Limitations of Bohr's Model Explained for NEET Students
Limitations of Bohr’s Model (NEET Level Notes)
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Key limitations of Bohr's atomic model explained for NEET and Class 11 students. |
Introduction
- Bohr’s model was an improvement over Rutherford’s model.
- It successfully explained:
- Stability of the hydrogen atom.
- Line spectrum of hydrogen and hydrogen-like ions such as He⁺, Li²⁺, Be³⁺, etc.
- However, Bohr’s model was too simple and could not explain many experimental observations.
1. Could Not Explain Fine Structure of Hydrogen Spectrum
What Bohr Explained
- Bohr explained that electrons move in fixed energy levels.
- When electrons jump between energy levels, spectral lines are produced.
Limitation
- Modern spectroscopes showed that some spectral lines are actually split into two or more very closely spaced lines.
Doublet
- A doublet means two very closely spaced spectral lines.
- Bohr's model could not explain why a single spectral line splits into a doublet.
NEET Point:
Bohr's model fails to explain the fine structure (doublet) of the hydrogen spectrum.
2. Could Not Explain Spectra of Multi-Electron Atoms
What Bohr Explained
- Bohr's theory works well only for:
- Hydrogen atom (1 electron)
- Hydrogen-like ions (only 1 electron)
Limitation
- It could not explain the spectra of atoms having more than one electron.
Example
- Helium atom (He) contains 2 electrons.
- Bohr's model fails to explain the spectral lines of helium and other multi-electron atoms.
NEET Point:
Bohr's model is applicable only to single-electron species and fails for multi-electron atoms.
3. Could Not Explain Zeeman Effect
What is Zeeman Effect?
- When spectral lines are observed in the presence of a magnetic field, they split into multiple lines.
- This phenomenon is called the Zeeman Effect.
Limitation
- Bohr's theory could not explain why spectral lines split in a magnetic field.
Definition:
Zeeman Effect: Splitting of spectral lines in the presence of a magnetic field.
NEET Point:
Bohr's model fails to explain the Zeeman Effect.
4. Could Not Explain Stark Effect
What is Stark Effect?
- When spectral lines are observed in the presence of an electric field, they split into multiple lines.
- This phenomenon is called the Stark Effect.
Limitation
- Bohr's model could not explain the splitting of spectral lines due to an electric field.
Definition:
Stark Effect: Splitting of spectral lines in the presence of an electric field.
NEET Point:
Bohr's model fails to explain the Stark Effect.
5. Could Not Explain Chemical Bonding
Limitation
- Bohr's model only described electrons revolving around the nucleus.
- It did not explain how atoms combine with each other.
Result
- The formation of molecules and chemical bonds could not be explained.
Examples
- H₂ molecule
- O₂ molecule
- H₂O molecule
Bohr's model gives no explanation for how these molecules are formed.
NEET Point:
Bohr's theory cannot explain the formation of chemical bonds and molecules.
Quick NEET Revision Table
| Limitation | Explanation |
|---|---|
| Fine structure (Doublet) | Cannot explain closely spaced spectral lines |
| Multi-electron atoms | Cannot explain spectra of He, Li, etc. |
| Zeeman Effect | Cannot explain splitting in magnetic field |
| Stark Effect | Cannot explain splitting in electric field |
| Chemical Bonding | Cannot explain formation of molecules and bonds |
One-Line NEET Summary
Bohr’s model successfully explains hydrogen and hydrogen-like ions but fails to explain fine spectral details, multi-electron atoms, Zeeman effect, Stark effect, and chemical bonding. ✅
Limitations of Bohr’s Model
│
├── 1. Fine Structure Not Explained
│ │
│ ├── Doublet lines observed
│ ├── Two closely spaced spectral lines
│ └── Bohr model cannot explain them
│
├── 2. Multi-Electron Atoms Not Explained
│ │
│ ├── Works only for single-electron species
│ ├── Hydrogen (H)
│ ├── He⁺, Li²⁺, Be³⁺
│ └── Fails for He, Li, Be atoms
│
├── 3. Zeeman Effect Not Explained
│ │
│ ├── Magnetic field applied
│ ├── Spectral lines split
│ └── Bohr model cannot explain splitting
│
├── 4. Stark Effect Not Explained
│ │
│ ├── Electric field applied
│ ├── Spectral lines split
│ └── Bohr model cannot explain splitting
│
├── 5. Chemical Bonding Not Explained
│ │
│ ├── How atoms combine
│ ├── Formation of molecules
│ ├── H₂, O₂, H₂O
│ └── Not explained by Bohr model
│
└── Conclusion
│
├── Explains hydrogen atom well
├── Explains hydrogen-like ions
└── Cannot explain complex atomic structure
and bonding phenomena
NEET Quick Memory Trick:
D-M-Z-S-C
D → Doublet (Fine Structure)
M → Multi-electron atoms
Z → Zeeman Effect
S → Stark Effect
C → Chemical Bonding
CBSE Class 11 Chemistry Question Bank on Limitations of Bohr's Model with answers.
MCQs (1 Mark Each)
1. Bohr's model successfully explains the spectrum of:
a) Helium atom
b) Hydrogen atom
c) Neon atom
d) Argon atom
Answer: b) Hydrogen atom
2. The splitting of spectral lines in a magnetic field is called:
a) Stark Effect
b) Zeeman Effect
c) Photoelectric Effect
d) Compton Effect
Answer: b) Zeeman Effect
3. The splitting of spectral lines in an electric field is called:
a) Zeeman Effect
b) Stark Effect
c) Raman Effect
d) Doppler Effect
Answer: b) Stark Effect
4. Bohr's model cannot explain the spectrum of:
a) H atom
b) He⁺ ion
c) Li²⁺ ion
d) He atom
Answer: d) He atom
5. Which of the following is NOT a limitation of Bohr's model?
a) Explains hydrogen spectrum
b) Cannot explain chemical bonding
c) Cannot explain Zeeman effect
d) Cannot explain Stark effect
Answer: a) Explains hydrogen spectrum
Very Short Answer Questions (1 Mark)
1. What is a doublet?
Answer: Two closely spaced spectral lines are called a doublet.
2. Define Zeeman Effect.
Answer: Splitting of spectral lines in the presence of a magnetic field is called Zeeman Effect.
3. Define Stark Effect.
Answer: Splitting of spectral lines in the presence of an electric field is called Stark Effect.
4. For which type of atoms is Bohr's model most successful?
Answer: Single-electron atoms and ions.
5. Name one phenomenon that Bohr's model could not explain.
Answer: Zeeman Effect.
Short Answer Questions (2–3 Marks)
1. Why is Bohr's model considered incomplete?
Answer:
- It cannot explain fine spectral details.
- It fails for multi-electron atoms.
- It cannot explain Zeeman and Stark effects.
- It cannot explain chemical bonding.
2. Why does Bohr's model fail for helium atom?
Answer: Bohr's model is applicable only to single-electron systems. Helium contains two electrons, and the interactions between electrons cannot be explained by Bohr's theory.
3. What are hydrogen-like ions? Give examples.
Answer: Hydrogen-like ions contain only one electron.
Examples:
- He⁺
- Li²⁺
- Be³⁺
Long Answer Questions (5 Marks)
1. Discuss the limitations of Bohr's atomic model.
Answer:
Although Bohr's model successfully explained the hydrogen atom spectrum and atomic stability, it has several limitations:
- It cannot explain the fine structure (doublets) of hydrogen spectral lines.
- It fails to explain spectra of multi-electron atoms like helium.
- It cannot explain the Zeeman Effect (splitting in magnetic field).
- It cannot explain the Stark Effect (splitting in electric field).
- It fails to explain the formation of chemical bonds and molecules.
Therefore, a more advanced atomic theory was needed.
Assertion and Reason Questions
1.
Assertion (A): Bohr's model explains the hydrogen atom spectrum.
Reason (R): Hydrogen atom contains only one electron.
a) Both A and R are true and R is the correct explanation.
b) Both A and R are true but R is not the correct explanation.
c) A is true but R is false.
d) A is false but R is true.
Answer: a)
2.
Assertion (A): Bohr's model explains the Zeeman Effect.
Reason (R): Zeeman Effect is splitting of spectral lines in a magnetic field.
Answer: d) A is false but R is true.
3.
Assertion (A): Bohr's theory cannot explain chemical bonding.
Reason (R): Bohr's model deals only with electrons revolving around the nucleus.
Answer: a)
Fill in the Blanks
-
Splitting of spectral lines in a magnetic field is called Zeeman Effect.
-
Splitting of spectral lines in an electric field is called Stark Effect.
-
Bohr's model is most successful for single-electron atoms.
-
A helium atom contains two electrons.
-
Bohr's model cannot explain chemical bonding.
Statement-Based Questions
Question 1
Read the statements:
I. Bohr's model explains hydrogen spectrum.
II. Bohr's model explains spectra of all atoms.
Choose the correct option:
a) Both statements are true
b) Both statements are false
c) Statement I is true, Statement II is false
d) Statement I is false, Statement II is true
Answer: c)
Question 2
I. Zeeman Effect occurs in magnetic field.
II. Stark Effect occurs in electric field.
a) Both are true
b) Both are false
c) Only I is true
d) Only II is true
Answer: a)
Match the Columns
| Column A | Column B |
|---|---|
| A. Zeeman Effect | 1. Electric Field |
| B. Stark Effect | 2. Magnetic Field |
| C. Doublet | 3. Closely Spaced Lines |
| D. He⁺ | 4. One Electron |
Answer
A → 2
B → 1
C → 3
D → 4
Case Study-Based Questions
Case Study
Bohr proposed that electrons revolve around the nucleus in fixed energy levels. His theory successfully explained the stability and spectrum of hydrogen atom. However, later experiments showed that spectral lines split into multiple lines under magnetic and electric fields. The theory also failed to explain spectra of multi-electron atoms and chemical bonding.
Questions
1. Which atom's spectrum was successfully explained by Bohr's model?
Answer: Hydrogen atom.
2. What is the splitting of spectral lines in a magnetic field called?
Answer: Zeeman Effect.
3. What is the splitting of spectral lines in an electric field called?
Answer: Stark Effect.
4. Why does Bohr's model fail for helium atom?
Answer: Helium has two electrons, while Bohr's model works only for single-electron systems.
HOTS (Higher Order Thinking Skills)
1.
Why was a new atomic theory required even after the success of Bohr's model?
Answer: A new theory was needed because Bohr's model could not explain fine spectral details, multi-electron atoms, Zeeman effect, Stark effect, and chemical bonding.
2.
A scientist observes splitting of spectral lines when a magnetic field is applied. Which limitation of Bohr's model is demonstrated?
Answer: Its inability to explain the Zeeman Effect.
Important CBSE Exam One-Liners
- Bohr's model works only for single-electron species.
- It fails to explain doublets in hydrogen spectrum.
- It cannot explain spectra of multi-electron atoms.
- It cannot explain Zeeman Effect.
- It cannot explain Stark Effect.
- It cannot explain chemical bonding.
- These limitations led to the development of Quantum Mechanical Model of Atom.
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