Given below are two statements: Assertion A: The first ionisation enthalpy decreases across a period. Reason R: The increasing nuclear charge outweighs the shielding across the period.
Both A and R are true and R is the correct explanation of A
A is true but R is false
A is false but R is true
Both A and R are true but R is NOT the correct explanation of A
Correct answer:C
Standard Method
Given: Assertion A says that the first ionisation enthalpy decreases across a period. Reason R says that increasing nuclear charge outweighs shielding across a period.
Find: Which option correctly identifies the truth of Assertion A and Reason R.
Across a period, atomic number increases from left to right. Hence nuclear charge increases. The added electrons enter the same shell, so shielding does not increase enough to offset the increased attraction by the nucleus.
Therefore, electrons are held more strongly and the first ionisation enthalpy generally increases across a period, not decreases. So Assertion A is false.
The statement in Reason R is correct because the increasing nuclear charge outweighs the shielding effect across the period. So Reason R is true.
Conclusion: The correct option is C. Therefore, A is false but R is true.
Detailed Explanation
Given:
Find: Decide the correct relation between Assertion A and Reason R.
As we move across a period, the number of protons in the nucleus increases. This increases nuclear charge. Although electrons are also added, they are added to the same principal shell, so the shielding effect does not increase significantly.
Because of this, the effective attraction on the outer electron increases. Removing an electron becomes more difficult, so the first ionisation enthalpy generally increases across a period.
Hence the assertion saying it decreases across a period is incorrect, while the reason correctly explains the actual trend.
Conclusion: Assertion A is false and Reason R is true. So the correct answer is Option (3), that is, C.
Assuming ionisation enthalpy decreases across a period because atomic size changes without considering effective nuclear charge. This is wrong because increasing nuclear charge dominates. Instead, track how strongly the nucleus holds the valence electron.
Confusing shielding across a period with shielding down a group. Across a period, electrons enter the same shell, so shielding does not rise much. Instead, remember that the weak change in shielding makes effective nuclear attraction larger.
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