(A) : Nitrogen has higher ionization energy than that of oxygen.
(R ) : Nitrogen atom has smaller atomic size than that of oxygen.

Options

A. Both A & R are true, R is the correct explanation of A
B. Both A & R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Answer

To analyze the assertion and reason provided in the question, we will break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "Nitrogen has higher ionization energy than that of oxygen."
- **Ionization energy (IE)** is defined as the energy required to remove an electron from an atom in its gaseous state.

### Step 2: Compare the Ionization Energies
- The electronic configuration of nitrogen (atomic number 7) is \(1s^2 2s^2 2p^3\).
- The electronic configuration of oxygen (atomic number 8) is \(1s^2 2s^2 2p^4\).
- Nitrogen has three electrons in its 2p orbital (which is half-filled), while oxygen has four electrons in its 2p orbital (which is not half-filled).

### Step 3: Stability of Half-filled Orbitals
- Half-filled orbitals (like in nitrogen) are generally more stable due to symmetry and exchange energy.
- This stability means that more energy is required to remove an electron from nitrogen than from oxygen, leading to a higher ionization energy for nitrogen.

### Step 4: Conclusion on the Assertion
- Therefore, the assertion that "Nitrogen has higher ionization energy than that of oxygen" is **true**.

### Step 5: Understand the Reason
The reason states that "Nitrogen atom has smaller atomic size than that of oxygen."
- As we move across a period in the periodic table from left to right, the atomic size generally decreases due to the increasing nuclear charge, which pulls the electrons closer to the nucleus.

### Step 6: Compare Atomic Sizes
- In this case, nitrogen (N) is to the left of oxygen (O) in the periodic table.
- Hence, nitrogen has a larger atomic size compared to oxygen.

### Step 7: Conclusion on the Reason
- Therefore, the reason that "Nitrogen atom has smaller atomic size than that of oxygen" is **false**.

### Final Conclusion
- The assertion is true, but the reason is false. Thus, the correct answer is that the assertion is true, but the reason is false.

### Summary
- Assertion (A): True
- Reason (R): False