To solve the question regarding the set of elements that have one electron in their valence shell, we can follow these steps:
### Step-by-Step Solution:
1. **Understanding Valence Electrons**:
- Valence electrons are the electrons in the outermost shell of an atom. The number of valence electrons determines the chemical properties of the element.
2. **Identifying Groups**:
- Elements are organized in groups in the periodic table. Group 1 elements, known as alkali metals, have one electron in their outermost shell.
3. **Analyzing the Given Sets**:
- **Set 1**: Chlorine (Cl), Bromine (Br), Iodine (I) - These are halogens (Group 17) with the configuration ns²np⁵. They have 7 valence electrons, so this set is incorrect.
- **Set 2**: Sodium (Na), Magnesium (Mg), Aluminium (Al) - Sodium is in Group 1 (1 valence electron), Magnesium is in Group 2 (2 valence electrons), and Aluminium is in Group 13 (3 valence electrons). This set is incorrect as they do not all have 1 valence electron.
- **Set 3**: Boron (B), Aluminium (Al), Gallium (Ga) - Boron has 3 valence electrons, Aluminium has 3 valence electrons, and Gallium has 3 valence electrons. This set is also incorrect.
- **Set 4**: Potassium (K), Rubidium (Rb), Cesium (Cs) - All these elements belong to Group 1 (alkali metals) and have the outer electronic configuration ns¹, meaning they each have 1 valence electron.
4. **Conclusion**:
- The correct set of elements that have one electron in their valence shell is **Potassium (K), Rubidium (Rb), and Cesium (Cs)**.
### Final Answer:
The set of elements having one electron in their valence shell is **Potassium (K), Rubidium (Rb), and Cesium (Cs)**.
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