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:
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:
- 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
The Correct Option is C
Approach Solution - 1
To solve this problem, we need to analyze the given statements and decide which option appropriately reflects the relationship between the assertion and the reason. Let's break it down:
- Assertion (A): "The first ionisation enthalpy decreases across a period."
- Reason (R): "The increasing nuclear charge outweighs the shielding across the period."
Explanation:
- Across a period in the periodic table, as we go from left to right, elements have increasing atomic numbers. This means more protons are added to the nucleus, increasing the nuclear charge.
- As the nuclear charge increases, electrons are pulled closer to the nucleus, making it more difficult to remove an electron. Thus, the first ionisation enthalpy generally increases across a period, not decreases. Therefore, Assertion A is false.
- The reason provided, "The increasing nuclear charge outweighs the shielding across the period," is true. Although electrons are being added as we move across a period, they are added to the same shell. Therefore, the shielding effect does not increase significantly, and the increased nuclear charge effectively holds the electrons more strongly.
Based on the above explanation:
- The Assertion A is incorrect because the first ionisation enthalpy actually increases across a period.
- The Reason R is correct because it correctly describes why ionisation enthalpy changes as it does across a period.
Conclusion:
The correct choice is: A is false but R is true.
Approach Solution -2
- The statement in Assertion A is incorrect because the first ionisation enthalpy actually increases across a period, not decreases. This is due to the increasing effective nuclear charge as we move across a period, which holds the electrons more tightly, making it harder to remove them.
- The statement in Reason R is true; the increase in nuclear charge indeed outweighs the shielding effect across a period, leading to a higher ionisation enthalpy.
Thus, A is false but R is true.
So, the correct answer is: Option (3)
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