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Classification of Elements and Periodicity in Properties appeared 36 times across 3 years — 4.2% of Chemistry. This question is from Atomic Radii Trends.

Year 2026 2025 2024 Total
Questions 9 16 11 36
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The incorrect decreasing order of atomic radii is :

Solution & Explanation

Related Formula

Atomic radius decreases across a period due to increase in effective nuclear charge (Zeff) and increases down a group due to addition of new electronic shells.

Core Logic

Let us analyze the elements in option (3):

  • Be and Mg belong to Group 2. Down the group, atomic radius increases: Mg > Be.
  • Mg, Al, and Si belong to Period 3. Across the period, atomic radius decreases: Mg > Al > Si.

    • Combining these trends, the correct decreasing order is:

Mg > Be > Al > Si

Therefore, the order given in option (3) Be > Mg > Al > Si is incorrect.

Pattern Recognition

Sees: Group 2 and Period 3 comparison. Trap: Assuming atomic radius always increases down a group regardless of cross-period shifts; Mg is larger than Be, making Be > Mg fundamentally incorrect. Shortcut: Check adjacent group/period boundaries to immediately isolate inversions.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Reference Study Guides

Classification Of Elements And Periodicity In Properties

More Classification of Elements and Periodicity in Properties Previous-Year Questions — Page 8

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 (ΔegH) values are considered with their signs when ordering from largest to smallest algebraically.

ElementΔegH (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.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

More Classification of Elements and Periodicity in Properties Questions — jee_main_2025_28_jan_morning

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JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%) | JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%)