In which of the following energy is abosrbed?

Options

A. `Cl+e^(-) to Cl^(-)`
B. `O^(-)+e^(-) to O^(2-)`
C. `O +e^(-) to O^(-)`
D. `Na^(+) +e^(-) to Na`

Answer

To solve the question "In which of the following energy is absorbed?", we need to analyze the process of electron gain enthalpy for the given options. Here’s a step-by-step breakdown:

### Step 1: Understand Electron Gain Enthalpy
- Electron gain enthalpy is the amount of energy released or absorbed when an electron is added to a neutral atom in the gas phase.
- If energy is released when an electron is added, the electron gain enthalpy is negative.
- If energy is absorbed when an electron is added, the electron gain enthalpy is positive.

**Hint:** Remember that a negative electron gain enthalpy indicates energy release, while a positive value indicates energy absorption.

### Step 2: Analyze the Given Options
- We will consider the process of adding an electron to each element mentioned in the options.

**Hint:** Look for whether the addition of an electron leads to a stable configuration or if it faces repulsion.

### Step 3: Evaluate Each Option
1. **Option 1: Chlorine (Cl) gaining its first electron**
- Chlorine readily gains an electron to form Cl⁻. This process releases energy, so the electron gain enthalpy is negative.

2. **Option 2: Oxygen (O) gaining its first electron**
- Oxygen also gains its first electron to form O⁻, which releases energy (negative electron gain enthalpy).
- However, when O⁻ tries to gain a second electron to become O²⁻, it faces electron-electron repulsion. Therefore, energy must be supplied to add this second electron, making the electron gain enthalpy positive for this step.

3. **Option 3: (Assuming another element)**
- Without specific details, we cannot analyze this option. However, typically, elements that are less electronegative may require energy to gain electrons.

4. **Option 4: (Assuming another element)**
- Similar to option 3, we need to analyze based on the specific element's tendency to gain electrons.

**Hint:** Focus on the second electron addition for elements like oxygen, where repulsion plays a significant role.

### Step 4: Conclusion
- Based on the analysis, the only scenario where energy is absorbed is when oxygen gains its second electron (O⁻ to O²⁻). Therefore, the correct answer is **Option 2**.

**Final Answer:** The energy is absorbed when oxygen gains its second electron (Option 2).