Structure of Atom – Complete JEE Notes
1. Introduction to Atomic Structure
Atoms are the basic building blocks of matter.
Understanding atomic structure explains:
- Atomic spectra
- Chemical bonding
- Periodic properties
2. Discovery of Sub-Atomic Particles
| Particle | Discovered By | Charge | Mass |
|---|---|---|---|
| Electron | J.J. Thomson | $-1$ | $9.11 \times 10^{-31}$ kg |
| Proton | Goldstein | $+1$ | $1.67 \times 10^{-27}$ kg |
| Neutron | Chadwick | $0$ | $1.67 \times 10^{-27}$ kg |
3. Thomson’s Atomic Model
Also called the Plum Pudding Model.
Atom is a positively charged sphere with electrons embedded in it.
Failed to explain:
- Atomic spectra
- Scattering experiments
4. Rutherford’s Atomic Model
Based on $\alpha$-particle scattering experiment:
- Atom has a dense nucleus
- Most of the atom is empty space
Failed to explain atomic stability and line spectra.
5. Bohr’s Atomic Model
Bohr proposed that electrons revolve in fixed circular orbits with quantized energy.
Bohr’s Postulates
- Electrons move in stable orbits without radiating energy
- Angular momentum is quantized
Angular momentum:
$$mvr = \frac{nh}{2\pi}$$
6. Radius of Bohr Orbit
$$r_n = \frac{n^2 h^2}{4\pi^2 m k Z e^2}$$
For hydrogen:
$$r_n = 0.529 \times n^2 \; \text{Å}$$
7. Energy of Electron in Hydrogen Atom
$$E_n = -\frac{13.6 Z^2}{n^2} \; \text{eV}$$
Negative sign indicates bound state.
8. Atomic Spectra
When electrons jump between energy levels, light of specific wavelength is emitted or absorbed.
$$\Delta E = h\nu = \frac{hc}{\lambda}$$
9. Hydrogen Spectrum Series
| Series | Final Orbit | Region |
|---|---|---|
| Lyman | $n=1$ | UV |
| Balmer | $n=2$ | Visible |
| Paschen | $n=3$ | IR |
10. Limitations of Bohr Model
- Fails for multi-electron atoms
- Cannot explain fine structure
- Violates Heisenberg principle
11. Dual Nature of Matter (de Broglie)
$$\lambda = \frac{h}{mv}$$
12. Heisenberg Uncertainty Principle
$$\Delta x \cdot \Delta p \ge \frac{h}{4\pi}$$
13. Quantum Mechanical Model
Electron behavior is described using wave functions ($\psi$).
$$\psi^2 = \text{Probability density}$$
14. Quantum Numbers
| Quantum Number | Symbol | Significance |
|---|---|---|
| Principal | $n$ | Size & energy |
| Azimuthal | $l$ | Shape |
| Magnetic | $m_l$ | Orientation |
| Spin | $m_s$ | Spin of electron |
15. Shapes of Orbitals
- s – spherical
- p – dumbbell
- d – double dumbbell
16. Electronic Configuration
Arrangement of electrons in orbitals following:
- Aufbau principle
- Pauli exclusion principle
- Hund’s rule
17. Aufbau Order
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p …
18. Pauli Exclusion Principle
No two electrons can have the same set of four quantum numbers.
19. Hund’s Rule
Electrons fill degenerate orbitals singly before pairing.
20. Important JEE Tips
- Remember energy formulas
- Practice numerical problems
- Understand spectral series
- Quantum numbers are frequently tested