AIIMS 2005 Chemistry Periodic Properties Assertion Reason Question

To determine the correct answer to this question, we need to analyze both the assertion and the reason provided in context of periodic trends in ionization energy.

Assertion: First ionization energy for nitrogen is lower than oxygen.

Explanation: The assertion is false. The first ionization energy is the energy required to remove the outermost electron from a neutral atom in the gaseous state. As we move across a period in the periodic table (from left to right), the ionization energy generally increases due to an increase in effective nuclear charge.

For nitrogen (atomic number 7) and oxygen (atomic number 8), the first ionization energies are:

• For nitrogen: Approximately 1402 kJ/mol
• For oxygen: Approximately 1314 kJ/mol

Although oxygen has a higher atomic number, it has a lower first ionization energy than nitrogen because the added electron in oxygen enters the same p subshell, leading to increased electron-electron repulsion which slightly offsets the increase in nuclear charge. However, nitrogen's ionization energy is indeed higher than oxygen's, making the assertion false.

Reason: Across a period effective nuclear charge decreases.

Explanation: The reason is also false. Effective nuclear charge (Z_eff) actually increases across a period. This is due to the fact that as protons are added to the nucleus, the increased positive charge attracts the electrons more strongly, which is not significantly offset by the added electron shielding (as electrons are being added to the same energy level).

The effective nuclear charge can be estimated using the formula:

$Z_{\text{eff}} = Z - S$

where $Z$ is the atomic number and $S$ is the shielding constant. As $Z$ increases, $Z_{\text{eff}}$ also increases across a period.

Conclusion:

Since both the assertion and the reason are false, the correct answer is D) If both assertion and reason are false.

Why Other Options Are Incorrect:

• Option A: This option states that both assertion and reason are true and that the reason is the correct explanation of the assertion, which is incorrect as both are false.
• Option B: This option also states that both assertion and reason are true but the reason is not the correct explanation, which is incorrect for the same reason.
• Option C: This states that the assertion is true but the reason is false, which is not applicable since the assertion is false.

Summary:

• Both the assertion and reason are false.
• The first ionization energy of nitrogen is actually higher than that of oxygen.
• The effective nuclear charge increases across a period, not decreases.

Thus, the correct answer is D) If both assertion and reason are false.