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ATOMS AND NUCLEI

STRUCTURE OF ATOM

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Prepared & Presented by

MATHEW ABRAHAM

PGT PHYSICS

KENDRIYA VIDYALAYA

EAC UPPER SHILLONG

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EARLY MODELS OF ATOM

J.J. THOMSON’S PLUM PUDDING MODEL

  • Thomson proposed that the electrons are embedded in a positively charged ‘pudding’’

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Rutherford’s Scattering Experiment (1911)

  • He found that, once in a while, the a-particles were scattered backwards by the target

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RATIONALIZING THE EXPERIMENTAL DATA

To explain the backscattering, the positive charge must be concentrated in a small region

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IMPACT PARAMETER

The perpendicular distance between the velocity vector of the alpha particle and the centre of the nucleus when the alpha particle is at infinite distance from the nucleus.

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DISTANCE OF CLOSEST APPROACH

The minimum distance upto which an energetic alpha particle travelling directly towards the nucleus can move before coming to rest and then retracing its path.

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Rutherford’s Solar System Model of the Atom

The atom consists of electrons orbiting around �a small but dense central nucleus

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WHAT’S WRONG?

  • It is known that accelerating charges emit radiation

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  • Thus, electron should emit radiation, lose energy and eventually fall into the nucleus!

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  • Why doesn’t this happen? Shows that something was wrong with this model of the hydrogen atom

RUTHERFORD’S ATOM MODEL COULDNOT EXPLAIN THE STABILITY OF ATOM

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Absorption spectrum of Sun

Emission spectra of various elements

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Bohr’s Model of the Hydrogen Atom�(1913)

  • Electrons can only take discrete energies �(energy is related to radius of the orbit)

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  • Electrons can jump between different orbits due to the absorption or emission of photons�
  • Dark lines in the absorption spectra are �due to photons being absorbed

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  • Bright lines in the emission spectra are �due to photons being emitted

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He proposed that only certain orbits for the electron are allowed

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Absorption / Emission of Photons�and Conservation of Energy

Ef - Ei = hf

Ei - Ef = hf

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REVISION QUESTIONS

  1. Describe Rutherford’s Alpha Scattering Experiment and write the conclusions drawn from it.
  2. Define Impact Parameter and write an expression for it.
  3. Define distance of closest approach and derive an expression for it.
  4. What are the drawbacks of Rutherford’s model; of atom.
  5. How did Bohr modified Rutherford’s model to overcome the drawbacks?