Chapter 6 — Structure of Atom

1. From indivisible atom to subatomic particles

Dalton regarded atoms as indivisible particles. Experiments involving electricity and discharge through gases showed that atoms have smaller constituents.

Three major subatomic particles discussed are:

Particle	Charge	Location
Electron	Negative	Outside nucleus
Proton	Positive	Nucleus
Neutron	Neutral	Nucleus

2. Discovery of electron

J. J. Thomson studied cathode rays.

Cathode rays:

• originate at the cathode;
• travel towards the anode;
• travel in straight lines under suitable conditions;
• are deflected by electric and magnetic fields;
• bend towards a positive plate, indicating negative charge.

Their behaviour remained essentially unchanged when the gas or electrode material was changed, supporting the conclusion that electrons are common constituents of atoms.

3. Discovery of proton

Eugen Goldstein observed canal rays using a perforated cathode.

They were positively charged, but unlike electrons, their properties depended on the gas present because canal rays consist of positive ions.

The lightest positive particle obtained with hydrogen was identified as the hydrogen ion and later recognised as the proton.

4. Discovery of neutron

Atomic masses could not be adequately explained using only protons and electrons.

James Chadwick discovered the neutron in 1932.

Neutrons:

• have no electric charge;
• have mass approximately comparable to a proton;
• occur in the nuclei of most atoms.

The chapter uses hydrogen isotopes to illustrate this:

• protium → no neutron;
• deuterium → one neutron;
• tritium → two neutrons.

5. Atomic spectra

White light produces a continuous spectrum.

Some elements produce line spectra, containing specific bright lines.

A line spectrum is characteristic of an element and can therefore help identify elements, including elements present in stars.

6. Hydrogen spectrum

Hydrogen produces several spectral series:

• Lyman
• Balmer
• Paschen
• Brackett
• Pfund

The chapter introduces the Rydberg relationship as an empirical description of hydrogen’s spectral lines.

7. Rutherford’s model

Rutherford’s nuclear model established the presence of a small positive nucleus but had important limitations.

It could not satisfactorily explain:

1. why an orbiting electron would remain stable;
2. why hydrogen produces a line spectrum rather than a continuous spectrum.

8. Bohr’s model

Bohr proposed that electrons can occupy specific allowed orbits with fixed energies.

An electron does not continuously radiate energy while remaining in an allowed orbit.

Radiation is emitted or absorbed when an electron moves between allowed energy levels.

If an electron moves from a higher-energy level to a lower-energy level, energy is released as radiation.

This explains why only certain spectral lines are observed.

However, Bohr’s model has limitations: it cannot fully explain the spectra of multi-electron atoms, fine spectral details, or effects such as Zeeman and Stark splitting.

Key idea

The development of atomic models shows an important scientific principle: a model is useful only as long as it explains experimental observations.

Chapter 6 — Structure of Atom

The chapter covers cathode rays, subatomic particles, spectra, Rutherford’s model and Bohr’s model, including their achievements and limitations.

Questions

A. Multiple Choice Questions

1. J. J. Thomson is credited with the discovery of the:
a) Proton
b) Electron
c) Neutron
d) Nucleus

2. Cathode rays travel from the:
a) Anode towards cathode
b) Cathode towards anode
c) Nucleus towards shell
d) Positive plate towards negative plate

3. Cathode rays are deflected toward a positive plate because they consist of:
a) Positive particles
b) Neutral particles
c) Negatively charged particles
d) Photons

4. The neutron was discovered by:
a) Thomson
b) Rutherford
c) Chadwick
d) Bohr

5. A spectrum containing distinct bright lines is called a:
a) Continuous spectrum
b) Line spectrum
c) Thermal spectrum
d) Mixed spectrum

6. The Balmer series of hydrogen involves transitions ending at:
a) $n=1$
b) $n=2$
c) $n=3$
d) $n=4$

7. Rutherford’s model failed mainly to explain:
a) The positive nucleus
b) Atomic stability and line spectra
c) The existence of electrons
d) The mass of the atom

8. According to Bohr’s model, electrons can occupy:
a) Any energy value continuously
b) Certain allowed energy levels
c) Only the nucleus
d) No fixed energy states

B. Fill in the Blanks

9. J. J. Thomson discovered the ______.

10. James Chadwick discovered the ______.

11. The nucleus is positively ______.

12. A spectrum consisting of separated bright lines is called a ______ spectrum.

13. The hydrogen spectrum is characteristic of the ______ atom.

14. Bohr proposed that electrons can exist in certain ______ energy levels.

C. True or False

15. Cathode rays are positively charged.

16. Cathode-ray behaviour remained essentially unchanged when the gas was changed.

17. Rutherford’s model completely explained atomic stability.

18. Bohr’s model explained the hydrogen line spectrum more successfully than Rutherford’s model.

D. Assertion–Reason

19. Assertion: Hydrogen produces a line spectrum rather than a continuous spectrum.
Reason: Electrons undergo transitions between specific allowed energy levels.

20. Assertion: Rutherford’s model could not explain atomic stability.
Reason: According to classical ideas, an accelerating orbiting electron would lose energy.

E. Short Answer

21. What observation from cathode-ray experiments established that electrons are common constituents of atoms?

22. Differentiate between a continuous spectrum and a line spectrum.

23. Why could a line spectrum be used to identify elements in stars?

24. State two major postulates of Bohr’s model.

25. Give two limitations of Bohr’s model.

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Answers

A. MCQ Answers

1. b) Electron
2. b) Cathode towards anode
3. c) Negatively charged particles
4. c) Chadwick
5. b) Line spectrum
6. b) n=2n=2
7. b) Atomic stability and line spectra
8. b) Certain allowed energy levels

B. Fill in the Blanks

9. electron
10. neutron
11. charged
12. line
13. hydrogen
14. allowed/fixed

C. True/False

15. False
16. True
17. False
18. True

D. Assertion–Reason

19. Both are true, and the reason correctly explains the assertion.
20. Both are true, and the reason correctly explains the assertion.

E. Answers

21. When different gases and electrode materials were used, the properties of cathode rays remained unchanged. This indicated that electrons are common constituents of atoms.
22. A continuous spectrum contains an uninterrupted range of wavelengths/colours, while a line spectrum contains distinct separated lines.
23. Each element has a characteristic line spectrum. Therefore, the spectral lines observed from starlight can reveal which elements are present.

• Electrons can occupy only certain allowed orbits/energy levels without continuously radiating energy.
• Radiation is emitted or absorbed when an electron changes from one allowed energy level to another.

25. Bohr’s model could not adequately explain the fine structure of hydrogen and the spectra of atoms containing more than one electron. It also could not explain certain spectral splittings such as the Zeeman and Stark effects.

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