Explanation:

In the case of metals, the valence and conduction bands have an overlap, and the energy gap between them is zero.

1. Valence Band:
- The valence band is the highest energy band that is fully occupied by electrons in a material at absolute zero temperature.
- It consists of the valence electrons, which are tightly bound to the atomic nuclei.
- These electrons are not free to move and contribute to the electrical conductivity of the material.

2. Conduction Band:
- The conduction band is the energy band located just above the valence band.
- It contains empty or partially filled energy states that are available for electrons to move freely and contribute to the electrical conductivity of the material.
- Electrons in the conduction band have higher energy and are not bound to any specific atom.

3. Energy Gap:
- The energy gap is the energy difference between the valence and conduction bands.
- In insulators, this gap is large, typically on the order of several electron volts, which makes it difficult for electrons to move from the valence band to the conduction band.
- In semiconductors, the energy gap is smaller than in insulators, allowing some electrons to acquire enough energy to move from the valence band to the conduction band, contributing to electrical conductivity.
- In metals, the energy gap between the valence and conduction bands is zero, meaning that there is no energy barrier for electrons to move from the valence band to the conduction band.
- This overlap allows electrons to move freely throughout the material, resulting in high electrical conductivity.

Conclusion:
In summary, in the case of metals, the valence and conduction bands have an overlap, and the energy gap between them is zero. This overlap allows electrons to move freely, contributing to the high electrical conductivity observed in metals.