Octet rule

While the octet rule is very useful for describing bonding in a large number of compounds, there are many exceptions to this rule.

Where duplet is formed?

A hydrogen atom has only one electron in its valence shell. It needs one more electron to fill its valence shell. The completed shell has the electronic arrangement of the noble gas helium, yet an octet is not completed. However we still get a stable molecule.

Where the octet remains incomplete?

The elements of group 1, 2 and 13 contain less than four electrons in their valence shell. An octet cannot be achieved by electron sharing for these elements. As a result, elements of these groups should not ordinarily form covalent compounds, but some elements do. Boron halides (BF₃ and BCl₃) have incomplete octets (where only six electrons surround the boron atom), yet they form covalent compounds. These compounds are thus electron-deficient compounds.

Where the octet is expanded?

The elements belonging to groups 15, 16 and 17 have more than four electrons in their valence (outermost) shell. The elements of these groups form stable compounds in which there are more than eight electrons around the central atom. For example PF₅, PCl_5,IF₆ and SF₆ are some typical compounds of this type. Here, Phosphorus atom has five electrons, and sulphur atom has six electrons in their valence shells. The Cl and F atoms have 7 electrons in their valence shells. Therefore, they need one more electron each to attain noble gas configuration.In doing so,Phosphorous has 10 electrons,Iodine has 14 electrons and Sulphur has 12 electrons in the outermost shell in

PF₅ ,PCl₅ ,IF₇ and SF₆ respectively.