Question 51. 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 option.
(1) Both A and R are true and R is the correct explanation of A
(2) A is true but R is false
(3) A is false but R is true
(4) Both A and R are true but R is NOT the correct explanation of A
Answer: (3)
Explanation:
Across a period, the nuclear charge Z increases.
The shielding effect remains almost constant.
The increased nuclear charge outweighs the shielding effect.
As a result, the first ionisation enthalpy increases across a period.
Therefore, Assertion A is false. Reason R is true.