what is octet rule? Briefly explain
its significance and limitations
Answers
Answer: The octet rule dictates that atoms are most stable when their valence shells are filled with eight electrons. It is based on the observation that the atoms of the main group elements have a tendency to participate in chemical bonding in such a way that each atom of the resulting molecule has eight electrons in the valence shell. The octet rule is only applicable to the main group elements.
The molecules of the halogens, oxygen, nitrogen, and carbon are known to obey the octet rule. In general, the elements that obey this rule include the s-block elements and the p-block elements (with the exception of hydrogen, helium, and lithium).
The octet rule can be observed in the bonding between the carbon and oxygen atoms in a carbon dioxide molecule
The shared electrons fulfil the valency requirements of both the bonded atoms. Thus, it can be noted that both the oxygen atoms and the carbon atom have an octet configuration in the CO2 molecule.
Upon observing that the noble gases were chemically inert, the electronic theory of valency was proposed by the German physicist Walther Kossel and the American chemist Gilbert Lewis. It was based on the tendency of atoms to assume the most stable state possible.
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:
(a) The rule failed to predict the shape and relative stability of molecules.
(b) 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.
(c) 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. )
(d) 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.
(e) 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.
Explanation: