Lewis Concept of Acids and Bases- Definition, Types, Practice Problems and FAQs

You might have identified substances as acid or a base based on hydrogen or hydroxide ions. But, have you heard of It does not have hydrogen or hydroxide ions. If so, is it an acid or a base?

If we consider , it can not release or OH- ions. Hence, according to Arrhenius and Bronsted Lowry concept, it can neither an acid nor a base. There are many molecules like this.

Let me tell you that behaves like an acid. So, this confusion was resolved by Lewis. He considered acid as a person in need of money and wants to get it at any cost and base as a person who does not bother about sharing the money.

In comparison, Lewis acids are those which want to accept electrons as much as they can to complete their octet and lewis bases are those which want to share their lone pair of electrons.

You will see that there are molecules that need electrons to complete their shell with eight electrons and they will behave like an acid. is one such molecule, that accepts electrons and can be classified as an acid according to Lewis's proposal. Let's discuss this more in detail.

Table of contents

Limitation of Brönsted-Lowry Concepts of Acid and Base
Lewis Concept
Types of Lewis acid
Types of Lewis base
Practice Problems
Frequently Asked Questions – FAQs

Limitation of Brönsted-Lowry concept

1. , like substance can’t be explained as acids by using this concept.
2. It is not able to explain the reaction between acidic and basic oxides.

Lewis concept (electronic concept)

G.N. Lewis, in 1923, states that a species which accepts electron pair is an acid and a species which donates electron pair is a base.

Accordingly any substance not having Hydrogen or releasing proton but accepting an electron pair is called as a Lewis acid.

Similarly, any substance not having Hydroxyl group or releasing hydroxide ion but sharing an electron pair is called as a Lewis base.

Lewis Acid : , , etc.

Lewis Base: , , etc.

This concept is also known as the electronic concept according to which a Lewis base donates an electron pair to be accepted by a Lewis acid.

acts as a Lewis acid as it has vacant p orbital. acts as a Lewis base, as it has a lone pair of electrons that makes them capable of donating a pair of electrons.

Hence, an adduct (→ ) is formed that has a coordinate covalent bond. It is also known as Dative bond.

Other examples

Types of Lewis acids

Molecules with incomplete octets

Examples: , , , etc.

Positively charged species

Example: ,

Although cations such as , , , and , the ability to accept electrons is low

because they possess an inert gas or noble gas configuration.

Molecules with multiple bonds

Example: Molecules having multiple bond(s) between the atoms of dissimilar electronegativity,

i.e., , , , etc.

Molecules in which the central atom can add additional pairs of electrons

Usually, atoms of the elements in the third period and below. Example:

Empty d-orbitals on the central atom

Molecules in which the central atom has empty d-orbitals can accept electron pairs

Example: Halides such as , , , , etc. (Where, X = F, Cl, Br, I)

Types of Lewis base

With a lone pair

Example: , , etc.

With a negative charge

Example: , , etc.

With π electrons

Example:

Practice Problems

Q 1. Which of the following is a Lewis Acid?

a.
b.
c.
d.

Answer: (D)

Molecules in which the central atom has empty d-orbitals can accept electron pairs and act as Lewis acids. Example: Halides such as , , , , etc. (Where, X = F, Cl, Br, I)

Q 2. Which of the following is not a Lewis Acid?

a.
b.
c.
d. All are lewis acids

Answer: (A)

In , because of the complete octet of the oxygen atom, despite having a positive charge, is not able to accept electrons. Hence, it is not lewis acid.

Q 3. Which group from P-Block is likely to form Lewis Acid?

Answer: Group 13 is more likely to form lewis acid because there exist electron deficient compounds like , , etc. which are in eager wait to accept electrons.

Q 4. Which of the following are Lewis acids?

, , , and

Answer: Out of , , , and , behaves as lewis base because it contains two lone pairs. Because of the complete octet of N in , it will also not behave as lewis acid. Hence, the remaining , are lewis acids.

Frequently Asked Questions – FAQs

Q 1. Classify the following species into Lewis acids and Lewis bases and show how these act as Lewis acid/base: (A) (B) (C)

Answer: Any substance accepting an electron pair is called a Lewis acid. Similarly, any substance sharing an electron pair is called a Lewis base.

a. ions can donate an electron pair. Hence, it acts as Lewis base.

b. ions can donate an electron pair. Hence, it acts as Lewis base.

c. ions can accept an electron pair. Hence, it acts as Lewis acid.

Q 2. Explain the concept of a Coordinate Covalent bond?

Answer: When a Lewis base donates an electron pair to be accepted by a Lewis acid. The two get linked to each other by a bond known as coordinate bond

Q 3 . Although has 3 H present in it, it is known as monobasic Lewis acid instead of tribasic Arrhenius acid. Explain.

Answer: When is dissolved in water, the ion will be taken by the empty p-orbital of boron and will form a bond, and the ion will remain in the solution. Therefore, in water, accepts ions acting as Lewis acid.

So, it is monobasic Lewis acid instead of tribasic Arrhenius acid.

Q 4. is not electron deficient and has a complete octet. Comment how it can be a lewis acid?

Answer: Consider as a person who has a nice home but he is not satisfied. He wants to purchase a luxury home with a swimming pool. is having complete octet and no electron deficiency but it is not satisfied. Due to the presence of a vacant d orbital, it still tries to accept the electrons.

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