Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R:
Assertion A: The first ionisation enthalpy decreases across a period.
Reason R: The increasing nuclear charge outweighs the shielding across the period.
In the light of the above statements, choose the most appropriate from the options given below:

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

Answer 1

The Correct Option is C

To address this issue, we will evaluate the provided statements and determine the correct relationship between the assertion and the reason. The breakdown is as follows:

1. Assertion (A): "First ionisation enthalpy decreases across a period."
2. Reason (R): "The increasing nuclear charge overrides the shielding effect across the period."

Analysis:

1. As one moves from left to right across a period in the periodic table, atomic numbers increase. This indicates an increase in protons within the nucleus, leading to a higher nuclear charge.
2. With an augmented nuclear charge, electrons are drawn nearer to the nucleus, escalating the difficulty of electron removal. Consequently, the first ionisation enthalpy typically increases across a period, not decreases. Thus, Assertion A is false.
3. The stated reason, "The increasing nuclear charge outweighs the shielding across the period," is accurate. Although electrons are added as one progresses across a period, they are added to the same electron shell. Therefore, the shielding effect does not significantly increase, and the enhanced nuclear charge more effectively binds the electrons.

Based on this analysis:

1. Assertion A is inaccurate because first ionisation enthalpy actually rises across a period.
2. Reason R is accurate as it correctly explains the factors influencing ionisation enthalpy across a period.

Conclusion:

The correct selection is: A is false but R is true.