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). 
 

Answer 2

Step 1: Understand what electron affinity is
Electron affinity (EA) is the amount of energy released or absorbed when an atom or ion in the gaseous state accepts an electron.
EA is usually negative because energy is released when an electron is added to a neutral atom.

Step 2: When is electron affinity positive?
A positive electron affinity means that energy is required (absorbed) to add an electron, making the process unfavorable.
This usually happens when adding an electron to an already negatively charged ion, due to strong electron-electron repulsion.

Step 3: Analyze the given case
In the case of \( O^- \) (oxide ion with one extra electron), adding another electron to form \( O^{2-} \) involves adding a second negative charge.
This increases electron-electron repulsion significantly.
As a result, the system requires external energy input to accommodate the second electron — the process is endothermic.

Step 4: Final Answer
Electron affinity is positive when:
\( O^- \) changes to \( O^{2-} \)