Chapter 4 – Structure of the Atom - Short Notes
1. Introduction to the Atom
Key Definitions
- Atom: The smallest particle of an element that takes part in a chemical reaction.
- Dalton’s Atomic Theory: A theory proposed by John Dalton which stated that matter is made up of indivisible particles called atoms.
Explanation (Point-wise)
- Earlier, atoms were considered indivisible.
- Later discoveries showed that atoms are made up of sub-atomic particles.
- This led to new models explaining the internal structure of the atom.
Diagram / Flowchart (Description)
- Flowchart showing:
Matter → Elements → Atoms → Sub-atomic particles
Important Points / Exam Tips
- Dalton’s theory failed to explain:
- Presence of sub-atomic particles
- Isotopes and isobars
2. Discovery of Sub-Atomic Particles
2.1 Discovery of Electron
Key Definitions
- Electron (e⁻): A negatively charged sub-atomic particle.
Explanation
- Discovered by J.J. Thomson using cathode ray experiments.
- Cathode rays:
- Travel in straight lines
- Are deflected by electric and magnetic fields
- This proved the presence of negatively charged particles.
Diagram Description
- Cathode ray tube showing cathode, anode, and deflection of rays.
Important Points / Exam Tips
- Charge of electron = –1
- Mass of electron is very small compared to proton
2.2 Discovery of Proton
Key Definitions
- Proton (p⁺): A positively charged sub-atomic particle.
Explanation
- Discovered by E. Goldstein.
- Positive rays (canal rays) were observed.
- Protons are present inside the nucleus.
Diagram Description
- Discharge tube experiment showing canal rays.
Important Points
- Charge of proton = +1
- Mass ≈ 1 amu
2.3 Discovery of Neutron
Key Definitions
- Neutron (n⁰): A neutral sub-atomic particle.
Explanation
- Discovered by James Chadwick.
- Neutrons are present in the nucleus.
Important Points
3. Atomic Models
3.1 Thomson’s Model of Atom
Key Definitions
- Known as Plum Pudding Model.
Explanation
- Atom is a positively charged sphere.
- Electrons are embedded like plums in pudding.
- Overall atom is electrically neutral.
Diagram Description
- Sphere of positive charge with electrons embedded inside.
Limitations
- Could not explain scattering of alpha particles.
Exam Tip
- Limitations are important for exams.
3.2 Rutherford’s Nuclear Model
Key Definitions
- Proposed by Ernest Rutherford.
Explanation
- Based on alpha particle scattering experiment.
- Findings:
- Most of the atom is empty space
- Positive charge is concentrated at the center (nucleus)
- Electrons revolve around the nucleus
Diagram Description
- Alpha particles striking a thin gold foil and scattering.
Limitations
- Could not explain stability of atom.
- Failed to explain electronic arrangement.
3.3 Bohr’s Model of Atom
Key Definitions
- Energy levels / Orbits: Fixed circular paths in which electrons revolve.
Postulates (Very Important)
- Electrons revolve in fixed orbits.
- Each orbit has fixed energy.
- Energy is emitted or absorbed when electrons jump between orbits.
Diagram Description
- Nucleus at center with circular shells K, L, M, N.
Important Points / Exam Tips
- Energy level increases away from nucleus.
- Bohr’s model explains atomic stability.
4. Distribution of Electrons in Different Orbits (Bohr-Bury Scheme)
Rules
- Maximum electrons in shell = 2n²
- Outer shell cannot have more than 8 electrons
- Shells fill step by step from inner to outer
Table
| Shell |
n |
Max electrons |
| K |
1 |
2 |
| L |
2 |
8 |
| M |
3 |
18 |
| N |
4 |
32 |
Exam Tip
- Electron configuration questions are common.
5. Valency
Key Definitions
- Valency: Combining capacity of an element.
Explanation
- Depends on number of electrons in outermost shell.
- If valence electrons ≤ 4 → valency = same
- If > 4 → valency = 8 – valence electrons
Examples
- Oxygen (6 electrons) → Valency = 2
- Sodium (1 electron) → Valency = 1
Important Points
- Noble gases have valency zero.
6. Atomic Number and Mass Number
Key Definitions
- Atomic Number (Z): Number of protons in nucleus.
- Mass Number (A): Protons + Neutrons
Formula
Example
- Sodium:
Z = 11, A = 23
Neutrons = 12
7. Isotopes
Key Definitions
- Isotopes: Atoms of same element with same atomic number but different mass numbers.
Examples
- Hydrogen: Protium, Deuterium, Tritium
Uses
- Uranium-235 → nuclear fuel
- Cobalt-60 → cancer treatment
Exam Tip
- Definition + one use is important.
8. Isobars
Key Definitions
- Isobars: Atoms of different elements with same mass number but different atomic numbers.
Example
Difference Table
| Isotopes |
Isobars |
| Same element |
Different elements |
| Same Z |
Same A |
Quick Revision Summary (Last-Minute)
- Atom is made of electron, proton, neutron
- Bohr’s model explains stability
- Electron distribution follows 2n² rule
- Atomic number = protons
- Mass number = protons + neutrons
- Isotopes → same element
- Isobars → different elements
- Valency depends on outer shell electrons
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