1. Meaning of negative electron affinity

Electron affinity is the enthalpy change for adding an electron to a gaseous species:

$X(g) + e^- \to X^-(g)$

If the value is negative, the process is endothermic, i.e. energy must be supplied.

For adding the first electron, negative electron affinity is expected for atoms with especially stable configurations such as:

• filled subshells: $ns^2$ (e.g. Be)
• half-filled subshells: $np^3$ (e.g. N)

For adding an electron to an already negative ion, the process is strongly opposed due to electron-electron repulsion, so it is endothermic.

2. Check each given case

(A) $\mathrm{Be} \to \mathrm{Be}^-$

Be has configuration:

$1s^2 2s^2$

This is a filled $2s$ subshell. Adding an electron forces it into the higher-energy $2p$ orbital, so the process is unfavorable.

Hence electron affinity is negative.

✅ A is correct

(B) $\mathrm{N} \to \mathrm{N}^-$

N has configuration:

$1s^2 2s^2 2p^3$

This is a half-filled $2p$ subshell, which is especially stable. Adding one electron disturbs this stability, so the process is unfavorable.

Hence electron affinity is negative.

✅ B is correct

(C) $\mathrm{O} \to \mathrm{O}^{2-}$

This corresponds effectively to adding an electron to $\mathrm{O}^-$:

$\mathrm{O}^-(g) + e^- \to \mathrm{O}^{2-}(g)$

Adding an electron to an already negatively charged ion faces strong repulsion, so this second electron affinity is always endothermic.

Hence the value is negative.

✅ C is correct

(D) $\mathrm{Na} \to \mathrm{Na}^-$

Na has configuration:

$[Ne]3s^1$

Adding an electron gives:

$[Ne]3s^2$

This is a more stable filled $3s$ configuration, so energy is released.

Hence electron affinity is positive (not negative).

❌ D is incorrect

(E) $\mathrm{Al} \to \mathrm{Al}^-$

Al has configuration:

$[Ne]3s^2 3p^1$

Adding an electron gives:

$[Ne]3s^2 3p^2$

This process is generally favorable; first electron affinity of Al is not negative.

❌ E is incorrect

3. Correct set

Negative electron affinity values occur for:

$A, B, C$

So the correct option is:

$\boxed{\text{D: A, B and C only}}$

4. Comparison with stored answer

Stored correct answer: D

My derived answer: D

They match.