Lewis Acid Base Theory

You have studied the Lewis dot structure in earlier posts. The same G.N. Lewis who has given the Lewis dot structure has defined acid and base in more generalized term. Every species has electron pairs, so he gave his definition in the terms of electron pairs.

Lewis acids are those which accept electron pairs and Lewis bases are those which donate electron pairs. Let’s check that BF₃ fits in which category?

Lewis Acid and Base

Draw the Lewis dot structure of BF₃molecule. You will find that B has 6 electrons it has incomplete octet. That means it is ready to accept an electron pair to fulfill its octet. Thus it falls under the category of Lewis acids. It means that the species which have incomplete octet (electron deficit) can act as Lewis acid like AlCl₃, BCl₃, Mg²⁺ and so on.

Lewis Vs Bronsted theory

Lewis theory Vs Bronsted theory

Let’s check whether the Brönsted base NH₃ is also a Lewis base or not? When you draw the Lewis dot structure of NH₃ you will find that is has 3 bonded pairs and 1 lone pair of electrons. It can donate one lone pair of electron and acts as Lewis base. Species those are electron rich can act as Lewis base like H₂O, OH^-, Cl^-, O^2- and so on.

If you observe closely, the Brönsted concept and Lewis theory apparently do not have much difference. Brönsted base accepts H⁺(proton) and Lewis base donate electron pair which is the same thing, only difference is in the language.

NH_3(aq) ⟶ NH₄⁺_(aq) 

Brönsted base NH_3(aq) accepts H⁺and becomes  NH₄⁺_(aq). When you draw the structure you will find that NH₃ provides electrons to make bond with H⁺. It means that in either case (Brönsted and Lewis) base provides electrons. Similarly acid accepts electrons in both cases.

How do we know which acid or base is stronger? Is there any method to measure the strength?  In the next post we will explore it.

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