Question

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R. 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. In light of the above statements, choose the most appropriate answer from the options given below:

• A. A is not correct but R is correct
• B. Both A and R are correct and R is the correct explanation of A
• C. Both A and R are correct and R is NOT the correct explanation of A
• D. A is correct but R is not correct

Hint:

Exception in ionization energy trend: Nitrogen = stable half-filled configuration Oxygen = paired electron repulsion lowers ionization energy Always remember: \[ IE_O \lt IE_N \] despite moving left to right in periodic table.

Answer 1

The Correct Option is B

Concept: Ionization enthalpy is the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom. Generally, ionization enthalpy increases across a period because effective nuclear charge increases. However, some exceptional electronic configurations create irregularities. Stability of half-filled and fully-filled subshells plays a major role in such exceptions.

Step 1: Write electronic configurations.
Nitrogen: \[ N = 1s^2 2s^2 2p^3 \] Oxygen: \[ O = 1s^2 2s^2 2p^4 \] Fluorine: \[ F = 1s^2 2s^2 2p^5 \]

Step 2: Compare nitrogen and oxygen.
Nitrogen has exactly half-filled p orbital configuration: \[ 2p^3 \] This half-filled arrangement gives extra stability because electrons occupy separate orbitals with parallel spins according to Hund’s rule. Hence removing an electron from nitrogen requires relatively high energy.

Step 3: Analyze oxygen.
Oxygen has configuration \[ 2p^4 \] One p orbital contains paired electrons. Electron-electron repulsion exists inside the paired orbital. Therefore removal of one electron becomes easier. After losing one electron oxygen becomes \[ O^+ = 1s^2 2s^2 2p^3 \] This is a stable half-filled configuration.

Step 4: Compare with fluorine.
Fluorine has higher nuclear charge than oxygen. Therefore attraction between nucleus and electrons is stronger, so more energy is required to remove electron. Thus experimentally \[ IE_O \lt IE_N \lt IE_F \]

Step 5: Evaluate assertion and reason.
Assertion: Oxygen has lower ionization enthalpy than nitrogen and fluorine → True Reason: Removal of electron from oxygen creates stable half-filled orbital → True Reason directly explains assertion. \[ \boxed{\text{Correct Answer = Option (2)}} \]