Which of the following elements has highest ionisation energy?

Options

A. Li
B. Na
C. K
D. Cs

Explanation

To determine which of the given elements has the highest ionization energy, we will analyze the properties of each element in the context of their atomic structure.

### Step-by-Step Solution:

1. **Understand Ionization Energy**:
- Ionization energy is defined as the amount of energy required to remove the most loosely bound electron (valence electron) from an atom in its gaseous state.

2. **Identify the Elements**:
- The elements provided in the question are lithium (Li), sodium (Na), potassium (K), and cesium (Cs).

3. **Determine the Period and Group of Each Element**:
- Lithium (Li) is in Period 2, Group 1.
- Sodium (Na) is in Period 3, Group 1.
- Potassium (K) is in Period 4, Group 1.
- Cesium (Cs) is in Period 6, Group 1.

4. **Analyze the Position in the Periodic Table**:
- As we move down a group in the periodic table, the atomic size increases because additional electron shells are added. This means that the valence electrons are further away from the nucleus.

5. **Evaluate the Effect of Distance on Ionization Energy**:
- The further the valence electron is from the nucleus, the weaker the attraction between the nucleus and the valence electron. Therefore, it requires less energy to remove the valence electron.
- For lithium, the valence electron is in the second shell, which is closer to the nucleus compared to sodium (third shell), potassium (fourth shell), and cesium (sixth shell).

6. **Conclusion**:
- Since lithium has its valence electron in the closest shell to the nucleus, it has the highest ionization energy among the given options. The correct answer is lithium (Li).

### Final Answer:
The element with the highest ionization energy among lithium, sodium, potassium, and cesium is **lithium (Li)**.