Which of the following elements will not show the inert-pair effect ?

Options

A. Sn
B. Fe
C. Pb
D. In

Explanation

To determine which of the following elements will not show the inert-pair effect, we first need to understand what the inert-pair effect is and the conditions under which it occurs.

### Step-by-Step Solution:

1. **Understanding Inert Pair Effect**:
- The inert pair effect refers to the phenomenon where the outermost s-electrons (in the valence shell) of heavier elements become less reactive and do not participate in bonding. This is primarily due to the poor shielding of the s-electrons by the inner d and f electrons.

2. **Conditions for Inert Pair Effect**:
- The inert pair effect is observed in elements that have d or f electrons in their electron configuration. The poor shielding by these inner electrons leads to a stronger attraction between the nucleus and the outermost s-electrons, causing them to remain unreactive.

3. **Identifying the Elements**:
- The elements provided in the question include:
- Tin (Sn)
- Indium (In)
- Lead (Pb)
- Iron (Fe)

4. **Analyzing Each Element**:
- **Tin (Sn)**:
- Electron configuration: [Kr] 5s² 4d¹⁰
- Tin has d-electrons and can exhibit the inert pair effect.
- **Indium (In)**:
- Electron configuration: [Kr] 5s² 4d¹⁰ 5p¹
- Indium has d-electrons and can exhibit the inert pair effect.
- **Lead (Pb)**:
- Electron configuration: [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p²
- Lead has f-electrons and can exhibit the inert pair effect.
- **Iron (Fe)**:
- Electron configuration: [Ar] 4s² 3d⁶
- Iron does not have d or f electrons in the outermost shell that would lead to poor shielding, thus it does not show the inert pair effect.

5. **Conclusion**:
- Among the elements listed, **Iron (Fe)** will not show the inert-pair effect because it lacks the necessary d or f electrons that would cause poor shielding.

### Final Answer:
**Iron (Fe)** will not show the inert-pair effect.