Alkali metals used in photoelectric cells 

Options

A. Na
B. Mg
C. Al
D. Cs

Answer

To determine which alkali metal is used in photoelectric cells, we need to consider the properties of alkali metals and how they relate to the photoelectric effect.

### Step-by-Step Solution:

1. **Understanding the Photoelectric Effect**:
- The photoelectric effect occurs when light falls on a metal surface, causing the emission of electrons. The ability of a metal to emit electrons depends on how easily it can lose its outermost electron.

2. **Properties of Alkali Metals**:
- Alkali metals are located in Group 1 of the periodic table and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).
- These metals have one electron in their outermost shell, which they can lose relatively easily.

3. **Trends in the Periodic Table**:
- As we move down the group from lithium to francium, the atomic size increases. This increase in size means that the outer electron is further from the nucleus and experiences less nuclear attraction, making it easier to lose.
- Therefore, the reactivity of alkali metals increases down the group.

4. **Choosing the Best Alkali Metal**:
- Although francium is the most reactive, it is radioactive and not practical for use in photoelectric cells.
- Among the stable alkali metals, cesium (Cs) is the most effective for the photoelectric effect because it has a larger atomic size compared to the others, which means it requires less energy to emit the outer electron.

5. **Conclusion**:
- The alkali metal that is most commonly used in photoelectric cells is **cesium (Cs)**.