Atoms Class 12 Notes PDF Download | Quick Revision & Important Concepts

Atoms Class 12 Notes PDF Download

Chapter: Atoms – Class 12 Physics Notes

1. Introduction

• The concept of atom originated from Greek philosophers like Democritus and Dalton’s atomic theory.
• Modern atomic models are based on experimental studies on atomic structure, spectra, and radiation.

2. Rutherford’s Nuclear Model of Atom

Experiment: Alpha particle scattering experiment by Geiger and Marsden under Rutherford’s supervision.
Observations:

1. Most α-particles passed undeviated → Most of atom is empty space.
2. Few were deflected → Existence of a dense positive center (nucleus).
3. Very few bounced back → Nucleus is very small and massive.

Conclusions:

• Atom consists of a positively charged nucleus at center.
• Electrons revolve around nucleus in circular orbits.
• Electrostatic force provides centripetal force.

Drawbacks:

• Could not explain stability of atom.
• Could not explain discrete line spectra of elements.

Atoms Class 12 Notes PDF Download | Quick Revision & Important Concepts

3. Atomic Spectra

• Each element emits/absorbs light of definite wavelengths → line spectrum.
• Hydrogen spectrum has series of lines corresponding to transitions between energy levels.

Spectral Series of Hydrogen:

Series	Region	Transition (n₂ → n₁)
Lyman	Ultraviolet	n₂ → 1
Balmer	Visible	n₂ → 2
Paschen	Infrared	n₂ → 3
Brackett	Infrared	n₂ → 4
Pfund	Infrared	n₂ → 5

Wavelength formula (Rydberg formula):

4. Bohr’s Model of Atom

Postulates:

1. Electron revolves around nucleus in certain discrete orbits (stationary states) without radiating energy.
2. Angular momentum quantized: mvr=nh/2π, n=1,2,3,…
3. Energy is emitted or absorbed only when electron jumps between orbits: ΔE=E₂−E₁=hν

Energy of nth orbit: En=−13.6/n² eV

Radius of nth orbit: r_n=n²a₀, a₀=0.529 A˚

Velocity: v_n=2.18×10⁶n m/s

5. Energy Level Diagram

• Negative sign of energy shows electron is bound to nucleus.
• Energy levels converge as nnn increases.
• Ionization energy of hydrogen = 13.6 eV (energy required to remove electron from n = 1 to ∞).

6. Limitations of Bohr’s Model

• Could not explain fine structure of spectral lines.
• Failed for multi-electron atoms.
• Violated uncertainty principle.
• Could not explain Zeeman and Stark effects.

7. Important Formulas

8. Key Points to Remember

• n=1 → Ground state
• n>1 → Excited states
• Absorption → Electron jumps to higher orbit
• Emission → Electron falls to lower orbit
• Ionization potential of H=13.6 V

→ Red line of Balmer series.

9. Important Constants