The electropositive first increases from B to Al and then decreases from Al to Tl down the group because of

Options

A. non-metallic nature of B
B. discrepancies in atomic size of elements
C. ability of B and Al to form `p pi - p pi` multiple bonds
D. irregular trend in electronegativity throughout the periodic table.

Explanation

To solve the question regarding the trend of electropositivity in the boron family from Boron (B) to Thallium (Tl), we will analyze the factors affecting electropositivity step by step.

### Step-by-Step Solution:

1. **Understanding Electropositivity**:
- Electropositivity refers to the ability of an element to lose electrons and form positive ions. It is generally associated with metallic character; elements that can easily lose electrons are considered more electropositive.

2. **Trends in the Boron Family**:
- The boron family consists of the elements: Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl). The trend in electropositivity is observed as follows:
- Electropositivity increases from B to Al.
- Electropositivity decreases from Al to Tl.

3. **Increase from B to Al**:
- **Atomic Size**: As we move down from Boron to Aluminum, the atomic size increases. Larger atomic size means that the outermost electrons are farther from the nucleus, which reduces the effective nuclear charge felt by these electrons.
- **Ionization Energy**: The ionization energy decreases down the group, making it easier for Al to lose its outermost electron compared to B. This results in increased electropositivity.

4. **Decrease from Al to Tl**:
- **Atomic Size and Shielding Effect**: Although the atomic size continues to increase down the group, the presence of d and f orbitals in Ga, In, and Tl introduces poor shielding. This leads to a stronger effective nuclear charge on the outermost electrons.
- **Irregular Trends**: The presence of d and f electrons causes irregularities in the expected trends of ionization energy and electronegativity, which in turn affects the electropositivity. Thallium, being heavier, has a more complex electron configuration that results in higher ionization energy compared to Aluminum.

5. **Conclusion**:
- The overall trend of electropositivity in the boron family is influenced primarily by atomic size and the effective nuclear charge due to poor shielding from inner d and f electrons. Therefore, the correct explanation for the trend in electropositivity is based on the description of atomic size and its effects.

### Final Answer:
The electropositive nature first increases from B to Al due to the increase in atomic size and decrease in ionization energy, and then decreases from Al to Tl due to poor shielding and increased effective nuclear charge.