Q.Electronic configuration of some elements is given in Column I and their electron gain enthalpies are given in Column II. Match the electronic configuration with electron gain enthalpy.
Column I (Electronic configuration)

Concept: Electron gain enthalpy measures the energy change when an electron is added to a gaseous atom. It becomes more negative (more exothermic) when the added electron enters a stable, half-filled, or nearly complete subshell, and less negative or positive when electron–electron repulsion dominates or a stable configuration is disrupted.

Analysis:

1. 1s² 2s² 2p⁶ – Noble gas (Ne); completely filled shell. Adding an electron requires forcing it into the next shell (3s) against strong repulsion. Electron gain is endothermic: (D) +48 kJ mol⁻¹.
2. 1s² 2s² 2p⁶ 3s¹ – Alkali metal (Na); losing the lone 3s electron is favorable, but gaining one (to form 3s²) offers little stability. Moderately exothermic: (A) –53 kJ mol⁻¹. …

Matching Electronic Configurations with Electron Gain Enthalpies

Electron gain enthalpy measures the energy change when an isolated gaseous atom accepts an electron. A more negative value means the atom releases more energy—it "wants" that electron badly. The key is to recognize what makes an atom eager or reluctant to accept an extra electron: nuclear charge, orbital stability, and electron-electron repulsion all play roles.

Let me identify each configuration first, then reason through their electron gain behavior.

Identifying the Elements

1. Configuration (i): $1s^2 \, 2s^2 \, 2p^6$

   This is neon (Ne), a noble gas with a completely filled outer shell. Noble gases have stable, closed-shell configurations and strongly resist gaining electrons because any additional electron must enter a new, higher-energy shell ($3s$ in this case). The incoming electron experiences significant repulsion from the tightly held $2p^6$ core and minimal effective nuclear attraction. This makes electron gain endothermic—energy must be supplied.

2. Configuration (ii): $1s^2 \, 2s^2 \, 2p^6 \, 3s^1$

   This is sodium (Na). Sodium has one loosely held valence electron in the $3s$ orbital. Adding an electron means placing it in a $3p$ orbital, which is higher in energy and experiences significant shielding from the inner electrons. The nuclear pull is weak, and the atom gains little stability. Electron gain enthalpy is slightly negative but small in magnitude.

3. Configuration (iii): $1s^2 \, 2s^2 \, 2p^5$

   This is fluorine (F), a halogen one electron short of a noble-gas configuration. Gaining one electron completes the $2p$ subshell, achieving the stable $1s^2 \, 2s^2 \, 2p^6$ configuration. The small atomic size means the nucleus exerts a strong pull on the incoming electron, and the energy release is substantial. Halogens have the most negative electron gain enthalpies.

4. Configuration (iv): $1s^2 \, 2s^2 \, 2p^4$

   This is oxygen (O). Oxygen needs two electrons to complete its octet, so gaining one electron is favorable. However, the $2p$ subshell already contains four electrons in a compact space. When the fifth electron enters, it experiences significant electron-electron repulsion, especially since it must pair up in an already occupied orbital. This repulsion reduces the energy release compared to fluorine, making the electron gain enthalpy less negative than that of halogens but still substantially negative.

Matching with Electron Gain Enthalpies

Now I'll match each configuration with the appropriate value:

Configuration (i) – Neon:

Noble gas, closed shell. Electron gain is unfavorable.

Match: (D) $+48 \, \text{kJ mol}^{-1}$ (positive, endothermic)

Configuration (ii) – Sodium: …