Question

Q. State octet rule. Explain it's inadequencies with respect to
1. Incomplete octet
2. Expanded octet

Answer 1

1) The Octet rule is not applicable when the Central metal is expanding its octet.

Example of a compound is PCl5

Here P has 5 electrons in its valence shell. and five Cl atoms bind to the P to complete their octet. However for P the octet rule is not valid as there are excess of electrons as shown :

2) When there are odd no.of electrons in the valence shell:

Nitrous oxide NO molecule has 6+5 =11 electrons . [which is an odd number].

So when the Lewis structure for this molecule is drawn,

O has a complete octet, so N has [11-8= 3] electrons as shown below:

hence octet rule is not applicable when there are odd number of electrons in the valence shell.

3)Octet rule is not applicable when there is an incomplete octet of central atom:

In compounds like BF3, central atom has 6 valence electrons[ 3 its own electrons and 3 contributed by the three Flourine atoms]. Hence octet rule is not applicable here.

Hope this information will clear your doubts about the topic.