Question

Electron affinity is positive when:

• A. \( O \) changes into \( O^- \)
• B. \( O^- \) changes to \( O^{2-} \)
• C. \( O \) changes into \( O^+ \)
• D. \( O \) changes to \( O^{2+} \)

Hint:

The first electron affinity is typically exothermic because the atom stabilizes by gaining an electron. However, the second electron affinity is endothermic due to the repulsive forces between the negatively charged ion and the incoming electron.

Answer 1

The Correct Option is B

Step 1: {Understanding Electron Affinity} 
Electron affinity refers to the energy change when an electron is added to a neutral atom in the gas phase to form a negatively charged ion. The first electron affinity is usually exothermic, meaning energy is released when the atom gains an electron. However, the second electron affinity is generally endothermic, meaning energy is required to add a second electron. This is because, after the first electron is added, the resulting negatively charged ion experiences a repulsive force from the incoming electron. 
Step 2: {Explaining Positive Electron Affinity} 
In the case of the oxygen ion \( O^- \), the negative charge on the ion creates a repulsive force that works against the addition of another electron. The added electron would experience electrostatic repulsion due to the negative charge of the \( O^- \) ion. Therefore, energy must be supplied to overcome this repulsion, making the second electron affinity positive (endothermic). Thus, the correct answer is (B).