This chapter, "Structure of the Atom," is the heartbeat of Chemistry. In your competitive exam, questions from this chapter are frequent because they test your understanding of the fundamental nature of matter. We are moving from "what" matter is to "why" it behaves the way it does.

1. The Discovery of Sub-Atomic Particles

The atom is divisible. It is made of three main particles. You must memorize the discoverers and the properties:

• Electron (e^-): Negative charge (-1). Mass is negligible (1/2000 of H atom). Discovered by J.J. Thomson (1897).
• Proton (p⁺): Positive charge (+1). Mass is 1 unit (approx 2000x electron). Discovered by E. Goldstein (via Canal Rays).
• Neutron (n): Neutral (0). Mass is 1 unit (nearly equal to proton). Discovered by J. Chadwick (1932).

2. Evolution of Atomic Models

Scientists struggled to figure out how these particles were arranged. This evolution is a favorite topic for statement-based questions.

A. Thomson’s Model (The "Christmas Pudding" Model)

Proposal: The atom is a positively charged sphere (like the red part of a watermelon) with electrons embedded in it (like seeds).
Result: Explained that the atom is electrically neutral.
Failure: Could not explain the results of scattering experiments (like Rutherford’s).

B. Rutherford’s Model (The Nuclear Model)

The Experiment: He bombarded a very thin Gold Foil (1000 atoms thick) with fast-moving Alpha (α) particles (Doubly charged Helium ions, He²⁺).

• Observation 1: Most particles went straight through → Most space inside an atom is empty.
• Observation 2: Some deflected slightly → Positive charge is concentrated in a small space.
• Observation 3: 1 out of 12,000 rebounded (180°) → The mass is concentrated in a tiny center called the Nucleus.

Drawback (Crucial): Rutherford said electrons revolve in circular orbits. According to physics, an accelerating charged particle should radiate energy and fall into the nucleus, making the atom unstable. But atoms are stable.

C. Bohr’s Model (The Stabilizer)

Solution: Neils Bohr proposed that electrons revolve only in certain discrete orbits (Shells).
Key Rule: While revolving in these specific shells, electrons do not radiate energy.
Shell Naming: K (n=1), L (n=2), M (n=3), N (n=4)...

3. Electronic Configuration & Valency

Arranging the Electrons

How many electrons fit in each shell? The formula is 2n² (where 'n' is the shell number).

• K shell (n=1): 2(1)² = 2 electrons.
• L shell (n=2): 2(2)² = 8 electrons.
• M shell (n=3): 2(3)² = 18 electrons.
• N shell (n=4): 2(4)² = 32 electrons.

Octet Rule: The outermost shell cannot hold more than 8 electrons (even if the formula allows more for inner shells). A full outer shell = Stability (like Noble Gases).

Valency (Combining Capacity)

Determined by the Valence Electrons (electrons in the outermost shell).

• If outer electrons are 1, 2, 3, or 4: Valency = Number of electrons.
  Example: Aluminium (2, 8, 3) → Valency is 3.
• If outer electrons are 5, 6, 7: Valency = 8 minus Number of electrons.
  Example: Chlorine (2, 8, 7) → Valency is 8 - 7 = 1.
• If outer electrons are 8: Valency = 0 (Noble Gases like Neon, Argon).

4. Atomic Number vs. Mass Number

Do not get these confused. This is where calculation questions come from.

• Atomic Number (Z): Total number of Protons. (This defines the element identity).
• Mass Number (A): Sum of Protons + Neutrons. (Collectively called Nucleons).

5. Isotopes and Isobars (High-Scoring Zone)

Isotopes

Definition: Atoms of the same element (same Atomic Number Z) but different Mass Numbers (different neutrons).

• Example (Hydrogen): Protium (¹H), Deuterium (²H), Tritium (³H).
• Example (Chlorine): Occurs as ³⁵Cl and ³⁷Cl in a 3:1 ratio.

Isobars

Definition: Atoms of different elements (different Atomic Numbers) with the same Mass Number.
Example: Calcium (Z=20) and Argon (Z=18). Both have a mass number of 40.
Mentor’s Note: Their chemical properties are totally different, but their nuclei weigh the same.

6. Mentor’s Final Drill (Exam-Ready Questions)

Study Strategy: Focus on the Isotope uses and the difference between Atomic Number and Mass Number. These are the areas where examiners try to trick you. Master the definitions, and you will secure full marks in this section.