Understanding Electron Affinity

Electron affinity (EA) is the energy change when an atom gains an electron to form a negative ion. A more negative EA value indicates greater tendency to gain an electron. Trends are influenced by atomic size, effective nuclear charge, and electronic configuration.

Step 1: Analyze Option S > O > Se

Oxygen (O) and sulfur (S) are in Group 16. EA generally decreases down a group due to increasing atomic size, but O has lower EA than S due to small size and electron-electron repulsion in the compact 2p subshell. Selenium (Se) is below S, so EA: S > O > Se is correct.

Confirmed order: $\mathrm{EA}\left(S\right)>\mathrm{EA}\left(O\right)>\mathrm{EA}\left(\mathrm{Se}\right)$

Step 2: Analyze Option Cl > Br > F

Chlorine (Cl), bromine (Br), and fluorine (F) are halogens. EA decreases down the group, but F has lower EA than Cl due to its small size and high electron density, leading to repulsion. Correct order: Cl > F > Br.

Thus, Cl > Br > F is incorrect because F should not be last.

Step 3: Analyze Option N < P

Nitrogen (N) has very low EA due to its half-filled stable 2p³ configuration, making it reluctant to gain an electron. Phosphorus (P) has higher EA. So, EA(N) < EA(P) is correct.

Confirmed: $\mathrm{EA}\left(N\right)<\mathrm{EA}\left(P\right)$

Step 4: Analyze Option C > N > O

Carbon (C), nitrogen (N), and oxygen (O) are in Period 2. EA generally increases across a period, but N has lower EA due to half-filled stability. Correct order: O > C > N.

Thus, C > N > O is incorrect.

Final Answer

The correct orders are: S > O > Se and N < P.

Related Topics

• Periodic Trends: Atomic size, ionization energy, electronegativity
• Electronic Configuration and Stability
• Anion Formation and Energetics

Key Formulae and Theory

Electron Affinity: $X\left(g\right)+e^{-}\to X^{-}\left(g\right)+\mathrm{energy}$ (exothermic for most elements).

Factors affecting EA:

• Atomic size: Smaller atoms have higher EA (more negative).
• Nuclear charge: Higher effective nuclear charge increases EA.
• Electronic configuration: Half-filled or fully filled subshells may lower EA.