Assertion :First ionization energy of N2 is lower than oxygen.
Reason :Across a period effective charge decreases.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false

Explanation

To solve the assertion and reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Analyze the Assertion
The assertion states that the first ionization energy of nitrogen (N2) is lower than that of oxygen (O2).

- **Ionization Energy**: This is the energy required to remove an electron from an atom in its gaseous state.
- **Atomic Numbers**: Nitrogen has an atomic number of 7, and oxygen has an atomic number of 8.
- **Electronic Configurations**:
- Nitrogen: 1s² 2s² 2p³
- Oxygen: 1s² 2s² 2p⁴

From the electronic configurations, we see that nitrogen has a half-filled 2p subshell (2p³), which is a stable configuration. In contrast, oxygen has one more electron in the 2p subshell (2p⁴), which makes it less stable due to increased electron-electron repulsion.

### Conclusion for Assertion
Since nitrogen has a half-filled configuration, it is less likely to lose an electron compared to oxygen. Therefore, the assertion that the first ionization energy of nitrogen is lower than that of oxygen is **false**.

### Step 2: Analyze the Reason
The reason states that "across a period effective charge decreases."

- **Effective Nuclear Charge (Z_eff)**: This is the net positive charge experienced by an electron in a multi-electron atom. It generally increases across a period due to the increase in the number of protons in the nucleus while the shielding effect remains relatively constant.

### Conclusion for Reason
As we move from left to right across a period, the effective nuclear charge actually **increases** because the number of protons increases while the shielding effect from inner electrons does not increase significantly. Therefore, the reason provided is also **false**.

### Final Conclusion
Both the assertion and the reason are false. Thus, the correct answer is that both statements are incorrect.

### Summary of the Solution
- Assertion: False (N2 has a higher ionization energy than O2).
- Reason: False (Effective nuclear charge increases across a period).