The ionisation energy of alkali metals decreases from Li to Cs bacause

Options

A. the atomic size increases from Li to Cs
B. The sitance betweenn nucleus and outermost orbital decreases from Li to Cs
C. electropositive character decreases down the group
D. melting point decreases from Li to Cs.

Answer

To explain why the ionization energy of alkali metals decreases from lithium (Li) to cesium (Cs), we can break down the reasoning into several key points:

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom in the gaseous state. Higher ionization energy means it is harder to remove an electron.

2. **Trends in Alkali Metals**:
- Alkali metals are found in Group 1 of the periodic table. As we move down the group from Li to Cs, we observe a trend in ionization energy.

3. **Atomic Size Increase**:
- As we move down the group from Li to Cs, the atomic size increases. This is due to the addition of electron shells. Each subsequent element has an additional shell of electrons, which increases the distance between the nucleus and the outermost electron.

4. **Shielding Effect**:
- The inner electron shells (the electrons in the filled shells between the nucleus and the outermost electron) shield the outermost electron from the full effect of the nuclear charge. This is known as the shielding effect. As more inner electrons are added, the outermost electron feels less attraction from the nucleus.

5. **Reduced Nuclear Attraction**:
- Due to the increased distance (larger atomic size) and the shielding effect, the force of attraction between the nucleus and the outermost electron decreases. This makes it easier to remove the outermost electron.

6. **Comparison of Nuclear Charge and Shielding**:
- Although the nuclear charge increases as we move down the group (more protons in the nucleus), the effect of increased atomic size and shielding outweighs the increase in nuclear charge. Therefore, the ionization energy decreases.

7. **Conclusion**:
- Overall, the ionization energy of alkali metals decreases from Li to Cs primarily because the increase in atomic size and the shielding effect reduce the effective nuclear attraction on the outermost electron, making it easier to remove.

### Final Answer:
The ionization energy of alkali metals decreases from Li to Cs because the atomic size increases and the outermost electron experiences reduced attraction due to the shielding effect of inner electrons.

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