How does the ionisation energy of 1st group elements very ?

Options

A. increases down the group
B. decrease down the groups
C. remains unchanged
D. variation is not regular

Explanation

To understand how the ionization energy of the first group elements varies, we can follow these steps:

### Step-by-Step Solution:

1. **Define Ionization Energy**:
- Ionization energy is the energy required to remove the outermost electron from a gaseous atom.

2. **Identify the First Group Elements**:
- The first group elements, also known as alkali metals, include hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).

3. **Trend in Ionization Energy**:
- As we move down the group from hydrogen to cesium, the ionization energy decreases.

4. **Explain the Reason for the Trend**:
- The electron configuration of these elements shows that as we go down the group, the outermost electron is in a higher principal energy level (1s1 for H, 2s1 for Li, 3s1 for Na, etc.).
- With each step down the group, the outermost electron is further away from the nucleus, which results in a weaker attraction between the nucleus and the outermost electron.

5. **Conclusion**:
- Because the outermost electron is less tightly held due to the increased distance from the nucleus and the shielding effect of inner electrons, it becomes easier to remove this electron. Therefore, the ionization energy decreases as we move down the group.

### Final Answer:
The ionization energy of the first group elements decreases as we move from top to bottom in the group.

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