Introduction of Octet Rule
Abegg's rule was formulated by Richard Abegg in 1904. The rule states that the difference between the maximum negative and positive valence of an element is 8. This rule was later used for formulating the octet rule by Gilbert.N.Lewis in 1916 in his cubic atom theory. Thus, the atoms of different elements react with each other to get the most stable state. When all orbitals of an element are full then it acquires a stable octet or complete octet. The lesser the atom has stability. Thus a reaction occurs to do so and during that reaction as the stability of the atom increases it will release energy in the form of heat or light.
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 a compound that follows binding information 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 the main group elements which are oxygen, carbon, nitrogen. s-block and p-block elements obey the octet rule except for hydrogen, helium, and lithium.
Octet Rule Examples
A few examples which follow the 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 combined 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:-
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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 electrons and complete their octet by forming Sodium Chloride (NaCl).
It is shown below with the help of Lewis dot structure:
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Lewis Dot Structure:
The way the electrons are coupled is reflected in Lewis dot structures. Lewis structures, which may be thought of as "electron bookkeeping" are a handy approach to summarise some information about bonding. Each dot during a Lewis dot structure represents an electron. Thus, a pair of dots represent the bond between the chemical symbols of the atom.
3. MgO
Magnesium reacts with oxygen to form magnesium oxide. Thus during this reaction, the Mg and O are bonded to each other by an ionic bond. The magnesium has two electrons in its outermost orbit i.e., M shell and oxygen needs two electrons to form a stable octet. The Mg loses two electrons and forms a stable octet with 12 protons and 10 electrons in the L shell. Whereas these two electrons lost by Mg are gained by oxygen to complete its stable octet. As an opposite charge is being applied electrostatically between the ions, an ionic bond is formed. Both the atoms Mg and O have a stable octet configuration.
Exceptions to the Octet Rule
Many elements do not follow the octet rule. Some of the exceptions to the octet rule are given below:
An electron or molecule which contains unpaired electrons in its outermost shell or valence shell is considered a free radical. These electrons are less stable and do not obey the octet rule.
Elements like hydrogen, lithium, helium do not obey the octet rule. They can only lose or gain one electron to become stable due to which they follow the octet rule.
Another exception of the octet rule is transition elements. Due to the presence of d-orbitals, they can hold 18 electrons in their 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 gasses are said to be highly stable elements. Those elements which follow the 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 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.
Hypervalent
Hypervalent compounds are formed by some main cluster elements. Sulphur hexafluoride (SF6) and phosphorus pentachloride are 2 examples (PCl5) in a big way. If all of the phosphorus-chlorine particularly links during a PCl5 molecule essentially by valency, then the phosphorus molecule would generally be breaking the octet rule by having a for all intents complete ten valence electrons, which is quite significant. The sp3d interbreeding in PCl5 will make a case for the creation of 5 bonds by phosphorus molecules, which essentially is quite significant. Now we associate the sp3d hybrid, generally created by the hybridization of one orbital 3p, and one d sort of orbital in a subtle way. With the five-element atoms, this produces five covalent connections.
FAQs on Octet Rule
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 its 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 its 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:
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3. In what way is the octet rule useful?
The octet rule helps us predict the chemical behaviour of the elements. It helps us understand that an atom is most stable when it has valence shells filled with 8 electrons. The octet corresponds to an electronic configuration of s2p6 because the octet rule only involves the ‘s’ and ‘p’ electrons. Thus, to obtain a stable octet configuration, the elements tend to form bonds in an order.
4. Does the O2 molecule satisfy the octet configurations?
The octet rule helps us predict the chemical behaviour of the main group elements. It helps us understand that an atom is most stable when it has valence shells filled with 8 electrons. When the atoms have less than 8 electrons they tend to react with other atoms and form more stable compounds. So, we can say that in an O2 molecule, each oxygen atom is surrounded by a total of 8 electrons. But as we know, an oxygen atom has only 6 electrons in its valence shell. Hence, to attain stability the oxygen molecule reacts with another oxygen molecule forming a double bond and sharing in total 4 electrons amongst themselves.
5. Name any four elements that disobey the octet rule.
We all know that 1p shell does not exist hence many atoms attain stability in the 1s2 configuration itself. While on the other hand, some elements can form hypervalent molecules as they exhibit the hypervalent property. Thus, the few elements that don't obey the octet rule are as follows: Hydrogen, Lithium, Phosphorus, Sulphur.