Chemistry Lecture: Atomic Structure (Chapter 2, Class 11)

Important Information

• This chapter is important from the perspective of IIT and NEET.
• Every year 2-4 questions are asked from this topic.

Main Points

Atoms and Subatomic Particles

• Subatomic Particles: Electron, Proton, Neutron.
• Ancient sage Kanad said: Everything is made of five elements.
• Dalton's Atomic Theory: In 1805, protons, neutrons, and electrons were not known.
  • Atom was considered the smallest particle.
  • Dalton's theory had many limitations.

Discovery of Electron

• J.J. Thomson's Cathode Ray Tube (CRT) experiment.
• Actions:
  • Used gas at very low pressure of 10^-3 mmHg.
  • High potential (1000 volts) applied from cathode to anode.
  • Cathode rays emitted on the screen, indicating the rays were actually electrons.

Properties of Cathode Rays

1. Negative Charge: Move towards the positive plate.
2. Travel in Straight Line: As per any obstacle.
3. Tiny Particles: They rotate a paddle wheel.
4. Energy: Evidence of Kinetic Energy.
5. Produce Fluorescence.
6. Generate X-rays when hitting heavy metals.

Discovery of Proton

• Proton was discovered.
• Credit goes to E. Goldstein.
• Process of Anode Rays.
• Properties of proton's charge and mass.

Rutherford's Experiment

• Alpha Particle Scattering Experiment:
  • 99.9% alpha particles went through without deflection.
  • Very few particles deflected at a large angle.
  • Some particles bounced back at 180 degrees.

Rutherford's Conclusions

1. Most of the atom is empty space.
2. Positive charge is concentrated in a small area (nucleus).
3. Nucleus is very small compared to the atom.

Rutherford's Model

• Solar System Model:
  • Atom imagined as electrons revolving around the nucleus.

Conclusion

• Rutherford's model does not explain the stability of the atom.
• There is a risk of energy loss due to the accelerated motion of electrons.
• Rutherford's theory has several limitations that need explanation.

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