Duplet Rule is similar to octet rule in its sense that it describes stability of the atom of an element using the number of electrons present in its valence shell.
According to this rule, an atom is stable if it has two electrons in its valence shell and that atoms participate in chemical bonding in order to achieve stability with two electrons in their valence shells.
This law was mainly proposed for explaining the stability and activity of elements, which have K-shell as their valence or penultimate shell.
Duplet rule was proposed in order to explain the stability of elements that could not be explained by the octet rule.
Actually, there were two rules proposed to cover for the elements which do not come under octet. These rules were the 18 electron rule for transition elements and the duplet rule for elements Hydrogen, Helium and Lithium.
When scientists proposed the octet rule they soon realised that the cause of Hydrogen participating in chemical reactions and forming chemical bonds with other elements was not explained by the octet rule.
Mainly due to this fact they proposed this rule, which not only explained the above phenomenon of Hydrogen, but also explained the stability of Helium and the existence of Li⁺¹.
Duplet Rule Explained
Duplet rule states that an element is stable if its atom has 2 electrons in its valence shell and to attain this state, elements lose, gain or share electrons and form chemical bonds. This rule is also called the duet rule.
The only elements known to follow the this rule are Hydrogen, Helium and Lithium. Where hydrogen gains and shares electrons as it has only one electron less than the duplet, and lithium loses an electron to attain a duplet.
There are a lot of limitations to the duplet rule. For example, there are many elements which have more than two electrons and are stable like transition elements. Also, there are a lot of elements with two electrons in their valence shell and are yet unstable.
Applications of Duplet Rule
- The stability of Helium is explained using this rule.
- The rule explains the participation of hydrogen in chemical reactions.
- It explains the stability of hydrogen molecules.
- It explains the stability of Li+1 and Be+2.
Let us consider the example of a hydrogen molecule to understand how elements follow the duet rule while bonding.
The electronic configuration of hydrogen(H): 1s1
In order to follow this rule, it has to gain one electron. But the ionisation enthalpy of a hydrogen atom is very high, due to this it cannot lose an electron. Though it is possible if the electronegativity of the other element in the reaction is very high, like F.
Hydrogen also cannot gain an electron because it does not have enough electronegativity.
So, the hydrogen atoms which bond share one electron each so that they both have two electrons. Thereby forming a covalent bond and hence following the rule. This bond is quite strong itself, 435.88 KJ/mol, which is greater than many other single bonds. From this we can say that elements with valence shell K follow the duplet rule for chemical bonding.
An element is stable when its atom has two electrons in its valence shell and the element forms chemical bonds according to this rule where it gets two electrons in its valence shell after bonding.
Limitations of the duplet rule are:
1. It cannot explain the stability of elements with more than two electrons in their valence shell.
2. There are elements with two electrons in their valence shell but are unstable.
The main differences between octet and duplet rule:
1. The number of electrons required by the valence shell of an atom to be stable in octet rule is 8 whereas in duplet rule it is 2.
2. The octet rule is applicable to p- and most s-block elements and duplet rule is applicable to hydrogen, helium lithium and beryllium only.
A duplet has 2 electrons in its valence shell and has the electronic configuration ns².
Lithium and beryllium are exception to which we apply the duplet rule instead of octet rule in order to check their chemical reactivity.
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