Q.136 Physics ⭐ 360° Option Elimination Active

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NEET 2020 Physics Question 136 Solution

Topic: Atoms · Official NTA Booklet Code E1

Question

For which one of the following, Bohr model is not valid?

(1)Hydrogen atom
(2)Singly ionised helium atom (He+)
(3)Deuteron atom
(4)Singly ionised neon atom (Ne+)

✓

Verified Against NTA Final Answer Key

Option (4) : Singly ionised neon atom (Ne+)

📚 Atoms Easy

📋 Given Parameters & Problem Setup

Theoretical Model Bohr Model of the Atom

Fundamental Assumption Applicable strictly to single-electron (hydrogenic) systems

Candidate Species H, He+, Deuteron atom (2_1H), Ne+

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The 30-Second Exam Hall Shortcut: ⚡ "Bohr Model is Valid ONLY for 1-Electron Systems: Ne+ Has 9 Electrons!"

⏱️ 20 seconds

The Bohr model accounts only for two-body electrostatic attraction between a nucleus and a SINGLE electron. Hydrogen (1e-), Singly ionized helium He+ (2 - 1 = 1e-), and Deuteron atom (isotope of hydrogen, 1e-) are all single-electron species. Singly ionized neon Ne+ has 10 - 1 = 9 electrons, where inter-electronic repulsions make the Bohr model completely invalid! Mark Ne+ (Option 4) in 5 seconds.

1 Bohr model postulate is valid only for hydrogen and hydrogen-like (single-electron) species.

2 Count electrons in each candidate: - Hydrogen atom (H): 1 electron (Valid). - Singly ionized helium (He+): 2 - 1 = 1 electron (Valid). - Deuteron atom (2_1H): isotope of H, 1 electron (Valid). - Singly ionized neon (Ne+): 10 - 1 = 9 electrons (Multi-electron system -> NOT valid).

3 Select Option (4) Singly ionised neon atom (Ne+).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Physics · Atoms (Bohr's Model of the Hydrogen Atom: Limitations)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Fundamental Principle of Bohr's Model

Niels Bohr formulated his quantized atomic model based on Coulomb's inverse-square law between the nuclear positive charge ( $+Ze$ ) and a single orbiting electron ( $-e$ ): $\frac{m v^2}{r} = \frac{1}{4\pi\varepsilon_0} \frac{Z e^2}{r^2}$ Because it considers only two-body central force interactions, it cannot account for multi-electron repulsions.

Step 2

Evaluate Number of Electrons in Each System

- Hydrogen atom ( $H$ ): Atomic number $Z = 1$ , contains 1 electron. - Singly ionized helium ( $He^+$ ): Helium has $Z = 2$ . After removing one electron, $He^+$ retains $2 - 1 = 1$ electron. - Deuteron atom ( $^2_1H$ ): Isotope of hydrogen with 1 proton, 1 neutron, and 1 orbiting electron. - Singly ionized neon ( $Ne^+$ ): Neon has atomic number $Z = 10$ . Removal of one electron leaves $10 - 1 = 9$ electrons.

Step 3

Identify the Multi-Electron Ion

Because $Ne^+$ is a 9-electron system, electron-electron repulsions and screening effects dominate, rendering the simple hydrogenic Bohr quantization condition invalid.

Step 4

Conclude Option 4

Therefore, the Bohr model is not valid for singly ionized neon ( $Ne^+$ ).

⭐

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; Bohr model was formulated specifically for the 1-electron neutral hydrogen atom.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming Bohr model fails for neutral hydrogen.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; $He^+$ has only 1 electron ( $Z = 2$ ) and is a classic hydrogenic system obeying Bohr formulas with $Z = 2$ .

⚠️

Student Fallacy (Why candidates pick this trap): Confusing nuclear charge with electron count.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; deuteron atom (deuterium) has a nucleus of 1 proton and 1 neutron, with 1 orbital electron, making it hydrogenic and fully valid under Bohr model.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing an isotope of hydrogen with a multi-electron species.

4 D

✓ 100% Correct

Verdict & Scientific Reason:

Factually exact; $Ne^+$ has 9 electrons, making it a multi-electron atom for which the Bohr model fails completely.

💡

Why this is airtight: None; core limitation of the Bohr model.

⚠️

NTA Trap Alert: Hydrogenic Species Definition

A hydrogenic (hydrogen-like) species must have EXACTLY ONE ELECTRON ( $H, He^+, Li^{2+}, Be^{3+}$ ). $Ne^+$ has 9 electrons, so it is NOT hydrogenic!

🛡️ How to avoid the -1 mark: Count remaining electrons:

Z - \text{charge}

. If

> 1

, Bohr model is NOT valid!

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Core NEET Syllabus Standard

Physics · Atoms Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Applicability of Bohr Atomic Model: Single-Electron Species Constraint

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Physics Question 136 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Bohr Model is Valid ONLY for 1-Electron Systems: Ne+ Has 9 Electrons!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Fundamental Principle of Bohr's Model

Evaluate Number of Electrons in Each System

Identify the Multi-Electron Ion

Conclude Option 4

360° Option Elimination Engine Key Feature

NTA Trap Alert: Hydrogenic Species Definition

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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