Physics Notes for Class 12 - Chapter 12: Atoms
Dalton's Atomic Theory
- Elements consist of small invisible particles called atoms.
- Atoms of the same element are identical; atoms of different elements differ.
Thomson's Atomic Model
- Atoms are uniformly positively charged spheres with embedded electrons.
- The atom is neutral overall.
Limitations
- Could not explain the spectral series of hydrogen and other atoms.
- Failed to explain large angle scattering of particles.
Rutherford's Atomic Model
- Positive charge and mass are concentrated in a nucleus (~10^-15 m).
- Electrons revolve around the nucleus in orbits.
- Atom is neutral due to equal positive and negative charges.
- Centripetal force is provided by electrostatic attraction.
Key Concepts
- Distance of Closest Approach: ( r_o = \frac{1}{4\pi\varepsilon_0} \frac{2Ze^2}{E_k} )
- Impact Parameter: Perpendicular distance from the velocity vector of an alpha particle to the nucleus.
- Rutherford's Scattering Formula: Explains scattering observed in experiments.
Limitations
- Couldn't explain atomic stability; electrons should emit energy and spiral into the nucleus.
- Failed to explain atomic line spectrum.
Bohr's Atomic Model
- Electrons revolve in non-radiating orbits (stationary orbits).
- Angular momentum ( mvr = \frac{nh}{2\pi} )
- Energy emission occurs when electrons jump between orbits.
- Radius of Orbit: ( r = \frac{n^2h^2}{4\pi^2mKZe^2} )
- Velocity: ( v = \frac{2KZe^2}{nh} )
- Kinetic Energy: ( E_k = 13.6 \frac{Z^2}{n^2} ) eV
- Potential and Total Energy Equations
- Hydrogen Spectrum Series: Lyman, Balmer, Paschen, Brackett, Pfund
- Depend on electron transitions between orbits.
Wave Model
- Based on wave mechanics.
- Quantum Numbers:
- Principal (n)
- Orbital angular momentum (l)
- Magnetic (m_e)
- Magnetic spin angular momentum (m_s)
Hydrogen Spectrum
- Lyman Series: Transitions to n=1, UV region.
- Balmer Series: Transitions to n=2, visible region.
- Paschen, Brackett, Pfund Series: Infrared region transitions.
These notes cover the essential concepts related to atomic models and spectra, providing a detailed study aid for understanding the development and limitations of atomic theory.