Class 12 Physics Atoms Notes

12.1 Introduction

Atoms are the fundamental building blocks of matter. Understanding atomic structure explains chemical properties, spectra, and quantum phenomena. This chapter covers Rutherford’s nuclear model, Bohr’s model, and atomic spectra.

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12.2 Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom

• Rutherford Experiment (1911): Alpha particles were directed at thin gold foil.
• Observations:
  • Most particles passed through → atom is mostly empty space.
  • Some deflected at large angles → presence of a dense, positively charged nucleus.
• Rutherford Model:
  • Atom has a small, dense nucleus containing protons.
  • Electrons revolve around the nucleus in orbits.
• Limitations: Could not explain atomic spectra.

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12.3 Atomic Spectra

• When atoms absorb or emit energy, they produce light of specific wavelengths, forming line spectra.
• Emission spectrum: Light emitted by excited atoms.
• Absorption spectrum: Light absorbed by atoms from a continuous source.
• Hydrogen spectrum is well-studied, forming the Balmer, Lyman, and Paschen series.

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12.4 Bohr Model of the Hydrogen Atom

• Postulates:
  1. Electrons revolve around the nucleus in stable orbits without radiating energy.
  2. Angular momentum of electrons is quantized:

$$mvr = n\frac{h}{2\pi}, \quad n = 1, 2, 3, \dots$$mvr=n2πh​,n=1,2,3,…

3. Radiation is emitted or absorbed when an electron jumps between orbits:

$$\Delta E = E_i – E_f = h\nu$$ΔE=Ei​−Ef​=hν

• Explains stability of atoms and discrete spectral lines.

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12.5 The Line Spectra of the Hydrogen Atom

• Energy of electron in nth orbit:

$$E_n = – \frac{13.6 \text{ eV}}{n^2}$$En​=−n213.6 eV​

• Frequency of emitted radiation:

$$\nu = \frac{E_i – E_f}{h}$$ν=hEi​−Ef​​

• Explains Balmer series in visible region and other series in UV/IR regions.

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12.6 de Broglie’s Explanation of Bohr’s Second Postulate

• de Broglie Hypothesis: Electrons exhibit wave nature.
• Wavelength of electron:

$$\lambda = \frac{h}{p} = \frac{h}{mv}$$λ=ph​=mvh​

• Bohr’s quantization condition explained as standing waves around the orbit:

$$2 \pi r = n \lambda$$2πr=nλ

• Provides wave-mechanical justification of Bohr’s orbits.