The order of electron gain enthalpy (magnitude) of `O, S` and `Se` is:

Options

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

Explanation

To determine the order of electron gain enthalpy (magnitude) for the elements oxygen (O), sulfur (S), and selenium (Se), we can follow these steps:

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

### Step 2: Analyze the Group Trend
Oxygen, sulfur, and selenium belong to Group 16 of the periodic table. As we move down the group from oxygen to selenium, the atomic size increases. This increase in size leads to a decrease in the effective nuclear charge experienced by the incoming electron due to increased distance from the nucleus.

### Step 3: Compare the Elements
- **Oxygen (O)**: Being the smallest atom in this group, it has a high effective nuclear charge relative to its size. However, the small size also leads to significant electron-electron repulsion when an additional electron is added, making it less favorable to gain an electron.

- **Sulfur (S)**: Sulfur is larger than oxygen, which means that the incoming electron experiences less repulsion compared to oxygen. Therefore, sulfur has a more favorable electron gain enthalpy compared to oxygen.

- **Selenium (Se)**: Selenium is larger than sulfur, and while it has a lower effective nuclear charge, the electron repulsion is also less significant. However, the tendency to gain an electron is even lower than that of sulfur due to its larger size.

### Step 4: Order of Electron Gain Enthalpy
Based on the analysis:
- The order of electron gain enthalpy (magnitude) is:
**S > O > Se**

### Final Answer
The order of electron gain enthalpy (magnitude) for O, S, and Se is: **S > O > Se**

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