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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 Ionization Enthalpy.

Year 2026 2025 2024 Total
Questions 9 16 11 36
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The correct trend in the first ionization enthalpies of the elements in the 3rd period of periodic table is:

Solution & Explanation

Core Logic

In general, on moving from left to right across a period, the first ionization energy increases due to an increase in effective nuclear charge (Zeff). However, there are exceptions due to stable electronic configurations.

Step 1: Configuration Analysis

For elements Al, Si, P, S, and Cl: Generally, Al < Si < P < S < Cl. But, Phosphorus (1s² 2s² 2p⁶ 3s² 3p³) has a half-filled, exceptionally stable 3p subshell compared to Sulfur (3s² 3p⁴). This makes it harder to remove an electron from P than from S.

Step 2: Final Trend Construction

Because of this half-filled stability, the ionization energy of P is greater than that of S. Therefore, the corrected trend becomes: Al < Si < S < P < Cl

Pattern Recognition

Always look for Group 15 (half-filled np³) vs Group 16 (np⁴) anomalies. Group 15 always has a higher first ionization energy than Group 16 in the same period.

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_2026_23_january_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%)