Define octet rule. Write its significance and limitations.
The 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. It was postulated by Kossel & Lewis. The atoms complete their octet by either loosing or gaining of electrons. They do so to attain the nearest noble gas stable electronic configuration for example, the octet rule successfully explained the formation of chemical bonds depending upon the nature of the element.
The octet rule successfully explained the formation of chemical bonds depending upon the nature of the element.
Limitations of the octet theory:
1) The main limitation to the rule is hydrogen, which is at its lowest energy when it has two electrons in its valence shell. Helium (He) is similar in that, it too only has room for two electrons in its valence shell.
Hydrogen and helium have only one electron shell. The first shell has only one- s orbital and no p orbital, so it holds only two electrons. Therefore, these elements are most stable when they have two electrons.
2) Second limitation are aluminium and boron, which can function well with six valence electrons. Consider BF3, The boron shares its three electrons with three fluorine atoms. The fluorine atoms follow the octet rule, but boron has only six electrons. Most elements to the left of the carbon group have so few valence electrons that they are in the same situation as boron, they are electron deficient.
3) In Period 3, the elements on the right side of the periodic table have empty d orbitals. The d orbitals may accept electrons, allowing elements like sulphur and phosphorus to have more than an octet. Compounds such as PCl5 and SF6 can form. These compounds have 10 and 12 electrons around their central atoms, respectively.
Xenon hexafluoride uses d-electrons to form more than an octet. This compound shows another exception: a noble gas compound.
4) 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.
5) The rule failed to predict the shape and relative stability of molecules.
6) It is based upon the inert nature of noble gases. However, some noble gases like xenon and krypton form compounds such as XeF2, KrF2 etc.
7) 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, BeH2, AlCl3 etc. do not obey the octet rule.
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