This question tests your understanding of electron gain enthalpy, which is the energy change when an atom gains an electron to form a negative ion. A more negative value means more energy is released.

Key Concept: The trend in electron gain enthalpy across a period and down a group is not perfectly uniform due to two competing factors: atomic size and electron-electron repulsion.

• Across a period (left to right): Atomic size decreases, making it easier for the nucleus to attract an extra electron. Therefore, electron gain enthalpy generally becomes more negative (more energy is released). However, an exception occurs between certain elements due to increased repulsion in a compact atom.
• Down a group (top to bottom): Atomic size increases, making it harder for the nucleus to attract an extra electron. Therefore, electron gain enthalpy generally becomes less negative (less energy is released).

Let's analyze each pair:

1. F & Cl (Group 17):

Fluorine has a smaller atomic size than chlorine, so you might expect it to have a more negative electron gain enthalpy. However, due to its very small size, the incoming electron experiences significant repulsion from the electrons already present in the compact 2p-subshell. This repulsion outweighs the increased nuclear attraction. Therefore, chlorine (Cl) releases more energy upon gaining an electron than fluorine (F).

Electron Gain Enthalpy: Cl < F (i.e., value for Cl is more negative).

2. S & Se (Group 16):

Sulfur (S) is above selenium (Se) in the group. Atomic size increases down the group. The smaller size of sulfur allows its nucleus to attract an incoming electron more effectively than the larger selenium atom. Therefore, sulfur (S) releases more energy upon gaining an electron than selenium (Se).

Electron Gain Enthalpy: S < Se (i.e., value for S is more negative).

3. Li & Na (Group 1):

Lithium (Li) is above sodium (Na) in the group. Similar to the previous case, the smaller atomic size of lithium means its nucleus can attract an incoming electron more effectively than the larger sodium atom. Therefore, lithium (Li) releases more energy upon gaining an electron than sodium (Na).

Electron Gain Enthalpy: Li < Na (i.e., value for Li is more negative).

Conclusion: The elements that release more energy upon electron gain are Cl (compared to F), S (compared to Se), and Li (compared to Na).

Final Answer: Cl, S and Li

Related Topics & Formulae

Electron Gain Enthalpy (Δ_egH): The enthalpy change for the process: $X\left(g\right)+e^{-}\to X^{-}\left(g\right)$

A more negative Δ_egH indicates a greater tendency to form an anion. It is influenced by:

• Atomic Size: Smaller atoms generally have more negative Δ_egH due to greater effective nuclear charge.
• Electronic Configuration: Stable half-filled or fully-filled orbitals have less negative Δ_egH.
• Nuclear Charge: Higher nuclear charge leads to a more negative Δ_egH.