JEE MAIN - Chemistry (2024 - 6th April Morning Shift - No. 5)
The electron affinity value are negative for
A. $$\mathrm{Be} \rightarrow \mathrm{Be}^{-}$$
B. $$\mathrm{N} \rightarrow \mathrm{N}^{-}$$
C. $$\mathrm{O} \rightarrow \mathrm{O}^{2-}$$
D. $$\mathrm{Na} \rightarrow \mathrm{Na}^{-}$$
E. $$\mathrm{Al} \rightarrow \mathrm{Al}^{-}$$
Choose the most appropriate answer from the options given below :
Explanation
To determine which elements have negative electron affinity values, we'll analyze each option individually.
Understanding Electron Affinity
Electron Affinity (EA): The energy change when an electron is added to a neutral atom in the gas phase to form a negative ion.
Convention:
If energy is released (exothermic process), the electron affinity is considered positive.
If energy is absorbed (endothermic process), the electron affinity is considered negative.
Option A: $\mathrm{Be} \rightarrow \mathrm{Be}^{-}$
Electronic Configuration of Be: $1s^2\,2s^2$
Analysis:
Adding an electron to beryllium means placing it into the higher energy $2p$ orbital.
Beryllium has a filled $2s$ subshell, so the added electron experiences higher energy and electron-electron repulsion.
Result: Energy is absorbed; the process is endothermic.
Conclusion: Electron affinity of Be is negative.
Option B: $\mathrm{N} \rightarrow \mathrm{N}^{-}$
Electronic Configuration of N: $1s^2\,2s^2\,2p^3$
Analysis:
Nitrogen has a half-filled $2p$ subshell.
Adding an electron introduces repulsion in the half-filled orbital.
Result: Energy is absorbed; the process is endothermic.
Conclusion: Electron affinity of N is negative.
Option C: $\mathrm{O} \rightarrow \mathrm{O}^{2-}$
Electronic Configuration of O: $1s^2\,2s^2\,2p^4$
Analysis:
First Electron Affinity (O to O⁻): Exothermic (energy released).
Second Electron Affinity (O⁻ to O²⁻): Endothermic because adding an electron to a negative ion requires energy due to electron-electron repulsion.
Overall Process (O to O²⁻): The second step dominates, making the overall process endothermic.
Conclusion: Electron affinity for forming $\mathrm{O}^{2-}$ is negative.
Option D: $\mathrm{Na} \rightarrow \mathrm{Na}^{-}$
Electronic Configuration of Na: $1s^2\,2s^2\,2p^6\,3s^1$
Analysis:
Adding an electron fills the $3s$ orbital.
Sodium tends to lose an electron to form $\mathrm{Na}^+$, not gain one.
However, adding an electron is still an exothermic process due to the low energy of the $3s$ orbital.
Result: Energy is released; the process is exothermic.
Conclusion: Electron affinity of Na is positive.
Option E: $\mathrm{Al} \rightarrow \mathrm{Al}^{-}$
Electronic Configuration of Al: $1s^2\,2s^2\,2p^6\,3s^2\,3p^1$
Analysis:
Adding an electron to the $3p$ orbital.
The process releases energy due to the addition of an electron to a partially filled orbital.
Result: Energy is released; the process is exothermic.
Conclusion: Electron affinity of Al is positive.
Final Conclusion
Negative Electron Affinity Values (Endothermic Processes):
Option A: Beryllium ($\mathrm{Be}$)
Option B: Nitrogen ($\mathrm{N}$)
Option C: Oxygen forming $\mathrm{O}^{2-}$
Therefore, the correct answer is:
Option D: A, B, and C only
Answer: Option D
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