Assertion A: The first ionization enthalpy of O is lower than that of N and F.

Reason R: The loss of an electron from O leads to stable half-filled p orbital.

Analyzing Ionization Enthalpy

First ionization enthalpy ($\Delta H_{IE1}$) is the minimum energy required to remove the most loosely bound electron from a neutral gaseous atom in its ground state.

• General Trend: Across a period, ionization enthalpy generally increases due to increasing effective nuclear charge. Therefore, we expect $\Delta H_{IE1}$(N) < $\Delta H_{IE1}$(O) < $\Delta H_{IE1}$(F).
• Electron Configurations:
  • Nitrogen (N, Z=7): $1s^2 2s^2 2p^3$ (half-filled $2p$ subshell - stable)
  • Oxygen (O, Z=8): $1s^2 2s^2 2p^4$ (paired electron in $2p$ subshell)
  • Fluorine (F, Z=9): $1s^2 2s^2 2p^5$
• Comparison:
  • N vs O: Removing an electron from Nitrogen requires breaking the stable half-filled $2p$ configuration. Removing the first electron from Oxygen results in a stable half-filled $2p^3$ configuration ($2p^4 \rightarrow 2p^3$). This stabilization makes it easier to remove an electron from Oxygen compared to Nitrogen. Thus, $\Delta H_{IE1}$(O) < $\Delta H_{IE1}$(N).
  • O vs F: While Fluorine has a higher nuclear charge, the stability of the half-filled $2p$ orbital in Oxygen affects the trend. However, the general increase across the period holds, and $\Delta H_{IE1}$(O) < $\Delta H_{IE1}$(F).
• Conclusion on Assertion A: Assertion A states that $\Delta H_{IE1}$(O) is lower than both $\Delta H_{IE1}$(N) and $\Delta H_{IE1}$(F). This is correct due to the specific stability considerations.

Evaluating the Reason

Reason R states that losing an electron from Oxygen leads to a stable half-filled p orbital. Oxygen's electron configuration is $2p^4$. When one electron is removed, the configuration becomes $2p^3$, which is a half-filled $p$ subshell and is known for its stability. This statement is factually correct.

Connecting Assertion and Reason

The reason explains the anomaly in the ionization enthalpy trend. The dip in ionization enthalpy from Nitrogen to Oxygen occurs precisely because removing an electron from Oxygen allows it to achieve the highly stable half-filled $2p$ orbital configuration ($2p^3$). This stability gained upon ionization makes the process energetically favorable compared to removing an electron from Nitrogen, which already possesses this stable configuration. Therefore, Reason R correctly explains Assertion A.

Final Choice

Both Assertion A and Reason R are correct, and Reason R provides the correct explanation for Assertion A.

This corresponds to Option 4.