The order of decreasing negative electron gain enthalpy of O. S, Se, is

Options

A. `O gt S gt Se`
B. `S gt O gt Se`
C. `Se gt O gt S`
D. `S gt Se gt O`

Explanation

To determine the order of decreasing negative electron gain enthalpy for the elements oxygen (O), sulfur (S), and selenium (Se), we need to consider the following steps:

### Step 1: Understand Electron Gain Enthalpy
Electron gain enthalpy is the amount of energy released when an electron is added to a neutral atom in the gaseous state. A more negative value indicates a greater tendency to gain an electron.

### Step 2: Consider Atomic Size
The electron gain enthalpy is inversely proportional to the size of the atom. As the size of the atom increases, the ability to attract an additional electron decreases, resulting in a less negative electron gain enthalpy.

### Step 3: Analyze the Elements
- **Oxygen (O)**: Being the smallest atom among the three, it has a high electron density and can attract an electron effectively. However, due to its small size, the added electron experiences significant inter-electronic repulsion, making its electron gain enthalpy less negative.

- **Sulfur (S)**: Larger than oxygen but smaller than selenium, sulfur has a more favorable electron gain enthalpy compared to oxygen. It can accommodate an additional electron more easily than oxygen.

- **Selenium (Se)**: The largest of the three, selenium has the least negative electron gain enthalpy. The larger size means that the added electron is less effectively attracted due to the increased distance from the nucleus.

### Step 4: Order of Decreasing Negative Electron Gain Enthalpy
Based on the above analysis, we can conclude the order of decreasing negative electron gain enthalpy is:
\[ \text{S} > \text{Se} > \text{O} \]

### Final Answer
The order of decreasing negative electron gain enthalpy for O, S, and Se is:
\[ \text{S} > \text{Se} > \text{O} \]

---