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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 from the options given below:

Solution & Explanation

### Core Logic Assertion A states that the first ionisation enthalpy decreases across a period. This is false, because first ionisation energy generally increases along a period from left to right. Reason R states that the increasing nuclear charge outweighs the shielding across the period. This is true, and it is the exact reason why the atomic radius decreases and ionisation enthalpy increases across a period. ### Step 1: Final Conclusion Since the assertion is incorrect and the reason is a factually correct statement, A is false but R is true. ### Pattern Recognition Recall the horizontal periodic trends: across a period, effective nuclear charge (Z_texteff) dominates, pulling valence electrons tighter, increasing ionisation energy. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity in Properties

More Classification of Elements and Periodicity in Properties Previous-Year Questions — Page 6

Q80 jee_main_2024_31_jan_evening Periodic Trends in Properties
Consider the following elements.
Group downarrowA^primeB^prime rightarrow Period
C^primeD^prime
Which of the following is/are true about A^prime, B^prime, C^prime and D^prime? A. Order of atomic radii: B^prime < A^prime < D^prime < C^prime B. Order of metallic character: B^prime < A^prime < D^prime < C^prime C. Size of the element: D^prime < C^prime < B^prime < A^prime D. Order of ionic radii: B^prime + < A^prime + < D^prime + < C^prime + Choose the correct answer from the options given below:
  • A. text1. A only
  • B. text2. A, B and D only
  • C. text3. A and B only
  • D. text4. B, C and D only

Solution

### Core Logic From the grid layout, A^prime and B^prime are in the same period (left to right), while C^prime and D^prime are below them in the next period. In general, moving along a period from left to right, size decreases and metallic character decreases. Moving down a group, size increases and metallic character increases. Atomic Radii / Size: B^prime < A^prime (across period) D^prime < C^prime (across period) A^prime < C^prime and B^prime < D^prime (down the group) Combining them: B^prime < A^prime < D^prime < C^prime. (Statement A is correct, C is incorrect). Metallic Character: B^prime < A^prime and D^prime < C^prime Overall trend follows size closely for these blocks: B^prime < A^prime < D^prime < C^prime. (Statement B is correct). Ionic Radii (+ ions): Follows the same basic trend as atomic radii: B^prime + < A^prime + < D^prime + < C^prime +. (Statement D is correct). ### Step 1: Final Selection Statements A, B, and D are correct. Statement C contradicts A, so it is incorrect. Option 2 is the right choice. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity in Properties
Q70 jee_main_2024_31_jan_morning Electron Gain Enthalpy
The correct sequence of electron gain enthalpy of the elements listed below is A. Ar B. Br C. F D. S Choose the most appropriate from the options given below:
  • A. C > B > D > A
  • B. A > D > B > C
  • C. A > D > C > B
  • D. D > C > B > A

Solution

### Core Logic Electron gain enthalpy (Delta_egH) values are considered with their signs when ordering from largest to smallest algebraically.
ElementDelta_egH text (kJ/mol)
F-333
S-200
Br-325
Ar+96
Arranging these algebraically from highest to lowest: +96 > -200 > -325 > -333 Thus, Ar > S > Br > F, which corresponds to A > D > B > C. ### Pattern Recognition Noble gases always have positive electron gain enthalpies (highest algebraically). Halogens have highly negative values (lowest algebraically). Fluorine is highly negative, Bromine is slightly less negative. Sulfur sits in between. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity in Properties

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