Which element of group `13` forms the most stable `+1` oxidation state.

Answer

To determine which element of group 13 forms the most stable +1 oxidation state, we can follow these steps:

### Step 1: Identify the Elements of Group 13
Group 13 of the periodic table consists of the following elements:
- Boron (B)
- Aluminium (Al)
- Gallium (Ga)
- Indium (In)
- Thallium (Tl)

### Step 2: Understand Oxidation States
Elements in group 13 can exhibit multiple oxidation states, primarily +3 and +1. The +3 oxidation state is common due to the loss of three valence electrons, while the +1 oxidation state results from the loss of only one valence electron.

### Step 3: Consider the Inert Pair Effect
As we move down the group from Boron to Thallium, the inert pair effect becomes more pronounced. The inert pair effect refers to the tendency of the s-electrons (from the outermost shell) to remain non-bonding or "inert" due to increased nuclear charge and poor shielding by d and f electrons.

### Step 4: Analyze the Stability of +1 Oxidation State
- **Boron (B)**: Typically exhibits +3 oxidation state.
- **Aluminium (Al)**: Can show +1 but is more stable in +3.
- **Gallium (Ga)**: Similar to Al, prefers +3.
- **Indium (In)**: Shows +1 but less stable than +3.
- **Thallium (Tl)**: The +1 oxidation state becomes more stable due to the inert pair effect, as the s-electrons are less likely to participate in bonding.

### Step 5: Conclusion
Among the group 13 elements, Thallium (Tl) forms the most stable +1 oxidation state due to the significant inert pair effect, which stabilizes the +1 state as we move down the group.

### Final Answer
The element of group 13 that forms the most stable +1 oxidation state is **Thallium (Tl)**.

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