Which observation in Rutherford's gold foil experiment directly contradicts Thomson's plum pudding model?

A few alpha particles were deflected at large angles

Alpha particles are helium nuclei

The gold foil was extremely thin

Assertion (A): Cathode rays are streams of negatively charged particles. Reason (R): Cathode rays are deflected towards the negative electrode in an electric field.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Rutherford used a very thin gold foil. Predict how the observations would change if the foil were much thicker, and justify your answer.

Fewer alpha particles would pass through undeflected; more would be scattered at various angles, and some might even be absorbed or bounced back due to multiple encounters with nuclei.

According to Rutherford’s model, why does an orbiting electron lead to atomic instability?

An electron moving in a circular orbit is constantly accelerating. According to classical physics, an accelerating charged particle radiates energy, so the electron would lose energy, spiral inwards, and eventually fall into the nucleus.

Distinguish between Thomson's atomic model and Rutherford's atomic model with respect to the distribution of positive charge.

In Thomson's model, the positive charge is uniformly distributed throughout the atom, while in Rutherford's model, the positive charge is concentrated in a tiny central nucleus.

According to classical electromagnetic theory, an accelerating charged particle radiates energy. Explain how this leads to a flaw in Rutherford's planetary model of the atom, and state how Bohr's model resolved this instability.

An electron in circular motion is constantly accelerating, so it should lose energy and spiral into the nucleus. Bohr proposed that electrons can only exist in certain stable orbits where they do not radiate energy.

Compare J.J. Thomson's plum pudding model with Rutherford's planetary model of the atom. List two similarities and two differences. Also, explain why Rutherford's model could not explain the stability of an atom.

Similarities: (1) Both models recognize that atoms contain negatively charged electrons. (2) Both models attempt to explain the neutral nature of atoms. Differences: (1) Thomson's model has the positive charge spread uniformly throughout the atom, while Rutherford's model concentrates positive charge in the nucleus. (2) In Thomson's model, electrons are stationary within the positive sphere, whereas in Rutherford's model, electrons revolve around the nucleus. Rutherford's model could not explain stability because, according to classical electrodynamics, a charged electron revolving in a circular orbit should continuously lose energy and spiral into the nucleus, causing the atom to collapse. But atoms are stable, so the model was incomplete.

In the gold foil experiment, most alpha particles passed undeflected, some were deflected at small angles, and a few bounced back. Using the concepts of atomic structure, explain these observations in detail. If the experiment were repeated with a beam of negatively charged beta particles (electrons) instead of alpha particles, predict how the scattering pattern would be different and justify your prediction.

Alpha particles are positively charged. Most pass through undeflected because the atom is mostly empty space. Some are deflected slightly when they pass near the nucleus due to Coulomb repulsion. A few bounce back if they collide head-on with the nucleus, indicating the nucleus is massive and dense. With beta particles (electrons), which are negatively charged and much lighter, the scattering pattern would change: (i) Many electrons would be absorbed or scattered at large angles by atomic electrons due to similar masses and opposite charge attraction. (ii) The negative nucleus interaction would be attractive, not repulsive, but since the nucleus is tiny and massive, the electrons would mostly be scattered by electron-electron interactions. Overall, fewer would pass straight, and there would be a broad background of scattered electrons, not sharp bouncing back, as the low mass of the beta particle cannot cause a heavy nucleus to recoil significantly. Also, beta particles have higher penetration, so they might pass through thicker foil but deflections would be more diffuse.

In a cathode ray tube, scientists apply a high voltage across two electrodes sealed in a glass tube at low pressure. They observe a beam of rays moving from the cathode to the anode. When an electric field is applied perpendicular to the path, the beam bends toward the positive plate. Further experiments show that the ratio of charge to mass (e/m) of the particles in the beam is a constant, regardless of the metal used for the cathode or the residual gas in the tube. (1) What does the bending of the beam toward the positive plate indicate about the nature of the particles? (2) Why is the constant e/m value a significant result? What conclusion can be drawn about the particles? (3) Given that an electron has a charge of -1.6 × 10⁻¹⁹ C and a mass of 9.1 × 10⁻³¹ kg, calculate the number of electrons required to make a total charge of -1 C.

The deflection shows the particles are negatively charged. The constant e/m proves electrons are universal. For -1 C, we need 6.25 × 10¹⁸ electrons.

Class 9 Science Chapter 8: Journey Inside the Atom — Important Questions & Sample Paper

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The chapter 'Journey Inside the Atom' traces the evolution of atomic theory from early speculations to Bohr's atomic model, making it a cornerstone of Class 9 Science. It begins with the discovery of the subatomic particles: electrons by J.J. Thomson, protons by Goldstein, and neutrons by Chadwick. Students explore Thomson’s ‘plum pudding’ model and then critically examine Rutherford’s gold foil experiment, which revealed the dense, positively charged nucleus and the vast empty space within the atom. The chapter explains why Rutherford’s model failed to account for a stable atom and how Bohr’s model introduced discrete energy levels to overcome this limitation. Core concepts include atomic number, mass number, isotopes, isobars, and the arrangement of electrons in shells (electronic configuration). Learners gain skills in calculating the number of protons, neutrons, and electrons in neutral atoms and ions, predicting valency, and representing atomic structures diagrammatically. Typical exam questions ask students to interpret the gold foil observations, compare atomic models, estimate relative sizes of the nucleus and atom, and solve numericals on atomic composition. This chapter builds a conceptual foundation that is essential for understanding chemical bonding and periodicity in higher classes.

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Science — Journey Inside the Atom

Class 9Time: 3 hrsMax Marks: 80

SECTION A

1.
Who proposed that all matter is composed of indivisible particles called atoms, based on scientific experiments?
(a) Democritus(b) J.J. Thomson(c) John Dalton(d) Ernest Rutherford
1
2.
If the diameter of an atom is approximately 10^{-10} m, how many atoms placed side by side would be needed to cover a distance of 1 mm?
(a) 10^4(b) 10^5(c) 10^6(d) 10^7
1
3.
According to Rutherford's atomic model, where is the positive charge of an atom located?
(a) Spread throughout the atom(b) In the electrons(c) Concentrated in the nucleus(d) Outside the atom
1

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In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

44 original, exam-style questions in our bank for this chapter — with answers.

Important & sample questions (with answers)

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Q1. Who proposed that all matter is composed of indivisible particles called atoms, based on scientific experiments?
1 mark
Multiple Choice (MCQ)
(A) Democritus(B) J.J. Thomson(C) John Dalton(D) Ernest Rutherford
▸ Answer▾ Answer
John Dalton
Q2. If the diameter of an atom is approximately 10^{-10} m, how many atoms placed side by side would be needed to cover a distance of 1 mm?
1 mark
Multiple Choice (MCQ)
(A) 10^4(B) 10^5(C) 10^6(D) 10^7
▸ Answer▾ Answer
10^7
Q3. According to Rutherford's atomic model, where is the positive charge of an atom located?
1 mark
Multiple Choice (MCQ)
(A) Spread throughout the atom(B) In the electrons(C) Concentrated in the nucleus(D) Outside the atom
▸ Answer▾ Answer
Concentrated in the nucleus
Q4. Which observation in Rutherford's gold foil experiment directly contradicts Thomson's plum pudding model?
1 mark
Multiple Choice (MCQ)
(A) Most alpha particles passed through(B) A few alpha particles were deflected at large angles(C) Alpha particles are helium nuclei(D) The gold foil was extremely thin
▸ Answer▾ Answer
A few alpha particles were deflected at large angles
Q5. Assertion (A): Cathode rays are streams of negatively charged particles. Reason (R): Cathode rays are deflected towards the negative electrode in an electric field.
1 mark
Assertion–Reason
(A) Both A and R are true and R is the correct explanation of A.(B) Both A and R are true but R is not the correct explanation of A.(C) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
A is true but R is false.
Q6. Rutherford used a very thin gold foil. Predict how the observations would change if the foil were much thicker, and justify your answer.
2 marks
Short Answer
▸ Answer▾ Answer
Fewer alpha particles would pass through undeflected; more would be scattered at various angles, and some might even be absorbed or bounced back due to multiple encounters with nuclei.
Q7. According to Rutherford’s model, why does an orbiting electron lead to atomic instability?
2 marks
Short Answer
▸ Answer▾ Answer
An electron moving in a circular orbit is constantly accelerating. According to classical physics, an accelerating charged particle radiates energy, so the electron would lose energy, spiral inwards, and eventually fall into the nucleus.
Q8. Distinguish between Thomson's atomic model and Rutherford's atomic model with respect to the distribution of positive charge.
3 marks
Short Answer
▸ Answer▾ Answer
In Thomson's model, the positive charge is uniformly distributed throughout the atom, while in Rutherford's model, the positive charge is concentrated in a tiny central nucleus.
Q9. According to classical electromagnetic theory, an accelerating charged particle radiates energy. Explain how this leads to a flaw in Rutherford's planetary model of the atom, and state how Bohr's model resolved this instability.
3 marks
Short Answer
▸ Answer▾ Answer
An electron in circular motion is constantly accelerating, so it should lose energy and spiral into the nucleus. Bohr proposed that electrons can only exist in certain stable orbits where they do not radiate energy.
Q10. Compare J.J. Thomson's plum pudding model with Rutherford's planetary model of the atom. List two similarities and two differences. Also, explain why Rutherford's model could not explain the stability of an atom.
5 marks
Long Answer
▸ Answer▾ Answer
Similarities: (1) Both models recognize that atoms contain negatively charged electrons. (2) Both models attempt to explain the neutral nature of atoms. Differences: (1) Thomson's model has the positive charge spread uniformly throughout the atom, while Rutherford's model concentrates positive charge in the nucleus. (2) In Thomson's model, electrons are stationary within the positive sphere, whereas in Rutherford's model, electrons revolve around the nucleus. Rutherford's model could not explain stability because, according to classical electrodynamics, a charged electron revolving in a circular orbit should continuously lose energy and spiral into the nucleus, causing the atom to collapse. But atoms are stable, so the model was incomplete.
Q11. In the gold foil experiment, most alpha particles passed undeflected, some were deflected at small angles, and a few bounced back. Using the concepts of atomic structure, explain these observations in detail. If the experiment were repeated with a beam of negatively charged beta particles (electrons) instead of alpha particles, predict how the scattering pattern would be different and justify your prediction.
5 marks
Long Answer
▸ Answer▾ Answer
Alpha particles are positively charged. Most pass through undeflected because the atom is mostly empty space. Some are deflected slightly when they pass near the nucleus due to Coulomb repulsion. A few bounce back if they collide head-on with the nucleus, indicating the nucleus is massive and dense. With beta particles (electrons), which are negatively charged and much lighter, the scattering pattern would change: (i) Many electrons would be absorbed or scattered at large angles by atomic electrons due to similar masses and opposite charge attraction. (ii) The negative nucleus interaction would be attractive, not repulsive, but since the nucleus is tiny and massive, the electrons would mostly be scattered by electron-electron interactions. Overall, fewer would pass straight, and there would be a broad background of scattered electrons, not sharp bouncing back, as the low mass of the beta particle cannot cause a heavy nucleus to recoil significantly. Also, beta particles have higher penetration, so they might pass through thicker foil but deflections would be more diffuse.
Q12. In a cathode ray tube, scientists apply a high voltage across two electrodes sealed in a glass tube at low pressure. They observe a beam of rays moving from the cathode to the anode. When an electric field is applied perpendicular to the path, the beam bends toward the positive plate. Further experiments show that the ratio of charge to mass (e/m) of the particles in the beam is a constant, regardless of the metal used for the cathode or the residual gas in the tube.
4 marks
Case Study
1. (i) What does the bending of the beam toward the positive plate indicate about the nature of the particles?1 mark
2. (ii) Why is the constant e/m value a significant result? What conclusion can be drawn about the particles?1 mark
3. (iii) Given that an electron has a charge of -1.6 × 10⁻¹⁹ C and a mass of 9.1 × 10⁻³¹ kg, calculate the number of electrons required to make a total charge of -1 C.2 marks
▸ Answer▾ Answer
The deflection shows the particles are negatively charged. The constant e/m proves electrons are universal. For -1 C, we need 6.25 × 10¹⁸ electrons.

Frequently asked questions

What did Rutherford’s gold foil experiment demonstrate, and why was it crucial?

Most alpha particles passed through the gold foil undeflected, indicating that the atom is mostly empty space. A few were deflected at small angles, suggesting the presence of a positive center. Very few bounced back, proving that this positive charge and most of the mass are concentrated in a tiny, dense nucleus. This overturned the earlier plum pudding model and established the nuclear model of the atom.

How did Bohr modify Rutherford’s model to make the atom stable?

Rutherford’s model could not explain why electrons, while accelerating around the nucleus, did not spiral in and collapse, as classical electromagnetism predicted. Bohr proposed that electrons move in fixed, discrete orbits (energy levels) and do not radiate energy while in these stationary states. Energy is emitted or absorbed only when an electron jumps between orbits, thus providing a stable structure.

Given an atom or ion, how do you find the numbers of protons, neutrons, and electrons?

The number of protons equals the atomic number (Z). In a neutral atom, the number of electrons equals the number of protons. For an ion, electrons = protons – (charge) (e.g., for a +2 ion, subtract 2 electrons). The number of neutrons is the mass number (A) minus the atomic number (Z). For example, an atom of element Z with 12 electrons (neutral) has 12 protons. If it loses two electrons, it becomes a cation with 10 electrons and a net charge of +2.

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