Electron affinity is positive when

Options

A. `O^(-)` is formed from O
B. `O^(2-)` is formed from `O^(-)`
C. `O^(+)` is formed from O
D. `O^(3-)` is formed from `O^(-)`

Explanation

To determine when the electron affinity (EA) is positive, let's analyze the concept step by step:

### Step 1: Understanding Electron Affinity
Electron affinity is the amount of energy released or absorbed when an electron is added to a neutral atom in the gas phase. It is typically negative for most elements because energy is released when an electron is added.

**Hint**: Remember that a negative electron affinity indicates that energy is released when an electron is added to an atom.

### Step 2: Conditions for Positive Electron Affinity
Electron affinity can be positive under certain conditions. Specifically, when adding an electron to an already negatively charged ion (anion), the process can require energy due to electron-electron repulsion.

**Hint**: Think about what happens when you try to add an electron to an already negatively charged species.

### Step 3: Example of Oxygen
Taking the example of oxygen:
- When an electron is added to a neutral oxygen atom (O), it forms O⁻ and this process is exothermic (negative EA).
- However, when trying to add a second electron to O⁻ to form O²⁻, the repulsion between the two negatively charged electrons makes this process endothermic, requiring energy input. Thus, the EA for this process is positive.

**Hint**: Consider how adding a second electron to an already negatively charged ion affects energy changes.

### Step 4: Conclusion
Therefore, electron affinity is positive when:
- An electron is added to a negatively charged ion (e.g., O⁻ → O²⁻), which requires energy due to repulsion between the electrons.

**Final Answer**: Electron affinity is positive when adding an electron to an already negatively charged ion.