Question

Electron affinity is positive, when:

• A. \( \text{O}^- \rightarrow \text{O}^- \)
• B. \( \text{O}^- \rightarrow \text{O}^{2-} \)
• C. \( \text{O} \rightarrow \text{O}^+ \)
• D. \( \text{O} \rightarrow \text{O}^{2+} \)

Hint:

Remember: First electron affinity is usually negative (energy released), while subsequent electron affinities (for anions) are always positive (energy required) due to inter-electronic repulsion.

Answer 1

The Correct Option is B

Concept: Electron affinity is the energy released when an electron is added to a neutral gaseous atom. Generally, the first electron addition is exothermic (negative value). However, when adding an electron to an already negatively charged ion, a strong electrostatic repulsion exists between the incoming electron and the existing negative charge.

Step 1: Understanding the energetics of ion formation. Adding an electron to a neutral atom (like O) is typically exothermic because the nucleus attracts the incoming electron. The reaction is: \[ \text{O}(g) + e^- \rightarrow \text{O}^-(g) \quad (\Delta H < 0) \] This process releases energy.

Step 2: Analyzing the second electron addition. To form the oxide ion (\(\text{O}^{2-}\)), an additional electron must be forced into the already negative \(\text{O}^-\) ion. The incoming electron faces significant electronic repulsion from the electron cloud of the \(\text{O}^-\) ion. \[ \text{O}^-(g) + e^- \rightarrow \text{O}^{2-}(g) \quad (\Delta H > 0) \] Because work must be done to overcome this repulsion, energy is absorbed, making the electron affinity value positive.

Step 3: Conclusion. Among the choices provided, the process where an electron is added to an anion (\(\text{O}^-\) changing into \(\text{O}^{2-}\)) is the only one that represents a second electron affinity, which is always endothermic (positive).