Atomic Structure – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Nature of Electromagnetic Radiation

Electromagnetic radiation consists of oscillating electric and magnetic fields that travel through space. It includes gamma rays, X-rays, UV, visible light, IR, microwaves, and radio waves.

2. Photoelectric Effect

When light of sufficient frequency strikes a metal surface, electrons are ejected. This phenomenon supports the particle nature of light, explained by Einstein using photons (E = hν).

3. Spectrum of Hydrogen Atom

The hydrogen spectrum consists of line spectra in different regions (Lyman, Balmer, Paschen series). These lines are due to electronic transitions between energy levels in hydrogen.

4. Bohr Model of Hydrogen Atom

• Postulates: Electrons revolve in fixed orbits without radiating energy. Angular momentum is quantized.
• Energy of Electron: ( E_n = -13.6 eV/n^2 )
• Radius of Orbit: ( r_n = 0.529 * n^2/Z Å )
• Limitations: Fails for multi-electron systems and doesn't explain Zeeman or Stark effect.

5. Dual Nature of Matter

Proposed by de Broglie: Matter exhibits both particle and wave-like properties. Wavelength λ = h/p = h/mv.

6. Heisenberg Uncertainty Principle

It is impossible to determine both position and momentum of an electron simultaneously with absolute accuracy:

Δx · Δp ≥ h / 4π

7. Quantum Mechanical Model of Atom

Developed by Schrödinger using wave equations. It describes electrons as wave functions (ψ). Important features include the probabilistic nature of electron position and energy quantization.

8. Atomic Orbitals as One-Electron Wave Functions

Orbitals are solutions to Schrödinger’s equation and represent regions with high probability of finding an electron. Each orbital corresponds to a unique set of quantum numbers.

9. Quantum Numbers and Their Significance

• Principal (n): Size and energy level of orbital
• Azimuthal (l): Shape of orbital (s, p, d, f)
• Magnetic (m): Orientation of orbital in space
• Spin (s): Spin of electron (+½ or –½)

10. Shapes of Orbitals

• s-orbitals: Spherical shape
• p-orbitals: Dumbbell shape (along x, y, z axes)
• d-orbitals: Cloverleaf shapes and donut-ring types

11. Radial Distribution Function (ψ and ψ²) for 1s and 2s Orbitals

ψ² represents the probability density of finding an electron at a distance r from the nucleus. For 1s and 2s, ψ² varies with r and shows a node in 2s orbital.

12. Rules for Filling Electrons in Orbitals

• Aufbau Principle: Electrons fill orbitals in increasing order of energy.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers.
• Hund’s Rule: Electrons occupy degenerate orbitals singly with parallel spins first.

13. Electronic Configuration and Stability

The arrangement of electrons in orbitals follows specific rules and results in greater stability for elements with half-filled and completely filled subshells (e.g., Cr, Cu).