Octet rule says, “atoms can combine either by transfer of valence electrons from one atom to another or by sharing their valence electrons in order to achieve the nearest inert gas configuration by having an octet in their valence shell.”
Octet rule explains chemical bond formation depending upon nature of element.

Limitations:

(a) Octet rule fails to predict the relative stability and shape of the molecules.

(b) It is based on inert nature of noble gases. But, some inert gases say, krypton(Kr) and xenon(Xe) form compounds like KrF2, XeF2 etc.

Definition of Octet Rule:
The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a stable configuration of eight valence electrons in their outermost energy level. This rule is based on the observation that elements within the main groups of the periodic table tend to form compounds in which they have eight electrons in their valence shell, similar to the noble gases.

Significance of Octet Rule:
1. Stability: By following the octet rule, atoms are able to achieve a stable electron configuration, which lowers their energy and increases their stability. This stability is crucial for the formation of chemical compounds.

2. Predictability: The octet rule provides a guideline for predicting the types of chemical bonds that atoms are likely to form. For example, elements that have fewer than eight valence electrons tend to lose electrons to achieve a stable configuration, forming positive ions. Conversely, elements that have more than eight valence electrons tend to gain electrons, forming negative ions.

3. Formation of Ionic and Covalent Bonds: The octet rule plays a key role in the formation of both ionic and covalent bonds. In ionic bonding, atoms transfer electrons to achieve a complete octet, resulting in the formation of positively and negatively charged ions. In covalent bonding, atoms share electrons to complete their octets, forming a stable molecule.

Limitations of Octet Rule:
1. Incomplete Octets: Not all elements are able to achieve a complete octet. For example, elements from the boron group (Group 13) tend to form compounds with only six valence electrons. Similarly, elements from the hydrogen and helium groups (Group 1 and 2) have incomplete octets as they only have one or two valence electrons, respectively.

2. Expanded Octets: Some elements, particularly those in the third period and beyond, are capable of accommodating more than eight valence electrons in their outer shell. This is known as an expanded octet. For example, elements like sulfur and phosphorus can form compounds where they have ten or twelve valence electrons.

3. Transition Metals: Transition metals and elements in the d-block often do not follow the octet rule due to the presence of additional d orbitals. These elements can form compounds with varying numbers of valence electrons, leading to a more complex electron configuration.

4. Electron Deficient Molecules: Certain compounds, such as boron hydrides (e.g., BH3), do not have enough electrons to satisfy the octet rule. These molecules are considered electron deficient and often form coordinate covalent bonds to compensate for the electron deficiency.

Overall, while the octet rule provides a useful framework for understanding chemical bonding and predicting molecular structures, it is not applicable to all elements and compounds. Exceptions to the octet rule are commonly observed, particularly for elements that do not have access to a complete octet or have the ability to accommodate more than eight valence electrons.