Q.47 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 47 Solution

Topic: Classification of Elements and Periodicity in Properties · Official NTA Booklet Code 45

Question

Which of the following statements are true? A. Unlike Ga that has a very high melting point, Cs has a very low melting point. B. On Pauling scale, the electronegativity values of N and Cl are not the same. C. Ar, K⁺, Cl⁻, Ca²⁺, and S²⁻ are all isoelectronic species. D. The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na. E. The atomic radius of Cs is greater than that of Li and Rb. Choose the correct answer from the options given below:

✓

Verified Against NTA Final Answer Key

Option (2) : C and E only

📚 Classification of Elements and Periodicity in Properties Medium

📋 Given Parameters & Problem Setup

Statement A Unlike Ga that has a very high melting point, Cs has a very low melting point.

Statement B On Pauling scale, the electronegativity values of N and Cl are not the same.

Statement C Ar, K+, Cl-, Ca2+, and S2- are all isoelectronic species.

Statement D The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na.

Statement E The atomic radius of Cs is greater than that of Li and Rb.

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The 30-Second Exam Hall Shortcut: ⚡ NCERT Periodic Counter-Fact Eliminator

⏱️ Save ~60 seconds

Ga melts in your hand (mp 303 K = 30°C, very low!) ⟹ A is FALSE! Both N and Cl have Pauling EN = 3.0 (identical!) ⟹ B is FALSE! Mg has full 3s² ⟹ Mg > Al ⟹ D is FALSE! Only C and E remain!

1 Check Statement A: Ga (303 K) and Cs (302 K) BOTH have unusually low melting points (liquid during summer). A claims Ga has very high mp ⟹ FALSE!

2 Eliminate options containing A: Options (1) and (4) are discarded instantly.

3 Check Statement B: On Pauling’s scale, both Nitrogen and Chlorine have the identical electronegativity value of 3.0. B claims they are not the same ⟹ FALSE!

4 Check Statement D: Due to stable filled 3s² subshell, Mg has higher IE₁ than Al (Mg = 737 kJ/mol, Al = 577 kJ/mol). The true order is Si > Mg > Al > Na. D claims Si > Al > Mg > Na ⟹ FALSE!

5 Eliminate Option (3) which includes D.

6 Only Option (2) (C and E only) survives!

7 Verify C: Ar(18), K+(18), Cl-(18), Ca2+(18), S2-(18) all have 18 electrons (isoelectronic, TRUE).

8 Verify E: Down Group 1, atomic radius increases: Li < Na < K < Rb < Cs (TRUE).

🛡️

Why You Can 100% Trust This Shortcut: Periodic Trends in Atomic and Ionic Radii (NCERT Chemistry · Classification of Elements and Periodicity in Properties).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Evaluate Statements A and B (Physical Properties & Electronegativity)

Analyze the canonical NCERT data for Gallium, Caesium, and Pauling electronegativities:

T_{mp}(\text{Ga}) = 303\text{ K} \,(30^\circ\text{C}), \quad T_{mp}(\text{Cs}) = 302\text{ K} \,(29^\circ\text{C})

• Both Ga and Cs have exceptionally LOW melting points and exist as liquids at room temperature during summer. Statement A is FALSE.
• Pauling scale values: EN(N) = 3.0 and EN(Cl) = 3.0. Their electronegativity values ARE identical. Statement B is FALSE.

Step 2

Evaluate Statement C (Isoelectronic Species)

Count the total number of electrons in each chemical species:

e^-(\text{Ar}) = 18, \; e^-(\text{K}^+) = 19 - 1 = 18, \; e^-(\text{Cl}^-) = 17 + 1 = 18, \; e^-(\text{Ca}^{2+}) = 20 - 2 = 18, \; e^-(\text{S}^{2-}) = 16 + 2 = 18

• All species have exactly 18 electrons and are therefore isoelectronic. Statement C is TRUE.

Step 3

Evaluate Statements D and E (Ionization Enthalpy & Atomic Radius)

Examine electronic configuration effects on ionization enthalpy and group trends in atomic radius:

\text{IE}_1: \; \text{Na}\,(496) < \text{Al}\,(577) < \text{Mg}\,(737) < \text{Si}\,(786\text{ kJ/mol})

• Mg ([Ne]3s²) has a fully filled 3s orbital with greater penetration than the 3p electron of Al ([Ne]3s² 3p¹), so IE₁(Mg) > IE₁(Al). Statement D is FALSE.
• Down the alkali metal group (Group 1), atomic radius increases monotonically with increasing principal quantum number: r(Li) < r(Na) < r(K) < r(Rb) < r(Cs). Statement E is TRUE.
• Conclusion: C and E only are true, matching Option (2).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Both A (Ga has low mp, not high) and B (N and Cl have same EN = 3.0) are false.

⚠️

Student Fallacy (Why candidates pick this trap): Believing Ga has a high melting point and N/Cl have different EN.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. C is isoelectronic (18e) and E follows group 1 atomic radius trend (Cs > Rb > Li).

💡

Why this is airtight: None; rigorous NCERT factual mastery.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Statement D is false because Mg > Al in first ionization enthalpy.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring the stable 3s² fully-filled configuration of Mg over Al.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Statement A is false.

⚠️

Student Fallacy (Why candidates pick this trap): Failing to recall the famously low melting point of Gallium.

⚠️

NTA Trap Alert: The Gallium Melting Point & Mg vs Al Ionization Trap

Gallium has an abnormally low melting point (30°C) due to its crystal structure of Ga₂ molecules, yet students often confuse it with Boron (high mp). Also, Mg > Al in IE₁ is the most common periodic trend trap in competitive exams.

🛡️ How to avoid the -1 mark: Remember: Ga melts on your palm! And s-penetration makes Mg > Al in IE₁.

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Classification of Elements and Periodicity in Properties Standard Benchmark

Core Scientific Principle Tested:

"Isoelectronic species have identical electron count; IE₁(Mg) > IE₁(Al); Pauling EN of N = Cl = 3.0."

Syllabus Focus: Periodic Trends in Chemical and Physical Properties

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ga melts in your hand, N and Cl equal 3.0, Mg beats Al in IE ⟹ C & E only!

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NEET 2025 Chemistry Question 47 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ NCERT Periodic Counter-Fact Eliminator

Step-by-Step Rigorous Workflow & Mathematical Proof

Evaluate Statements A and B (Physical Properties & Electronegativity)

Evaluate Statement C (Isoelectronic Species)

Evaluate Statements D and E (Ionization Enthalpy & Atomic Radius)

360° Option Elimination Engine Key Feature

NTA Trap Alert: The Gallium Melting Point & Mg vs Al Ionization Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Classification of Elements and Periodicity in Properties

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