Which of the following are thermions?

Options

A. Protons
B. Electrons
C. photons and neutral atoms/molecules
D. Positrons

Answer

To determine which of the following are thermions, we need to understand the concept of thermionic emission. Thermions are particles that can be emitted from a heated surface due to thermal energy. Let's analyze the options given in the question:

1. **Proton**
2. **Electron**
3. **Proton and neutral atom or molecule**
4. **Positron**

### Step-by-Step Solution:

**Step 1: Define Thermionic Emission**
- Thermionic emission refers to the phenomenon where charged particles (typically electrons) are emitted from a material when it is heated. The thermal energy provided to the material allows some electrons to overcome the attractive forces binding them to the atoms.

**Step 2: Analyze Each Option**
- **Proton**: Protons are located in the nucleus of an atom and are held there by strong nuclear forces. These forces are much stronger than the forces that hold electrons in their orbits. Therefore, protons cannot be emitted through thermionic emission.

- **Electron**: Electrons are the primary particles that can be emitted from a heated surface. When the surface is heated, some electrons gain enough energy to overcome the attractive forces from the nucleus and escape into the surrounding space. Thus, electrons are considered thermions.

- **Proton and neutral atom or molecule**: As previously stated, protons cannot be emitted due to strong nuclear forces. Neutral atoms or molecules do not typically emit particles through thermionic emission, as this process specifically involves charged particles.

- **Positron**: Positrons are the antiparticles of electrons. While they can be emitted in certain processes (like beta decay), they are not typically emitted through thermionic emission from a heated surface. Therefore, positrons are not considered thermions in this context.

**Step 3: Conclusion**
Based on the analysis, the only particle that qualifies as a thermion in this context is the **electron**.

### Final Answer:
The correct option is **Electron**.

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