Page 1: Introduction

Atom is the smallest unit of matter that retains chemical properties.

Atoms are divisible into subatomic particles: protons, neutrons, electrons.

This chapter traces discovery and models of atomic structure.

Page 2: Discovery of Electron (Cathode Rays)

J.J. Thomson (1897) — discharge tube experiments.

Page 3: Discovery of Proton (Anode Rays)

Goldstein — perforated cathode experiment.

Page 4: Discovery of Neutron

Chadwick (1932) — bombardment of beryllium by alpha particles.

Neutral particle with mass ≈ proton → neutron.

Page 5: Thomson’s Model (Plum Pudding)

Atom = positively charged sphere with electrons embedded like plums in pudding.

Explained neutrality but failed alpha scattering.

Page 6: Rutherford’s Alpha Scattering Experiment

Gold foil bombarded with alpha particles.

Observations:

• Most passed straight
• Few deflected
• Very few (1/20000) rebounded

Page 7: Rutherford’s Model

Drawbacks: Couldn’t explain stability (electrons should spiral in).

Page 8: Bohr’s Model (1913)

Explained hydrogen spectrum and stability.

Page 9: Distribution of Electrons (Bohr-Bury Scheme)

• K shell (n=1): max 2 electrons
• L shell (n=2): max 8
• M shell (n=3): max 18
• N shell (n=4): max 32
• General: 2n² rule
• Outermost shell ≤ 8 electrons

Page 10: Atomic Number and Mass Number

Atomic number Z = number of protons = number of electrons (neutral atom)

Mass number A = protons + neutrons

Notation: ¹²₆C

Page 11: Electronic Configuration

Distribution in shells.

Page 12: Valency

Combining capacity.

• Outermost electrons 1-4 → valency = number
• 5-8 → valency = 8 - number

Example: Na = 1, O = 2, Cl = 1

Page 13: Isotopes

Same atomic number, different mass number (different neutrons).

Example: ¹H, ²H, ³H; ³⁵Cl, ³⁷Cl

Same chemical properties.

Page 14: Isobars

Same mass number, different atomic number.

Example: ⁴⁰Ar, ⁴⁰Ca

Different chemical properties.

Page 15: Key Points Summary

• Subatomic particles: p⁺, n⁰, e⁻
• Rutherford: nucleus
• Bohr: fixed orbits
• 2n² rule
• Isotopes vs Isobars

Page 16: Practice Questions - Easy (1-10)

1. Charge of electron.
2. Who discovered neutron?
3. Atomic number meaning.
4. Valency of oxygen.
5. Electronic configuration of Na.
6. Isotopes example.
7. Mass number of ¹²C.
8. Bohr’s model main point.
9. Rutherford observation.
10. Maximum electrons in K shell.

Page 17: Practice Questions - Medium (11-20)

11. Electronic configuration Mg, Al, Cl.
12. Valency of elements Z=9 to 15.
13. Difference isotopes and isobars.
14. Draw Rutherford model description.
15. Explain alpha scattering results.
16. Bohr-Bury scheme for Ca (Z=20).
17. Why Thomson model failed.
18. Properties of cathode rays.
19. Neutron discovery experiment.
20. Write configuration Ne, Ar.

Page 18: Practice Questions - Hard (21-30)

21. Explain Bohr’s postulates.
22. Compare all models.
23. Find protons/neutrons/electrons given A and Z.
24. Applications of isotopes.
25. Limitations of Bohr model.
26. Draw electronic configuration diagram.
27. Historical timeline of discoveries.
28. Explain stability in Bohr model.
29. Valency and configuration link.
30. Advanced isobar example.

Page 19: NCERT Exercise Types

Electronic configuration, valency, isotopes/isobars, model differences.

Page 20: Common Mistakes

• Wrong electron distribution
• Confusing isotopes/isobars
• Forgetting 2n² rule
• Mixing models
• Wrong valency for transition

Page 21: Exam Tips

• Memorise first 20 elements configuration
• Draw diagrams for models
• Explain experiments clearly
• Write postulates exactly

Page 22: Quick Revision Sheet

All models, particles, formulas.

Page 23: Electronic Configuration Table

First 20 elements.

Page 24: Final Motivation

Chapter 4 complete! Atomic structure is foundation for Chemistry.

Master configurations and models.

Board Buddy Science unstoppable 🦖

Page 25: Models Comparison

Thomson vs Rutherford vs Bohr.

Page 26: Discovery Timeline

Key scientists and years.