to maintain eight electrons in their own valence shell, is called octet rule.

For example, carbon tetra chloride, methane , carbon dioxide, oxygen , nitrogen etc. 

Limitations of octet rule 

Although octet rule explained the formation of many covalent and co-ordinate compounds, yet it has some limitations like other rule.

There are five limitation of octet rule are known to us ,which are discussed below.

·        Incomplete electron octet 

There are few stable compounds in which the central atom does not attain octet rule. That is, the central atom contain less than eight electrons in their valence shell.

These type of compounds with incomplete octet are known as electron deficient compounds. 

For example, BeCl₂ , BF₃ , BCl₃ etc are example of incomplete electron octet.

·        Expansion of electron octet.

There are few stable compounds in which the central atom does not attain octet rule. That is, the central atom contain higher than eight electrons in their valence shell.

These type of compounds with higher than eight electron  are known as expansion of electron octet.

For example, PCl₅ , SF₆ , IF₇ etc are example of expansion of electron octet.

·        Odd electron molecules.

There are some molecules or ions which contain both two electron as well as three electron covalent bond.

Such type of molecules or ions does not follow octet rule. That is, the central atom have incomplete valence shell.

For example, NO, NO₂ , ClO₂ , O₂^– etc are example of odd electron molecules.

·        Noble gas compounds 

Noble gas elements are generally more stable elements. They have no tendency  to react with other elements.

But ,under certain condition Xe and Kr form few compounds. These compounds does not obey octet rule.

Because, in these compounds, the central atom contain more than eight electrons in their outer valence shell.

·        Beside the above limitation, octet rule can not explain the shape and relative stability of compounds.

·        What is hypovalent compounds?

There are few stable compounds in which the central atom does not attain octet rule. That is, the central atom contain less than eight electrons in their valence shell.

These type of compounds with incomplete octet are known as electron deficient or hypovalent compounds.

For example, BeCl₂ , BF₃ , BCl₃ etc are example of hypovalent compounds.

·        What is hypervalent compounds?

There are few stable compounds in which the central atom does not attain octet rule. That is, the central atom contain higher than eight electrons in their valence shell.

These type of compounds with higher than eight electron are known as hypervalent compounds.

For example, PCl₅ , SF₆ , IF₇ etc are example of hypervalent compounds.

·        What is odd electron molecules ?

There are some molecules or ions which contain both two electron as well as three electron covalent bond.

Such type of molecules or ions does not follow octet rule. That is, the central atom of these type of compounds have incomplete valence shell.

This type of bond is called odd electron bond and the concern molecules are called odd electron molecules.

For example, NO, NO₂ , ClO₂ , O₂^– etc  are example of odd electron molecules.

Summary 

     ·       Octet rule-definition in covalent molecules .
·        Limitations of octet rule 
·        What is hypovalent compounds ?
·        What is hypervalent compounds ?
·        What is odd electron compounds ?

Polarization and polarizability