Question

give two limitations for octet rule

Answer 1

One limitation of the octet rule is that it cannot be applied to the nonmetals after silicon in the Periodic Table.

These elements can “expand their octet” and have more than eight valence electrons around the central atom.

Examples are

PF

5

,

SF

6

, and

H

2

SO

4

.

The

P

atom in

PF

5

has 10 electrons in its valence shell:

Answer 2

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Limitations of the octet rule:

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.

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