Curriculum
Science Grade IX
Structure Of Atom
School Revise · Grade 9 · Advanced Science (Optional) · Chapter 6
Structure of Atom
Science at Advanced Level, Class 9. This optional chapter follows how the electron, proton and neutron were discovered, what a line spectrum reveals, and how Bohr’s model of fixed orbits explained it.
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What this chapter is about
The atom was once thought indivisible. Careful experiments with glowing tubes and glowing gases proved otherwise. In this chapter we trace the discovery of the three sub atomic particles, learn what the sharp lines of a spectrum tell us, and see how Bohr’s model made sense of them.
1. Discovering the electron, proton and neutron
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The three discoveries Thomson found the electron in cathode rays; Goldstein’s canal rays led to the proton; Chadwick found the neutron Cathode rays bent toward the positive plate, so they are negative electrons. Canal rays bent toward the negative plate, so they are positive, the lightest from hydrogen being the proton. Atoms weighed more than their protons and electrons, so Chadwick’s neutral neutron was needed. In real life: A television or an old computer monitor once glowed because a beam of these very electrons, cathode rays, struck the screen. |
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Worked example 1 Question. Cathode rays bend towards the positive plate. What does this show about them?
Answer: They are negatively charged particles, electrons. |
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Worked example 2 Question. Helium has 2 protons yet weighs about 4 times a hydrogen atom. What does this suggest?
Answer: There must be neutrons, the neutral particles Chadwick discovered. |
2. Spectra and the Bohr model
White light through a prism gives a continuous rainbow. Glowing hydrogen gives only a few sharp coloured lines, a line spectrum. This was a puzzle Rutherford’s model could not solve.
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Bohr’s model electrons move only in fixed orbits, and light is given out or taken in only when an electron jumps between them Because only certain orbits, and so certain energy jumps, are allowed, only certain colours appear, which is exactly a line spectrum. This also explained why the atom is stable, which Rutherford’s model could not. In real life: The orange glow of a sodium street lamp is light from electrons jumping between fixed orbits in sodium atoms, its own line spectrum. |
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Worked example 3 Question. Why does hydrogen give a line spectrum and not a continuous one?
Answer: Because only fixed energy jumps are allowed, giving only certain lines. |
Practise with the interactive
Try this chapter hands on: change the values and watch the result update live and animate. The interactive opens right here in the lesson.
Practice set A, multiple choice
1. Cathode rays are made of …
Electrons, negatively charged particles, shown by their bending towards the positive plate.
2. The neutron was discovered by …
Chadwick, who bombarded beryllium with alpha particles and found a neutral radiation.
3. A line spectrum shows that electron energies are …
Fixed, taking only certain values, so only certain energy jumps and certain colours appear.
4. Canal rays carry a … charge.
Positive charge, since they bend towards the negative plate; the lightest, from hydrogen, is the proton.
Practice set B, short answer
1. Why did scientists propose the neutron even after finding the electron and proton?
Because atoms weighed more than their protons and electrons alone could account for, so a neutral particle, the neutron, was needed to explain the extra mass.
2. State one thing Rutherford’s model could not explain.
It could not explain the stability of the atom, nor why hydrogen gives a line spectrum rather than a continuous one.
3. How does Bohr’s model explain a line spectrum?
Electrons occupy only fixed orbits, so only certain energy differences exist; light is emitted only in those fixed amounts, giving sharp lines of only certain colours.
Quick summary
| Idea | The idea |
| Electron | Cathode rays; negative; Thomson. |
| Proton | Canal rays; positive; from hydrogen. |
| Neutron | Neutral; Chadwick; explains mass. |
| Line spectrum | Only certain colours. |
| Bohr orbits | Fixed energy levels. |
| Light | Emitted on jumps between orbits. |
| Open the Virtual Lab |
These free Grade 9 Advanced Science notes explain the discovery of subatomic particles, spectra and the Bohr model, with clear step by step worked examples and labelled diagrams for every student using the optional Advanced Level book.
© 2026 School Revise. All rights reserved. This lesson is original content written by School Revise, aligned to the CBSE Class 9 Science at Advanced Level (Optional) syllabus. Unauthorised copying, reproduction or redistribution is not permitted. Curriculum names are used only to indicate alignment.
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