Q.136 Physics ⭐ 360° Option Elimination Active

Vault ↗

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+

⚡

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!

📖

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 ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 180-minute timed NTA simulator.

Launch NEET 2020 CBT Test →

All Physics Questions Grid Next (Q.137) →

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2020 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

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?

🔗 Related Questions — Atoms

Quantized Magnetic Flux in Magnetic Field & Electron Magnetic Moment

Bohr Quantization with Constant Inward Central Force

Wavelengths of Balmer Spectral Lines in Hydrogen

Electrical Neutrality, Stability, and Characteristic Emission Spectra of Atoms

Solve 4 NEET Questions a Day — One per Subject