Question

In the case of insulators, a band gap and conduction band is respectively

• A. very high, empty
• B. very low, partially filled
• C. very high, completely filled
• D. very low, empty

Hint:

If a conduction band is even partially filled, the material conducts electricity. An insulator's entire job is to block current, which mathematically guarantees its conduction band must be completely empty due to the massive energy barrier below it.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We must identify the defining band theory characteristics of an electrical insulator regarding its energy band gap and the state of its conduction band.

Step 2: Detailed Explanation:
According to energy band theory in solids:
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Conductors (Metals): Have overlapping valence and conduction bands (no band gap), or a partially filled conduction band, allowing electrons to move freely.
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Semiconductors: Have a small, finite energy band gap (usually $< 3\ \text{eV}$). At absolute zero, the conduction band is empty, but at room temperature, thermal energy can excite some electrons across the gap.
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Insulators: Are characterized by a very large energy band gap (typically $> 3\ \text{eV}$, often $6\ \text{eV}$ or more) between the valence band and the conduction band. Because this gap is so large, normal thermal energy cannot bridge it. Therefore, the valence band remains completely full, and the conduction band remains completely empty.

Step 3: Final Answer:
For insulators, the band gap is very high and the conduction band is empty, matching option (A).