🧪 What Are Valence Band and Conduction Band? | Class 12 Chemistry Notes
Understanding how electrons behave inside solids is a key concept in Solid State Physics and Chemistry. Two important terms that describe this behavior are the Valence Band and the Conduction Band. Let’s explore their meanings, differences, and importance in materials like conductors, semiconductors, and insulators.
🌟 Introduction
In solids, especially metals and semiconductors, atoms are arranged very closely together. The outer electrons of these atoms interact with each other, forming different energy levels. These levels are grouped into bands — mainly:
- Valence Band
- Conduction Band
The energy gap between these two bands determines whether a material will conduct electricity or not.
⚛️ What Is the Valence Band?
The Valence Band is the highest range of electron energies in a solid where electrons are normally present at absolute zero temperature.
📘 Key Points:
- Formed by valence (outermost) electrons of atoms.
- Electrons are tightly bound and cannot move freely.
- Energy is lower than conduction band electrons.
- With enough energy, electrons can jump to the conduction band.
⚡ What Is the Conduction Band?
The Conduction Band lies above the valence band and contains free electrons responsible for electrical conduction.
📘 Key Points:
- Electrons in this band move freely within the solid.
- Empty at 0 K in insulators and semiconductors.
- When electrons gain enough energy, they cross the energy gap.
🧭 The Energy Band Gap (Eg)
The Energy Band Gap (Eg) is the energy difference between the bottom of the conduction band and the top of the valence band.
Eg = Ec − Ev
(where Ec = Conduction Band Energy and Ev = Valence Band Energy)
⚙️ Types of Solids Based on Band Theory
| Type of Solid | Band Gap (Eg) | Electrical Property | Example |
|---|---|---|---|
| Conductor | Very small or overlapping | High conductivity | Copper, Silver |
| Semiconductor | Small (~1 eV) | Moderate conductivity | Silicon, Germanium |
| Insulator | Large (> 3 eV) | Poor conductivity | Diamond, Glass |
💡 How Electrons Move Between Bands
When energy is supplied (like heat, light, or electric field):
- Electrons in the valence band absorb energy.
- They jump across the band gap to the conduction band.
- These free electrons allow current to flow.
🧩 Difference Between Valence Band and Conduction Band
| Basis | Valence Band | Conduction Band |
|---|---|---|
| Definition | Highest energy range filled with electrons | Band above valence band with free electrons |
| Electron Mobility | Tightly bound | Free to move |
| Energy Level | Lower energy | Higher energy |
| Electric Conduction | No conduction | Responsible for conduction |
🧠 Real-Life Importance
- Semiconductor Devices: Transistors, diodes, and LEDs work on electron movement between bands.
- Solar Cells: Electrons jump from valence to conduction band when exposed to sunlight.
- Conductors vs Insulators: Explains why metals conduct and rubber does not.
✨ Conclusion
In simple terms, the Valence Band holds bound electrons, while the Conduction Band allows electrons to move freely. The Energy Gap between them decides whether a material is a conductor, semiconductor, or insulator. Understanding these bands helps us explain the electrical behavior of all materials — from metals to modern semiconductor chips.
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