Chapter 12 – Atomic Structure (JEE Chemistry)
1. Introduction to Atomic Structure
Atomic structure deals with the arrangement of sub-atomic particles
(electrons, protons, neutrons) inside an atom and explains
how electrons are distributed around the nucleus.
2. Discovery of Sub-Atomic Particles
| Particle | Discoverer | Charge | Mass |
|---|---|---|---|
| Electron | J. J. Thomson | $-1$ | $9.1 \times 10^{-31}\,\text{kg}$ |
| Proton | Rutherford | $+1$ | $1.67 \times 10^{-27}\,\text{kg}$ |
| Neutron | Chadwick | $0$ | $1.67 \times 10^{-27}\,\text{kg}$ |
3. Thomson’s Atomic Model
Atom is a positively charged sphere with electrons embedded in it
(like plums in pudding).
❌ Failed to explain Rutherford scattering experiment.
4. Rutherford’s Atomic Model
Atom consists of:
- Small, dense, positively charged nucleus
- Electrons revolving around the nucleus
❌ Could not explain atomic stability and line spectra.
5. Electromagnetic Radiation
Energy travels in the form of electromagnetic waves.
$c = \lambda \nu$
- $c$ = speed of light
- $\lambda$ = wavelength
- $\nu$ = frequency
6. Planck’s Quantum Theory
Energy is emitted or absorbed in discrete packets called quanta.
$E = h\nu$
This theory laid the foundation of quantum mechanics.
7. Photoelectric Effect
Electrons are emitted when light of sufficient frequency strikes a metal surface.
$E_k = h\nu - \phi$
8. Atomic Emission Spectrum
When excited atoms return to ground state, they emit radiation
of specific wavelengths forming line spectra.
9. Hydrogen Spectrum
$\frac{1}{\lambda} = R \left(\frac{1}{n_1^2} - \frac{1}{n_2^2}\right)$
R = Rydberg constant = $1.097 \times 10^7 \,\text{m}^{-1}$
10. Bohr’s Atomic Model
Electrons revolve in fixed circular orbits without radiating energy.
$mvr = \frac{nh}{2\pi}$
$E_n = -\frac{13.6}{n^2}\,\text{eV}$
11. Radius of Bohr Orbit
$r_n = \frac{n^2 h^2}{4\pi^2 m e^2 Z}$
12. Limitations of Bohr’s Model
- Fails for multi-electron atoms
- Cannot explain Zeeman and Stark effects
- Contradicts wave nature of electrons
13. Dual Nature of Matter (de Broglie)
$\lambda = \frac{h}{mv}$
Electrons show both particle and wave nature.
14. Heisenberg Uncertainty Principle
$\Delta x \cdot \Delta p \ge \frac{h}{4\pi}$
Exact position and momentum cannot be measured simultaneously.
15. Schrödinger Wave Equation
Describes electron behavior as a wave function $\psi$.
$\hat{H}\psi = E\psi$
16. Orbitals and Probability
$\psi^2$ represents probability of finding an electron.
17. Quantum Numbers
| Quantum Number | Symbol | Significance |
|---|---|---|
| Principal | $n$ | Size & energy |
| Azimuthal | $l$ | Shape |
| Magnetic | $m_l$ | Orientation |
| Spin | $m_s$ | Spin of electron |
18. Shapes of Orbitals
- s – spherical
- p – dumbbell shaped
- d – double dumbbell
19. Nodes in Orbitals
Total nodes = $n - 1$
Angular nodes = $l$
Radial nodes = $n - l - 1$
20. Important JEE Exam Tips
- Memorize Bohr formulas
- Practice numerical on spectra
- Quantum numbers are very high-weightage
- Understand limitations of models