Question

Assertion: If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound, and no free electrons are available in the conduction band.
Reason: Some of the electrons in the valence band may gain external energy to cross the gap between the conduction band and the valence band.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but the Reason is incorrect.
B. Both Assertion and Reason are incorrect.

Answer 1

The electrons in the outermost shell of an atom are called valence electrons. They are responsible for conduction in materials called conductors. However, the valence electrons acting as conduction electrons depend on the energy gap between two bands known as the valence band and conduction band.

Complete step by step answer:
The electrons in an atom contain energy levels with continuous energies forming an energy band.
The energy band that contains the energy levels of the valence electrons is called the valence band and the energy band that contains the energy levels of the conduction electrons.
Normally, the conduction band will be empty and the valence electrons will be excited to the conduction band helping in conducting electricity.
This can happen when there is a lesser energy gap from the valence and conduction band.

Now, let us study the Assertion and Reasoning statements.
(i) Assertion: If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound, and no free electrons are available in the conduction band.
This statement is correct, because the electrons in the valence band will be filled with valence electrons and since the gap exists, the electrons will be bound in the valence band itself.
(ii) Reason: Some of the electrons in the valence band may gain external energy to cross the gap between the conduction band and the valence band.
This statement is correct since the electrons can be excited through heat from the valence band to conduction band, thereby, helping in conduction.
However, it is not true that the heat energy is compulsorily required to excite the electrons from the valence to the conduction band. In some materials called conductors, there is no heat required at all, to excite the electrons since the electrons in the valence band, themselves, have overlapped with the conduction band.

Thus, both these statements are true however, the reason is not the correct explanation of the assertion statement. Hence, the correct option is Option B.

Note:
Based on the energy gap, the materials are classified into three types:
(i) Conductors
(ii) Insulators and
(iii) Semiconductors.
Conductors are those materials which have zero energy gap or overlapping energy bands
Insulators are those materials that have very high energy between the conduction and valence band thus, making it very difficult for the valence band to jump into the conduction band to conduct.
Semiconductors have a comparably lesser energy gap than the insulators, thus making them possible to conduct using some external influencers like doping or heat.