In each of the following questions a Statement of Assertion (A ) is given followed by a corresponding statement of Reason (R ) Just below it . Of the statement mark the correct answer as
Assertion. First ionization energy for nitrogen is lower than that of oxygen .
Reason . Across a period effective nuclear charge decreases.

Explanation

To analyze the given Assertion (A) and Reason (R), we need to break down the statements and evaluate their correctness.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "First ionization energy for nitrogen is lower than that of oxygen."
- Ionization energy is the energy required to remove an electron from an atom in its gaseous state.

2. **Evaluating the Ionization Energies**:
- The electronic configuration of nitrogen (N) is 1s² 2s² 2p³, while that of oxygen (O) is 1s² 2s² 2p⁴.
- Generally, ionization energy increases across a period due to increasing nuclear charge, which holds the electrons more tightly.
- However, between nitrogen and oxygen, the ionization energy of nitrogen is actually higher than that of oxygen due to the stability of the half-filled p subshell in nitrogen.

3. **Understanding the Reason**:
- The reason states that "Across a period effective nuclear charge decreases."
- This statement is incorrect. Effective nuclear charge actually increases across a period because as protons are added to the nucleus, the positive charge increases, pulling the electrons closer and making them harder to remove.

4. **Conclusion**:
- Since the assertion is false (first ionization energy of nitrogen is higher than that of oxygen) and the reason is also false (effective nuclear charge increases across a period), both statements are incorrect.

### Final Answer:
- Both Assertion (A) and Reason (R) are false.