SSC CGL - Detailed Guide 2025
Self-Paced Course
Atomic Structure
Reference: NCERT Class 9-12, Lucent GK
1. Dalton's Atomic Theory & Bohr's Model
Dalton's Atomic Theory (1808)
John Dalton proposed that:
| Postulate | Status Today |
|---|---|
| Matter is made of indivisible atoms | Partially True (atoms divisible) |
| Atoms of a given element are identical in mass and properties | False (due to isotopes) |
| Atoms cannot be created or destroyed | True in chemical reactions |
| Atoms combine in simple ratios to form compounds | True |
| Chemical reactions involve rearrangement of atoms | True |
This theory laid the foundation but was later modified after discovery of electrons, protons, and neutrons.
Bohr's Model of Atom (1913)
| Feature | Description |
|---|---|
| Nucleus | Contains positively charged protons and neutral neutrons |
| Electrons | Revolve in fixed circular paths (called shells or energy levels) around nucleus |
| Energy Levels (Shells) | K, L, M, N... (n = 1, 2, 3, ...) |
| Energy Absorption/ Emission | Electrons jump to higher/lower shells by absorbing/releasing energy (as photons) |
Formula for energy levels: Eₙ = –13.6 eV/n² (for hydrogen)
Key Models of the Atom
| Model | Scientist | Description |
|---|---|---|
| Dalton's Model | John Dalton (1803) | Atom as an indivisible solid sphere |
| Thomson's Model | J.J. Thomson (1897) | "Plum pudding" model: positive sphere with embedded electrons |
| Rutherford's Model | Ernest Rutherford (1911) | Atom has a dense, positive nucleus with electrons orbiting around it |
| Bohr's Model | Niels Bohr (1913) | Electrons move in fixed energy levels (shells) |
| Quantum Model | Erwin Schrödinger | Electrons exist in probability clouds (orbitals) |
This shows the evolution of atomic theory from solid spheres to quantum mechanical models.
2. Atomic Number and Mass Number
| Term | Symbol | Definition | Example (Carbon) |
|---|---|---|---|
| Atomic Number | Z | Number of protons (or electrons in neutral atom) | Z = 6 |
| Mass Number | A | Total number of protons + neutrons in nucleus | A = 12 (6p + 6n) |
| Neutrons | n = A – Z | Number of neutrons | n = 6 |
3. Isotopes and Isobars
Isotopes
| Definition | Example |
|---|---|
| Atoms of the same element (same Z) with different mass numbers (A) | ₁H¹, ₁H² (Deuterium), ₁H³ (Tritium) |
| Same chemical properties, different physical properties | Used in nuclear medicine, tracing, etc. |
All isotopes have same number of protons but different neutrons.
Isobars
| Definition | Example |
|---|---|
| Atoms with same mass number (A) but different atomic numbers (Z) | ₆C¹⁴ and ₇N¹⁴ |
| Different elements with different properties | Occur due to equal sum of protons + neutrons |
4. Quick Facts
- • The value of G was first measured by Henry Cavendish in 1798
- • g on Moon ≈ 1/6th of Earth's gravity (1.62 m/s²)
- • Escape velocity from Earth's surface ≈ 11.2 km/s
- • Gravitational force is the weakest fundamental force
- • Satellites in geostationary orbit have orbital period = Earth's rotation period (24 hrs)
- • Weightlessness occurs when the only force acting is gravity (free-fall)
- • Electron was discovered by J.J. Thomson
- • Proton was discovered by E. Goldstein
- • Neutron was discovered by James Chadwick
- • Nucleus was discovered by Ernest Rutherford
- • Mass of electron ≈ 1/1836 of proton
- • Bohr’s model applies best to Hydrogen-like atoms
Developed By Satyam Kumar
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