What is octet rule write its significance and limitations?
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The Octet rule is a general rule of thumb that applies to most atoms. Basically, it states that every atom wants to have eight valence electrons in its outermost electron shell.
Significance of octet rule: The octet rule successfully explains the formation of chemical bonds depending upon the nature of the element. It is used to determine stability in atoms. The octet rule helps to figure out how atoms will bond with one another.
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 only 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 aluminum 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 sulfur and phosphorus to have more than an octet. Compounds such as PCl5and SF6 can form. These compounds have 10 and 12 electrons around their central atoms, respectively.
Significance of octet rule: The octet rule successfully explains the formation of chemical bonds depending upon the nature of the element. It is used to determine stability in atoms. The octet rule helps to figure out how atoms will bond with one another.
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 only 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 aluminum 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 sulfur and phosphorus to have more than an octet. Compounds such as PCl5and SF6 can form. These compounds have 10 and 12 electrons around their central atoms, respectively.
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Answer:
Octet rule fails to explain the following:
(1) The stability of incomplete octet molecules, i.e., the molecules with the central atom containing less than eight electrons.
For example, .
(2) The stability of expanded octet molecules, i.e., the molecules with the central atom containing more than eight electrons.
For example, .
(3) Observed shape and geometry of the molecules.
(4) Difference in energies and reactivities of different molecules.
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