Journey Inside the Atom
Complete Class 9 Science Notes: Matter, Subatomic Particles & Atomic Models
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Matter • Early Ideas • Subatomic Particles • Thomson Model • Rutherford Model • Bohr Model • Practice Questions
📘 About This Chapter
Everything around us that has mass and occupies space is considered matter. This chapter takes us from early ideas about the smallest particles of matter to the discovery of electrons, protons and neutrons, and then to Thomson's, Rutherford's and Bohr's models of the atom.
1. What is Matter?
Anything in the universe that has mass, occupies space and can be felt by our sense organs is called matter.
2. Early Ideas About the Smallest Particle
🇮🇳 Acharya Kanada's Idea
Acharya Kanada proposed that continuous division of a substance would eventually lead to a smallest particle called Parmanu.
Two parmanus form a Dyad, while three parmanus form a Triad.
🇬🇷 Greek Philosophers
Leucippus and Democritus proposed a similar idea and used the term Atomos for an indivisible particle.
🔬 From Ideas to Experiments
Scientific experiments later provided evidence about the internal structure of the atom and led to the identification of subatomic particles.
3. Discovery of Subatomic Particles
| Particle | Scientist | Year | Charge | Mass in Notes |
|---|---|---|---|---|
| Electron | J. J. Thomson | 1897 | Negative | 9.1 × 10-31 kg |
| Proton | Ernest Rutherford | 1920 | Positive | 1.67 × 10-27 kg |
| Neutron | James Chadwick | 1932 | No charge | — |
4. Discovery of Electron – J. J. Thomson
In 1897, J. J. Thomson studied a discharge tube containing gas at very low pressure and high voltage. A stream of small negatively charged particles travelled from the cathode towards the anode. These were identified as electrons.
5. Discovery of Proton
Experiments involving gases at very low pressure produced positively charged anode rays, also called canal rays. The notes describe Rutherford's study of these particles and their identification as protons.
6. Discovery of Neutron – James Chadwick
In 1932, James Chadwick carried out experiments involving alpha particles and a beryllium sheet. The resulting particles were not attracted or repelled by electric or magnetic fields, leading to their identification as neutrons.
7. Thomson's Model of Atom
In 1903, J. J. Thomson proposed a model in which an atom was considered to contain positive charge with negatively charged electrons embedded in it. Equal positive and negative charges made the atom electrically neutral.
8. Limitations of Thomson's Model
- It could not explain the stability of the atom.
- It could not explain how positive charge holds electrons inside the atom.
- It could not explain the observations of Rutherford's scattering experiment.
9. Rutherford's Alpha Particle Gold Foil Experiment
In 1911, Geiger and Marsden, working under Ernest Rutherford, tested Thomson's model by directing alpha particles towards a thin sheet of gold foil.
| Observation | Conclusion |
|---|---|
| Most alpha particles passed straight through the gold foil. | Most of the atom is empty space. |
| Very few alpha particles were deflected. | Positive charge occupies a very small region. |
| A very few alpha particles returned along their original path. | Positive charge is concentrated at the centre, called the nucleus. |
10. Rutherford's Atomic Model
🔵 Mostly Empty Space
Most of the atom is empty space.
🎯 Central Nucleus
Positive charge is concentrated in a very small central region called the nucleus.
⚡ Electrons
Negatively charged electrons surround the nucleus.
11. Limitation of Rutherford's Model
The notes explain that an electron revolving around the nucleus would radiate energy. Continuous loss of energy would make the orbit shrink and could cause the electron to fall into the nucleus, suggesting an unstable atom.
12. Bohr's Atomic Model – 1913
To overcome the limitation of Rutherford's model, Neils Bohr proposed an atomic model based on specific energy levels or discrete orbits.
- Electrons revolve only in specific permitted orbits.
- Electrons do not radiate energy while in these permitted orbits.
- An electron can move from a lower to a higher energy level when energy is supplied.
- When an electron moves from a higher to a lower energy level, energy is released.
- Shells are represented as K, L, M, N and so on.
- The maximum number of electrons in a shell is represented by 2n², where n is the shell number.
| Shell | n | Maximum Electrons (2n²) |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
| N | 4 | 32 |
📝 Important Questions
Answer: Parmanu.
Answer: J. J. Thomson in 1897.
Answer: James Chadwick in 1932.
Answer: Most alpha particles passed straight through the gold foil without deviation.
Answer: Electrons were described as revolving around the nucleus, similar to planets revolving around the Sun.
🎯 Quick MCQ Practice
A) Light B) Sound C) Chair D) Heat
A) Atom B) Molecule C) Parmanu D) Electron
A) Electron B) Proton C) Neutron D) Positron
A) Dalton B) Rutherford C) James Chadwick D) Goldstein
A) Thomson B) Rutherford C) Bohr D) None
🔗 Match the Scientist with the Contribution
| Scientist / Philosopher | Associated Idea / Discovery |
|---|---|
| Acharya Kanada | Parmanu |
| Leucippus & Democritus | Atomos |
| J. J. Thomson | Electron |
| Ernest Rutherford | Proton / Nuclear model |
| James Chadwick | Neutron |
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