The correct decreasing order of `1^st` ionization energy of 13-group elements is

Options

A. `BgtAlgtGagtIngtTl`
B. `BgtTlgtGagtAlgtIn`
C. `BgtGagtAlgtTlgtIn`
D. `AlgtBgtGagtIngtTl`

Answer

To determine the correct decreasing order of the first ionization energy of the 13-group elements (also known as Group III elements), we need to analyze the elements in this group: Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

### Step-by-Step Solution:

1. **Identify the Elements in Group 13**:
The elements in Group 13 are Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

2. **Understand the Trend in Ionization Energy**:
Generally, ionization energy decreases as we move down a group in the periodic table due to the increase in atomic size and the shielding effect. However, there are exceptions based on electronic configurations.

3. **Consider the Electronic Configuration**:
- Boron (B): [He] 2s² 2p¹
- Aluminum (Al): [Ne] 3s² 3p¹
- Gallium (Ga): [Ar] 3d¹⁰ 4s² 4p¹
- Indium (In): [Kr] 4d¹⁰ 5s² 5p¹
- Thallium (Tl): [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

4. **Analyze Gallium's Ionization Energy**:
Gallium has a filled 3d subshell (10 d electrons) which affects its ionization energy. The presence of these d electrons leads to a greater effective nuclear charge felt by the outer electrons, making it harder to remove an electron compared to Indium and Aluminum.

5. **Establish the Order**:
Based on the above analysis, we can establish the following order:
- Boron has the highest ionization energy due to its small size and lack of shielding.
- Thallium has a relatively high ionization energy due to the effective nuclear charge despite being lower in the group.
- Gallium has a higher ionization energy than Aluminum and Indium due to the presence of d electrons.
- Indium has the lowest ionization energy in this group.

6. **Final Order**:
The correct decreasing order of first ionization energy for the 13-group elements is:
**Boron (B) > Thallium (Tl) > Gallium (Ga) > Aluminum (Al) > Indium (In)**.