Which of the following group `13` elements forms univalent halides which are more stable than the trihalides?

Options

A. `Al`
B. `Ga`
C. `Tl`
D. `In`

Explanation

To determine which of the group 13 elements forms univalent halides that are more stable than their trihalides, we can follow these steps:

### Step 1: Identify Group 13 Elements
Group 13 elements include Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

### Step 2: Understand Oxidation States
Group 13 elements can exhibit multiple oxidation states. The common oxidation states are +1 (univalent) and +3 (trivalent).

### Step 3: Analyze Stability of Halides
- **Univalent Halides (MX)**: These are formed when the element exhibits a +1 oxidation state.
- **Trihalides (MX3)**: These are formed when the element exhibits a +3 oxidation state.

### Step 4: Consider the Inert Pair Effect
The inert pair effect refers to the tendency of the s-electrons (the 'inert pair') to remain non-bonding in heavier elements of a group. As we move down the group, the +1 oxidation state becomes more stable compared to the +3 oxidation state due to poor shielding of the nucleus by d and f electrons.

### Step 5: Evaluate Each Element
- **Boron (B)**: Forms stable trihalides (BCl3, BBr3) but not stable univalent halides.
- **Aluminium (Al)**: Similar to Boron, AlCl3 is stable, while AlCl is less stable.
- **Gallium (Ga)**: GaCl3 is stable, GaCl is less stable.
- **Indium (In)**: InCl3 is stable, InCl is less stable.
- **Thallium (Tl)**: TlCl (univalent halide) is more stable than TlCl3 (trihalide) due to the inert pair effect.

### Conclusion
Among the group 13 elements, Thallium (Tl) forms univalent halides (TlCl) that are more stable than its trihalides (TlCl3).

### Final Answer
**Thallium (Tl)** forms univalent halides that are more stable than the trihalides.

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