Class 9 · Advanced ScienceNCERT 2026 Revised Syllabus
Chapter 6: Structure of Atom
Science Advanced
Discovery of electron, proton and neutron (cathode rays, canal rays, Chadwick), Rutherford’s model, Bohr’s model, and line vs continuous spectra.
Find complete NCERT solutions for Class 9 Advanced Science, Science Advanced Chapter 6. Every question has a clear, step-by-step answer.
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Quick Check (Discovery of Subatomic Particles)(10 questions)
1
Why do cathode rays bend towards the positive plate?
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Solution:- Cathode rays consist of negatively charged particles (electrons).
- They are attracted toward the positively charged plate in an electric field.Answer: Cathode rays are attracted to the positive plate because electrons are negatively charged.
2
What conclusion did Thomson draw from using different gases in discharge tubes?
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Solution:- Cathode ray properties remained the same regardless of the gas used.
- Thomson concluded electrons are a fundamental constituent present in atoms of all elements.Answer: Electrons are a fundamental constituent present in atoms of all elements.
3
Why are canal rays different from cathode rays in nature?
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Solution:- Canal rays are positively charged, and their mass depends on the type of gas used.
- Cathode rays are negatively charged electrons whose properties are independent of the gas.Answer: Canal rays are positive with gas-dependent mass; cathode rays are negative electrons independent of the gas.
4
Why was the discovery of the neutron necessary?
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Solution:- Atomic mass was found to be greater than the combined mass of protons and electrons.
- This indicated an additional neutral particle must contribute to the atom's mass.Answer: The extra mass beyond protons and electrons required a neutral particle - the neutron.
5
In a cathode ray experiment, rays bend towards a positively charged plate. What can we conclude about the nature of these rays?
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Solution:- The rays bend toward the positive plate.
- Opposite charges attract each other.Answer: The rays must carry a negative charge.
6
The gas is changed from hydrogen to neon, but cathode ray behaviour remains unchanged. What does this tell us about electrons?
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Solution:- Cathode ray behaviour is unchanged from hydrogen to neon.
- Electrons are a universal constituent of all atoms, regardless of the gas used.Answer: Electrons are a universal constituent of all atoms.
7
If cathode rays were neutral instead of negatively charged, how would their behaviour differ in an electric field?
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Solution:- Neutral rays carry no charge.
- They would travel undeflected in a straight line through the electric field, with no attraction toward either plate.Answer: Neutral rays would pass straight through undeflected.
8
In an experiment with canal rays, different gases give different masses of particles observed. What conclusion can be drawn?
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Solution:- Canal rays are not a single fundamental particle.
- They consist of positive ions of the specific gas used, so their mass varies with the gas.Answer: Canal rays are positive ions of the specific gas, so their mass varies with the gas.
9
Why did scientists feel the need to propose the existence of neutral particles even after discovering electrons and protons? Explain using helium.
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Solution:- Helium's mass is about four times that of hydrogen, yet it has only two protons.
- This extra mass could only be explained by two additional neutral particles (neutrons).Answer: Helium's mass needed two extra neutral particles - neutrons.
10
In Chadwick’s experiment, the emitted particles were not deflected by electric or magnetic fields. What does this indicate about the nature of these particles?
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Solution:- Particles were not deflected by electric or magnetic fields.
- They carried no electric charge.
- Confirmed as neutral particles, later named neutrons.Answer: The particles carried no charge - they were neutrons.
Check Your Understanding(17 questions)
1
The hydrogen spectrum consists of only a few sharp spectral lines instead of a continuous spectrum. What information does it provide about the energy of electrons in an atom?
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Solution:- Only a few sharp spectral lines are observed.
- Electrons can only have fixed, discrete energy values (quantised), not a continuous range.Answer: Electrons have fixed, quantised energy values.
2
Explain why Rutherford’s model would predict a continuous spectrum rather than a line spectrum.
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Solution:- Rutherford's model allowed electrons to have any continuous energy while revolving around the nucleus.
- Radiated energy would form a continuous, not discrete, spectrum.Answer: Continuous electron energies give a continuous spectrum.
3
A discharge tube filled with an unknown gas produces a line spectrum identical to hydrogen. What can you conclude about the gas? Give reason.
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Solution:- A line spectrum is characteristic and unique to each element.
- The unknown gas produced a line spectrum identical to hydrogen.Answer: The gas is likely hydrogen.
4
If electrons in an atom were allowed to have a continuous set of energy values, what kind of spectrum would you expect? Why is this not observed?
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Solution:- A continuous spectrum would be expected.
- This is not observed because electrons occupy only fixed, quantised energy levels, producing distinct line spectra.Answer: A continuous spectrum would be expected, but quantised energy levels give line spectra.
5
‘Bohr’s model solved all problems of atomic structure.’ Comment.
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Solution:- This statement is incorrect.
- Bohr's model explained only hydrogen's spectrum.
- It failed to explain multi-electron atom spectra, fine spectral lines, and effects of external fields.Answer: Bohr's model solved only hydrogen's spectrum, not all problems of atomic structure.
6
How does the concept of fixed energy levels explain the stability of atoms?
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Solution:- Electrons can exist only in specific allowed orbits.
- They do not continuously radiate energy.
- They do not spiral into the nucleus, keeping the atom stable.Answer: Fixed energy levels prevent electrons from spiralling into the nucleus, keeping atoms stable.
7
Why do different elements produce different line spectra? Give a conceptual explanation.
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Solution:- Each element has a unique set of allowed electron energy levels.
- The specific energy jumps and resulting emitted radiation wavelengths differ between elements.Answer: Different elements have different energy levels, so their line spectra differ.
8
Explain why Bohr’s model works well for hydrogen but not for multi-electron atoms.
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Solution:- Bohr's model considers only a single electron-nucleus interaction.
- Multi-electron atoms have additional electron-electron repulsions that this simple model cannot account for.Answer: Bohr's model ignores electron-electron repulsions, so it fails for multi-electron atoms.
9
State two limitations of Rutherford’s model.
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Solution:- It could not explain atomic stability - an orbiting electron would continuously lose energy.
- It could not explain the observed line spectrum of hydrogen.Answer: (1) Cannot explain atomic stability. (2) Cannot explain hydrogen's line spectrum.
10
Rutherford’s model explained the structure of the atom but failed to explain atomic stability and spectra. Discuss.
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Solution:- Rutherford correctly proposed a small, dense, positively charged nucleus with orbiting electrons.
- He could not explain why electrons do not spiral inward.
- He could not explain why only discrete spectral lines are observed.Answer: Correct nucleus model, but failed on atomic stability and line spectra.
11
What was the main drawback of Rutherford’s model regarding electron motion? What assumption did Bohr make to overcome this problem?
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Solution:- Electrons revolving continuously would lose energy and collapse into the nucleus.
- Bohr assumed electrons move only in fixed orbits without radiating energy.Answer: Drawback - electrons would collapse into the nucleus; Bohr assumed fixed, non-radiating orbits.
12
How does Bohr’s model explain the line spectrum of hydrogen?
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Solution:- Electrons jump between fixed energy levels.
- They release energy as radiation only in fixed amounts equal to the energy difference between levels.
- This produces distinct spectral lines.Answer: Fixed energy jumps between levels produce hydrogen's distinct spectral lines.
13
Outline the limitations of Bohr’s model.
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Solution:- Could not explain fine spectral line splitting.
- Could not explain the spectra of atoms other than hydrogen.
- Could not explain splitting in magnetic (Zeeman) or electric (Stark) fields.Answer: Bohr's model fails for fine structure, other atoms, and Zeeman/Stark effects.
14
Define line spectrum and continuous spectrum with one example each.
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Solution:- Continuous spectrum: an unbroken band of all colours, e.g., white light through a prism.
- Line spectrum: only distinct bright lines, e.g., the sodium vapour lamp spectrum.Answer: Continuous - white light through a prism; Line - sodium vapour lamp spectrum.
15
Write two main postulates of Bohr’s model.
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Solution:- Electrons revolve only in certain fixed orbits without radiating energy.
- Radiation is emitted or absorbed only when an electron jumps from one allowed orbit to another.Answer: (1) Fixed orbits without radiation. (2) Energy emitted/absorbed on jumping between orbits.
16
What is meant by fine structure in the hydrogen spectrum?
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Solution:- Fine structure refers to closely spaced additional spectral lines.
- Observed with sophisticated spectroscopic techniques.
- Bohr's simple model could not explain them.Answer: Fine structure is the closely spaced additional spectral lines Bohr's model could not explain.
17
What is the significance of the Rydberg equation?
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Solution:- The Rydberg equation empirically predicts the energies and wavelengths of all spectral lines in the hydrogen spectrum.
- It uses simple integer values representing the energy levels.Answer: It predicts the wavelengths of all hydrogen spectral lines using integer energy-level values.