Question

The electron affinity values are negative for:
A. Be → Be⁻
B. N → N⁻
C. O → O²⁻
D. Na → Na⁻
E. Al → Al⁻
Choose the most appropriate answer from the options given below:

• A. D and E only
• B. A, B, D and E only
• C. A and D only
• D. A, B and C only

Answer 1

The Correct Option is A

To determine which elements have negative electron affinity values, we need to understand the concept of electron affinity. Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom in the gas phase. A negative value indicates that energy is released when an electron is added, making the process exothermic.

Let's analyze each given option:

1. A. Be → Be⁻: Beryllium has a completely filled s-subshell, making it less likely to accept an additional electron. It has a positive electron affinity because adding an electron increases electron-electron repulsion without gaining significant stability.
2. B. N → N⁻: Nitrogen has a half-filled p-subshell, which is stable due to symmetrical electron distribution. Adding an electron to nitrogen introduces repulsion, resulting in a positive electron affinity.
3. C. O → O²⁻: Oxygen generally gains energy when forming a single anion (O⁻). However, adding a second electron (forming O²⁻) involves significant electron-electron repulsion, resulting in a positive electron affinity.
4. D. Na → Na⁻: Sodium has one electron in its outer shell. Losing this electron (to form Na⁺) is favorable, while adding an electron (to form Na⁻) is not, leading to a positive electron affinity.
5. E. Al → Al⁻: Aluminum has a similar situation to sodium where adding an electron is not favorable, resulting in a positive electron affinity.

Thus, the elements where electron affinity values are negative (indicating energy is required to add an electron) are Sodium (Na) and Aluminum (Al), i.e., options D and E.

Hence, the correct answer is: D and E only.

Answer 2

The electron affinity values for each case are determined as follows:
(A) Be + e$^-$ $\to$ Be$^-$: Electron affinity is negative (E.A. = --ive).
(B) N + e$^-$ $\to$ N$^-$: Electron affinity is negative (E.A. = --ive).
(C) O + e$^-$ $\to$ O$^-$: Electron affinity is positive for O $\to$ O$^-$ (E.A. = +ive).
(D) Na + e$^-$ $\to$ Na$^-$: Electron affinity is positive (E.A. = +ive).
(E) Al + e$^-$ $\to$ Al$^-$: Electron affinity is positive (E.A. = +ive).
Thus, the correct answer is: A, B, and C only.