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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 from the options given below :

1. (1) A is true but R is false
2. (2) A is false but R is true
3. (3) Both A and R are true but R is NOT the correct explanation of A
4. (4) Both A and R are true and R is the correct explanation of A

Correct answer: (2) A is false but R is true

Solution

The assertion is incorrect because the first ionisation enthalpy actually increases across a period due to the increasing nuclear charge, which attracts electrons more strongly. The reason is true as it correctly describes the effect of nuclear charge and shielding, but it does not support the assertion.

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