Chemistry : Chapter 01 : atomic structure

 
🔍 Summary

This chapter introduces the fundamental concepts of atomic structure, detailing the discovery and properties of subatomic particles (electrons, protons, neutrons). It outlines major atomic models proposed by scientists such as J.J. Thomson, Rutherford, and Bohr, and explains how these models evolved. It also covers key ideas like atomic number, mass number, isotopes, isobars, isotones, and electron configurations. Students also learn the Bohr model and how electrons are arranged in orbits or shells. Real-life applications of isotopes and the development of the periodic table are briefly introduced, linking atomic structure to chemical properties. The aim is to build a solid foundation for understanding how matter is organized at the microscopic level.
📌 Capsule Notes
🔬 Subatomic Particles

    Electron: Negatively charged; discovered via cathode ray experiments.

    Proton: Positively charged; found in anode rays/canal rays.

    Neutron: Neutral particle; discovered by James Chadwick.

⚗️ Cathode Ray Observations

    Travel in straight lines (cast shadows).

    Have mass (rotate paddle wheel).

    Negatively charged (deflect in electric/magnetic fields).

🧪 Important Experiments

    Crookes Tube → Discovered cathode rays (electrons).

    Goldstein → Discovered canal rays (protons).

    Rutherford’s Gold Foil Experiment → Discovery of nucleus.

    Millikan’s Oil Drop → Determined electron charge.

⚛️ Atomic Models

    Thomson’s Model: Plum pudding model.

    Rutherford’s Model: Planetary model; nucleus is small & dense.

    Bohr’s Model: Electrons revolve in fixed orbits (energy levels).

🧠 Atomic Number (Z) and Mass Number (A)

    Z = No. of protons (also electrons in neutral atom).

    A = No. of protons + neutrons

    Neutrons = A − Z

🔄 Electron Configuration

    Electrons fill shells: K (2), L (8), M (18), N (32)...

    Formula: Max electrons in shell = 2n²

    Outer shell can have max 8 electrons.

🔄 Concepts

    Isotopes: Same Z, different A (e.g., H-1, H-2, H-3).

    Isobars: Same A, different Z (e.g., Ar-40, Ca-40).

    Isotones: Same number of neutrons (e.g., N-15, C-14).

❓ Questions with Answers (Q&A)

    MCQ: Who discovered the electron?

        A. Rutherford

        B. J.J. Thomson ✅

        C. Bohr

        D. Chadwick

    Short Answer: What is an isotope?

        Atoms of the same element with same atomic number but different mass numbers.

    One-liner: What is the charge of a proton?

        +1

    Short Answer: Define atomic number.

        The number of protons in the nucleus of an atom.

    One-liner: What is the maximum number of electrons in the L shell?

        8

    Short Answer: What is the e/m value of an electron?

        1.76 × 10¹¹ C/kg

    MCQ: Who discovered the neutron?

        A. Bohr

        B. Goldstein

        C. Chadwick ✅

        D. Faraday

    One-liner: Give an example of isobars.

        Argon-40 and Calcium-40

    Short Answer: What is meant by mass number?

        The sum of protons and neutrons in an atom.

    Short Answer: Define electron configuration.

        The arrangement of electrons in different energy levels of an atom.

    MCQ: Which subatomic particle has no charge?

        A. Electron

        B. Proton

        C. Neutron ✅

        D. Positron

    Short Answer: Why are noble gases stable?

        They have a complete outer shell (duplet or octet configuration).

    One-liner: How many electrons can M shell hold?

        18

    Short Answer: Write the Bohr model features in two points.

        Electrons revolve in fixed energy levels.

        Energy is absorbed/released when electrons jump between levels.

    Short Answer: How are isotones different from isotopes?

        Isotones have the same number of neutrons; isotopes have same atomic number but different mass numbers.

📚 Definitions & Key Terms
Term    Definition    Example
Electron    Negatively charged subatomic particle    Found in cathode rays
Proton    Positively charged subatomic particle    Found in canal rays
Neutron    Neutral subatomic particle in nucleus    Chadwick (1932)
Atomic Number (Z)    No. of protons    Z of Oxygen = 8
Mass Number (A)    Protons + Neutrons    A of Oxygen = 16
Isotope    Same Z, different A    C-12, C-13, C-14
Isobar    Same A, different Z    Ar-40 and Ca-40
Isotone    Same no. of neutrons    N-15, C-14
Orbit/Shell    Path where electrons revolve    K, L, M, N
Electron Configuration    Distribution of electrons in shells    Oxygen: 2,6
Nucleus    Central positively charged core of atom    Discovered by Rutherford
e/m Ratio    Charge-to-mass ratio of electron    1.76 × 10¹¹ C/kg
🧠 Main Points to Remember (For Revision)

    🧪 Electrons discovered by J.J. Thomson (Cathode ray).

    ⚛️ Proton discovered by Goldstein (Anode ray).

    🧲 Neutrons are neutral; discovered by Chadwick.

    🌐 Atomic number = Protons = Electrons (in neutral atom).

    ⚖️ Mass number = Protons + Neutrons.

    🔁 Electron shell capacity = 2n².

    🌟 Noble gases have full outer shells → stable.

    🔄 Isotopes = Same Z, different A.

    🔄 Isobars = Same A, different Z.

    🔄 Isotones = Same neutrons.

    🧬 Bohr model: electrons revolve in fixed orbits.

    ⚠ Rutherford model failed to explain atomic stability.

    🧊 Neutron mass ≈ Proton mass, no charge.

📂 Topic-Wise Breakdown
Topic    Key Takeaways
Cathode Ray Experiments    Discovery of electrons, their properties.
Anode Rays    Discovery of protons.
Atomic Models    Evolution: Thomson → Rutherford → Bohr.
Bohr’s Model    Fixed orbits, energy levels, stable configuration.
Subatomic Particles    Charge, mass, and role in atom.
Atomic Number & Mass Number    Calculations and significance.
Electron Configuration    Filling rules, shell limits, periodicity.
Isotopes/Isobars/Isotones    Definitions, differences, and examples.