Which one of the following amongs each pair will release maximum energy on gaining one electron (A = F, Cl), (B = S, Se), (C = Li, Na)

Options

A. Cl, S and Li
B. F, Se and Na
C. F, S and Li
D. Cl, Se and Na

Answer

To solve the question of which element in each pair will release the maximum energy upon gaining one electron, we need to analyze the electron gain enthalpy (or electron affinity) of the elements in each pair. The pairs are:

1. (A) F, Cl
2. (B) S, Se
3. (C) Li, Na

### Step-by-Step Solution:

**Step 1: Analyze Pair A (F, Cl)**

- **Fluorine (F)**: It is a second-period element with a small atomic size. When an electron is added to fluorine, there is significant inter-electronic repulsion due to the small size and high electron density. This repulsion can hinder the release of energy.

- **Chlorine (Cl)**: Chlorine is a third-period element and has a larger atomic size compared to fluorine. The larger size reduces the inter-electronic repulsion when an electron is added, allowing for a more favorable electron gain process.

**Conclusion for Pair A**: Chlorine (Cl) will release more energy upon gaining an electron compared to fluorine (F).

**Step 2: Analyze Pair B (S, Se)**

- **Sulfur (S)**: Sulfur is in the third period and has a relatively smaller size compared to selenium. It has a higher electron affinity compared to selenium due to stronger nuclear attraction for the added electron.

- **Selenium (Se)**: Selenium is in the fourth period and has a larger atomic size. The increased distance from the nucleus reduces the nuclear attraction for the added electron, resulting in lower electron affinity.

**Conclusion for Pair B**: Sulfur (S) will release more energy upon gaining an electron compared to selenium (Se).

**Step 3: Analyze Pair C (Li, Na)**

- **Lithium (Li)**: Lithium is a smaller alkali metal with a higher effective nuclear charge compared to sodium. This results in a stronger attraction for the added electron, leading to a higher electron affinity.

- **Sodium (Na)**: Sodium is larger than lithium, and the increased size results in a weaker attraction for the added electron, leading to lower electron affinity.

**Conclusion for Pair C**: Lithium (Li) will release more energy upon gaining an electron compared to sodium (Na).

### Final Summary:

- For Pair A: Chlorine (Cl) releases maximum energy.
- For Pair B: Sulfur (S) releases maximum energy.
- For Pair C: Lithium (Li) releases maximum energy.

Thus, the correct answer is:
**Chlorine, Sulfur, Lithium** (Option A).