Among the following , number of metals which can be used as electrodes in the photoelectric cell is _____(Integer answer ) (A) Li (B) Na (C) Rb (D) Cs

Answer

To determine the number of metals among the given options that can be used as electrodes in a photoelectric cell, we need to analyze each of the alkali metals provided: Lithium (Li), Sodium (Na), Rubidium (Rb), and Cesium (Cs).

### Step-by-Step Solution:

1. **Understanding Photoelectric Cells:**
- Photoelectric cells utilize materials that can emit electrons when exposed to light. These materials are referred to as photoelectrodes.

2. **Properties of Alkali Metals:**
- Alkali metals are known for their low ionization energies, which decrease as you move down the group in the periodic table. This means that the ability to lose an outer electron increases as you go from Lithium to Cesium.

3. **Evaluating Each Metal:**
- **Lithium (Li):** Has a relatively high ionization energy compared to the other alkali metals. It is less likely to emit electrons efficiently in a photoelectric cell.
- **Sodium (Na):** While it has a lower ionization energy than lithium, it is still not as effective as the heavier alkali metals in emitting electrons under light exposure.
- **Rubidium (Rb):** Has a lower ionization energy than both lithium and sodium, making it more suitable for use in photoelectric applications.
- **Cesium (Cs):** Has the lowest ionization energy among the alkali metals listed, making it highly effective as a photoelectrode.

4. **Conclusion:**
- The metals that can be used as electrodes in a photoelectric cell from the options provided are Rubidium (Rb) and Cesium (Cs).
- Therefore, the total number of metals that can be used as electrodes in the photoelectric cell is **2**.

### Final Answer:
The number of metals which can be used as electrodes in the photoelectric cell is **2**.

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