Octet Rule – Definition, Examples, and Limitations

What is the Octet Rule? 

Lewis, Kossel, and Longmuir tried to explain why atoms combine based on the electronic configuration of noble gases. They assumed that the atoms of noble gas do not tend to form compounds with other atoms due to their stable configuration of eight electrons, which they called an octet. This tendency of atoms to complete their octet is called the octet rule.

Hence, they compared that when atoms of other elements combine to form molecules, the electrons in their outermost orbits are arranged so that they achieve an octet of stable electrons. As a result, a chemical bond is established between the atoms. 

History of the Octet Rule

In 1904, Richard Abegg introduced the valence theory that resembles the modern concept of oxidation states. He noted that the difference between positive and negative valences of an element frequently came as eight.

In 1916, based on Abegg’s rule, Lewis composed his “rule of eight” and the cubical atom model. In 1919, Irving Langmuir clarified all these concepts more and renamed them “octet theory” and “cubical octet atom”. The “octet theory” is further known as the “octet rule”.

During their research, Kossel and Lewis noted that noble gases did not appear apt to participate in chemical reactions under normal conditions. Based on this observation, they deduced that noble gas atoms are steady. Taking this conclusion as their base, in 1916, they proposed a theory of valency known as the “electronic theory of valency.”

According to this theory, “During the formation of a chemical bond, atoms combine by losing, gaining, or sharing electrons in this fashion that they acquire the nearest noble gas configuration.”

Octet Rule Definition Chemistry

As per the electronic theory of the formation of chemical bonds, atoms can unite either by passing, i.e., gaining or losing valence shell electrons from one element or atom to another, or by splitting or sharing them to attain a complete octet in their ultimate shells. It is known as the octet rule.

The octet rule is also called the rule of eight, the electronic theory of valence, or the octet theory of valence. The octet theory states:

In the chemical bond formation between atoms, they interact with each other by losing, gaining, or sharing electrons. They do acquire a stable outermost shell of eight electrons.

The main points of this theory are:

• Atoms with eight electrons in the outermost shell are chemically stable. Hence, they are incapable of carrying the chemical combination.
• Atoms with less than eight electrons in their outermost shell are chemically active. Hence, they tend to combine with other atoms.
• Atoms with below four electrons in their ultimate shell tend to lose electrons. In comparison, those having more than four electrons tend to gain electrons during the bond formation to attain the maximum stability and stable configuration of the nearest inert gas.
• Atoms form bonds due to transferring or passing electrons from the ultimate shell of one element or atom to another or by sharing or splitting one, two, or three electron pairs among the valence shells of both atoms.
• The propensity of an atom for transference or sharing its electron pairs is a measure of its chemical activity.

The Octet Rule Examples

Some octet rule examples are given below:

1. CO₂

In its valence shell, the C-atom contains four electrons. Hence, it requires four electrons to complete its octet and acquire the noble gas configuration. As a result, it combined with two oxygen molecules and formed a CO₂ molecule.

Here, each oxygen atom needed two electrons to complete its octet. Therefore, carbon and oxygen share their ultimate electron and form CO₂, which further completes their octet.

It is shown with the help of the Lewis dot structure:

 

2. NaCl

Here, the Cl-atom contains seven electrons in its ultimate shell and needs only one electron to complete its octet. In contrast, sodium, i.e., Na-atom, contains one electron in its valence shell. Both atoms share their electrons present in the outermost shell. As a result, they complete their octet by devising NaCl, i.e., sodium chloride.

The octet theory of valence is shown below with the help of the Lewis dot diagram:

3. MgO

The Mg-atom has two electrons in its valence shell. At the same time, the oxygen atom is short of two electrons for being a stable atom. Therefore, they share their electrons to complete their octet and become a stable compound. As a result, they form MgO, i.e., magnesium oxide.

An ionic bond is formed as an opposite charge is applied electrostatically between the ions of Mg and O. Both the atoms Mg and O have a stable octet configuration. Bond formation between Mg and O is shown below with the help of the Lewis dot structure:

The significance of the octet rule is given below:

• The octet rule clearly explains the establishment of chemical bonds depending on the element’s character.
• It is used to find out the stability of atoms.
• The octet rule aids in finding out how atoms will combine.
• The octet rule is helpful in terms of deriving chemical structures. For example, the octet rule will help you draw the molecular structure of CH₃
• It helps in predicting the stability and reactivity of the chemical structures. For example, the octet rule will help you predict the special reactivity of species that do not satisfy the octet rule, such as BF₃ (Lewis acidic).

Limitations of the Octet Rule

Not all elements or molecules follow the octet theory of valence. There are some limitations to the octet rule that stop all the elements or molecules from following it. These are given below:

• An ion, atom, or molecule that contains an unpaired valence electron called a free radical violates the octet rule. However, free radicals are very unstable and tend to form dimers spontaneously.
• As the first shell, i.e., the innermost shell, can only aid two electrons, elements such as helium (He), lithium (Li), and hydrogen (H) follow the duet rule, i.e., rule of two, instead of the octet rule. For instance, Li can drop an electron to have a secure configuration that results in an ultimate shell of two electrons.
• The transition elements do not follow the octet rule. It is because of the presence of a d-orbital. The outermost shells of these atoms can hold up to 18 electrons.
• Aromatic compounds entail a delocalization of pi electrons. Instead of following the octet rule, these electrons heed Huckel’s rule.

There are some electron-deficient molecules, such as boranes and carboranes. These molecules follow Wade’s rules to attain stability. These molecules feature three-centered or banana bonds in which three atoms split two electrons.

Conclusion

The octet ‘rule’ is handy and is like a ‘rule of thumb’. It is very helpful to quickly add valances using the Lewis dot diagrams and count valency up to 8. Most reactions and compounds you will look at probably contain elements like C, H, O, N, Si, Fe, K, P, Ca, F, Na, etc.

Many stable compounds follow the octet rule and have some underlying quantum mechanical reasons. But there are a lot of chemistry and chemical compounds that cannot be understood with octets. And they are certainly not a rule that all compounds must follow.

Frequently Asked Questions

1. What are the elements that follow the octet rule?

Atoms or elements that follow octet rules are the main group elements which are carbon (C), oxygen (O), and nitrogen (N). The p- and s-block elements heed the octet rule except for helium, hydrogen, and lithium (Li). Metals such as sodium or magnesium also obey the octet rule.

Some other rules, such as the duplet rule or rule of two and 18-electron rules, are followed by other elements. The duplet rule is for helium (He) and hydrogen (H), or the 18-electron rule for d-block or transition metals.

2. Is the Octet rule universal?

No, the octet rule is not applicable to all molecules and compounds. Some compounds are electron deficient like BF₃, XeF₄, etc., and many are electron-rich, i.e., they extend their octet like PCl₅, PF₅, etc. Therefore, it will be true to say that the octet rule is not universal.

3. What are the exceptions to the octet rule?

Lewis dot diagrams provide a simple model for justifying most familiar compounds’ bonding. However, there are three extensive exceptions to the octet rule. These are:

• Molecules, such as NO, etc., with 7, 9, or other odd numbers of electrons.
• Compounds or molecules in which one or other atoms have more than eight electrons, for instance, SF₆, etc.
• In molecules such as BCl₃, etc., one or extra atoms possess less than eight electrons.