This MCQ module is based on: Atomic Models — Thomson, Rutherford and Bohr
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Atomic Models — Thomson, Rutherford and Bohr
🎓 Class 9
Science
CBSE
Theory
Ch 8 — Journey Inside the Atom
⏱ ~13 min
🌐 Language: [gtranslate]
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This assessment will be based on: Atomic Models — Thomson, Rutherford and Bohr
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Introduction: Picturing the Inside of an Atom
Once experiments had revealed that the atom contains charged particles, the obvious next question was: how are these particles arranged inside the atom? A scientific picture or theoretical sketch of the structure of an atom is called an atomic model. In this part we look at four landmark models — Dalton, Thomson, Rutherford and Bohr — and see how each one improved on the previous one.
8.7 Dalton's Atomic Theory (1808)
John Dalton was the first to propose a clear scientific theory of the atom. His main statements were:
- All matter is made up of extremely tiny particles called atoms.
- Atoms cannot be created, destroyed or divided into smaller parts during a chemical reaction.
- All atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in small whole-number ratios to form compounds.
Dalton's theory beautifully explained the laws of conservation of mass and constant proportions, and it remained the working picture of the atom for nearly a hundred years. Its weakness, however, was the assumption that atoms are indivisible — an assumption proven wrong by the discovery of the electron.
8.8 Thomson's Plum-Pudding Model (1898)
After discovering the electron, J.J. Thomson knew the atom must contain something to balance the negative charge of its electrons, because atoms as a whole are electrically neutral. He proposed the plum-pudding model:
- An atom is a sphere of uniform positive charge, like a ball of pudding.
- Electrons are scattered throughout this positive sphere like raisins (or plums) in the pudding.
- The total positive charge equals the total negative charge, so the atom is electrically neutral.
This model successfully explained why atoms are neutral, but it could not explain why electrons stay in fixed positions, nor what would happen if a fast-moving particle were fired at an atom. The next experiment exposed exactly that weakness.
8.9 Rutherford's α-Scattering (Gold-Foil) Experiment, 1911
Ernest Rutherford and his students Geiger and Marsden directed a narrow beam of fast-moving α-particles (positively charged helium nuclei) at a very thin sheet of gold foil — only about 1000 atoms thick. A circular fluorescent screen surrounded the foil so that wherever an α-particle struck, a small flash of light was produced.
Observations
- Most α-particles passed straight through the foil with little or no deflection.
- A small fraction (about 1 in 8000) were deflected by small angles.
- An extremely tiny fraction bounced back along nearly the path they had come from.
Rutherford's reaction: "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Conclusions — the Nuclear Model
- Most of the atom is empty space — that is why most α-particles went straight through.
- The entire positive charge and almost the entire mass of the atom is concentrated in a very tiny central region called the nucleus.
- The size of the nucleus is extremely small compared to the whole atom — roughly \(10^{-15}\) m versus \(10^{-10}\) m for the atom.
- The electrons revolve around the nucleus in circular paths called orbits.
Drawback of Rutherford's Model
Classical electromagnetism predicts that any charged particle moving in a circular path continuously radiates energy. So a revolving electron should lose energy, slow down, spiral inward and crash into the nucleus within a fraction of a second. But atoms are clearly stable — they do not collapse. Rutherford's model could not explain this stability. A new idea was needed.
8.10 Activity — Modelling Rutherford's Experiment
Activity 8.2 — The Marble and the Cardboard BoxL3 Apply
Predict first: A box has many cards stuck inside it with a few small marbles glued in the centres. If you roll a steel ball through the box, how often will it hit a marble?
- Take a long flat cardboard box and stick five or six small marbles to its base, well separated.
- Cover the top of the box with a sheet of paper that has a thin slit at one end.
- Roll a steel ball through the slit so it travels across the box.
- Observe how often the ball goes straight through, how often it slightly changes direction, and how often it bounces back.
- Compare this with α-particles striking gold atoms.
Observations: Most attempts result in the ball travelling straight through the box without hitting anything. Occasionally the ball glances off a marble and changes direction by a small angle. Very rarely it strikes a marble head-on and rolls straight back.
Conclusion: Just as the cardboard box is mostly empty space with rare hard centres (the marbles), the atom is mostly empty space with a tiny massive nucleus at its centre. Rutherford's nuclear model is supported by exactly this kind of scattering pattern.
Conclusion: Just as the cardboard box is mostly empty space with rare hard centres (the marbles), the atom is mostly empty space with a tiny massive nucleus at its centre. Rutherford's nuclear model is supported by exactly this kind of scattering pattern.
8.11 Bohr's Model of the Atom (1913)
The Danish physicist Niels Bohr fixed Rutherford's stability problem by combining the nuclear atom with quantum ideas. The key statements (postulates) of Bohr's model are:
- Electrons revolve around the nucleus only in certain permitted orbits of definite energy. These orbits are called energy levels or shells.
- While moving in any one of these allowed orbits, an electron does not radiate energy. The atom is therefore stable.
- The shells are labelled K, L, M, N, … (counting outward from the nucleus) or numbered \(n = 1, 2, 3, 4, \ldots\)
- An electron can jump from one orbit to another. When it jumps to a higher orbit it absorbs a fixed amount of energy; when it falls back to a lower orbit it emits the same amount of energy as light.
🔬 Atomic Model Showdown — Click each model to compare L4 Analyse
Three big atomic models in one frame. Click each picture below — Thomson · Rutherford · Bohr — and compare what each captures correctly and where each one breaks down.
Click any of the three models above to compare what it gets right with what it leaves out.
Quick Recap
| Model | Year | Key Idea | Limitation |
|---|---|---|---|
| Dalton | 1808 | Atom is an indivisible solid sphere | Could not explain electron / charged particles |
| Thomson | 1898 | Electrons embedded in a sphere of positive charge | Failed to explain α-scattering |
| Rutherford | 1911 | Tiny dense nucleus + electrons in orbits | Could not explain atomic stability |
| Bohr | 1913 | Electrons in fixed energy shells, no radiation | Worked best for hydrogen-like atoms |
Competency-Based Questions
In a classroom demonstration, students are shown three model-atoms made of foam: (i) a uniform pink ball with blue beads stuck inside, (ii) a tiny red bead at the centre of a wire cage with blue beads moving along the wires, and (iii) the same red bead at the centre but with blue beads sitting only on three concentric rings.
Q1. Which model corresponds to Thomson's plum-pudding picture? L2
(a) Model (i) — a uniform positive sphere with electrons embedded inside, like raisins in a pudding.
Q2. State two observations of Rutherford's α-scattering experiment and what each observation implied. L3
(a) Most α-particles passed straight through ⇒ atom is mostly empty space. (b) A few α-particles bounced back ⇒ all the positive charge and nearly all the mass is concentrated in a very small region (the nucleus).
Q3. Why was Rutherford's model considered incomplete? L3
According to classical theory, an electron revolving around the nucleus should continuously radiate energy, spiral inward and crash into the nucleus. But real atoms are stable, which Rutherford could not explain. Bohr fixed this by proposing fixed non-radiating orbits.
Q4. State whether True or False: "In Bohr's model, an electron continuously emits energy as it moves in its orbit." L1
False. An electron moving in any one of its allowed Bohr orbits does not radiate energy. Energy is exchanged only when the electron jumps from one orbit to another.
Q5. If gold foil were replaced with a thin sheet of an element with much lighter nuclei, predict how the α-scattering pattern would change. L4
Lighter nuclei carry less positive charge and have less mass, so they would deflect α-particles by smaller angles and would rarely send them back. The number of large-angle scattering events would drop sharply.
Assertion–Reason Questions
Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.
A: Most α-particles passed undeflected through the gold foil.
R: The atom contains a large amount of empty space.
(A) Both true and R correctly explains A. Empty space allows most α-particles to fly through unaffected.
A: Bohr's model successfully explains atomic stability.
R: An electron in a permitted orbit does not radiate energy.
(A) Both true and R is the correct explanation. No energy loss in fixed orbits ⇒ the atom does not collapse.
A: Thomson's model could explain Rutherford's α-scattering results.
R: In Thomson's model the positive charge is spread uniformly inside the atom.
(D) Assertion is false — uniformly spread positive charge cannot deflect α-particles by large angles. Reason is true and is exactly why the model fails.
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