Octet Rule

A Brief on What is the Octet Rule 

As per the chemical rule of thumb which is considered to be the octet rule, it is asserted that an electron should have eight electrons in its outermost shell. Moreover, it is seen that this rule is followed by most of the elements from s-block and p-block apart from a few elements from the p block which are known as hydrogen, lithium, and helium. For example, Carbon Dioxide is the compound which follows the 'Octet Rule'. As per the chemical rule of thumb which is the octet rule it is asserted that an electron should have eight electrons in its outermost shell.

Octet Rule in Chemistry 

The octet rule states that an atom tends to have eight electrons in its outermost valence shell by forming covalent bonds through gaining or losing electrons from its outermost shell. Elements that obey octet rules are main group elements which are oxygen, carbon, nitrogen. s-block and p-block elements obey octet rule except for hydrogen, helium, and lithium.

Octet Rule Examples

Few examples which follow octet rule are :

CO2, NaCl, MgO

1.CO2

Carbon contains four electrons in its outermost shell. Also, carbon should have four electrons to complete its octet when it is combining with two molecules of oxygen. Here each carbon atom requires two electrons to complete its octet. Carbon and oxygen share their outermost electron and form CO2 which further completes the octet.

This is shown with the help of Lewis dot structure:-

o    ::O: :C: :O::

2. NaCl

Chlorine contains seven electrons in its outermost shell and requires only one electron to complete its octet whereas sodium contains one electron in its outermost shell. Both sodium and chlorine share their electron and complete their octet by forming  Sodium Chloride (NaCl). 

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

                            Na. .::: Cl

Exceptions to the Octet Rule

Many elements do not follow the octet rule. Some of the exceptions about octet rule are given below:

  1. An electron or molecule which contains unpaired electrons in its outermost shell or valence shell is considered as free radical. These electrons are less stable and do not obey the octet rule.

  2. Elements like hydrogen, lithium, helium do not obey the octet rule. They can only lose or gain one electron in order to become stable due to which they follow the octet rule. 

  3. Another exception of octet rule is transition elements. Due to the presence of d-orbitals, they can hold 18 electrons in its outermost shell. 

Stability of an Atom

If the force between electron and proton is balanced, then an atom is said to be stable whereas if the force is unstable then it is said to be unstable. An electron that contains a complete outermost shell or half-filled is said to be more stable whereas electrons less than half-filled are unstable and can easily lose their electrons.

Noble gases are said to be highly stable elements. Those elements which follow octet rule are said to be more stable. 

Octet Rule and Valence Electron

The octet rule states that the elements which can lose, gain, or share electrons from its outermost shell to complete the valence shell with a set of eight electrons. Valence electrons mean the total number of electrons present in the outermost shell of an element that can participate in the bond formation. Such electrons are considered as the valence electron of that particular element.

For Example:

Let us take an example of NaCl:

Sodium has one electron in its outermost shell. So we can say that the valence electron of Na is 1. Whereas Cl has seven electrons in its outermost shell. 

So we can say the Valence electron for Cl is 7. When both sodium and chlorine combine and share their electron and have eight electrons in their outermost shell then it is said that the compound formed follows the octet rule.

FAQ (Frequently Asked Questions)

1. What is the Difference Between the Octet of an Electron and a Valence Electron?

An octet of an electron means the presence of eight electrons in its outermost shell. This is the ability of an electron to gain, lose, or share their electrons with other elements to complete its octet. 

The valence of an electron means the total number of electrons present in the outermost shell of an electron which can be shared with the other elements to form a chemical bond.

For example:

Valence electrons in Na are 1

Valence electrons in Cl are 7

Valence electrons in H are 1

2. Why do Elements Complete their Octet?

The octet rule states that when an element loses, gains, or shares their outermost electrons to complete their octet state with a set of eight electrons then it Is said that they are following the octet rule. Elements follow the octet rule to become more stable as complete filled outermost shells have a strong and balanced force between protons and the electrons. 

For example NaCl

Chlorine contains seven electrons in its outermost shell and requires only one electron to complete its octet whereas sodium contains one electron in its outermost shell. Both sodium and chlorine share their electron and complete their octet by forming  Sodium Chloride (NaCl) as shown below with the help of Lewis dot structure:   

Na. .::: Cl