Evolution of Atomic Models Plum Pudding Model YouTube Lecture Handouts

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Questions?

  • Plum Pudding Model (Thomson՚s-1904)
  • Rutherford Experiment (1908 - 1910)
  • Findings (Rutherford Model- 1911)
  • Bohr՚s (or Bohr-Rutherford) Model (1913)

Plum Pudding Model (J. J. Thomson 1904)

Electrons surrounded by a volume of positive charge, like negatively charged “plums” embedded in a positively charged “pudding” .

Superseded

Larmor՚s solar system model (1897)

Explains

  • Electrons are negatively charged particles.
  • Atoms have no net electric charge.
  • Two scales: Macroscopic and microscopic
  • Classical Physics deals mainly with macroscopic phenomena and includes subjects like Mechanics, Electrostatics, Electrodynamics, Optics and Thermodynamics.
  • Thermodynamics, in contrast to mechanics, does not deal with the motion of bodies as a whole. Rather, it deals with systems in macroscopic equilibrium and is concerned with changes in internal energy, temperature, entropy, etc. , of the system through external work and transfer of heat. The efficiency of heat engines and refrigerators, the direction of a physical or chemical process, etc. , are problems of interest in thermodynamics.

The Story of Rutherford Experiment

Illustration: The Story of Rutherford Experiment
Illustration: The Story of Rutherford Experiment
  • A radioactive source emitting alpha particles enclosed within a protective lead shield.
  • A thin section of gold foil was placed in front of the slit, and
  • A screen coated with zinc sulfide to render it fluorescent served as a counter to detect alpha particles.
  • As each alpha particle struck the fluorescent screen, it produced a burst of light called a scintillation, which was visible through a viewing microscope attached to the back of the screen.
  • We can assume that the screen itself was movable.
  • Rutherford began his graduate work by studying the effect of x-rays on various materials. Shortly after the discovery of radioactivity, he turned to the study of the -particles emitted by uranium metal and its compounds.
  • Alpha particles i.e.. , positively charged particles, identical to the helium atom nucleus and 7,000 times more massive than electrons and high speeds
  • Before he could study the effect of -particles on matter, Rutherford had to develop a way of counting individual -particles. He found that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an -particle. Rutherford and his assistant, Hans Geiger, would sit in the dark until his eyes became sensitive enough. They would then try to count the flashes of light given off by the ZnS screen. (It is not surprising that Geiger was motivated to develop the electronic radioactivity counter that carries his name.)
  • Rutherford found that a narrow beam of -particles was broadened when it passed through a thin film of mica or metal. He therefore had Geiger measure the angle through which these -particles were scattered by a thin piece of metal foil. Because it is unusually ductile, gold can be made into a foil that is only 0.00004 cm thick. When this foil was bombarded with -particles, Geiger found that the scattering was small, on the order of one degree.
  • These results were consistent with Rutherford՚s expectations. He knew that the -particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the -particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the -particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.
  • One day, Geiger suggested that a research project should be given to Ernest Marsden, who was working in Rutherford՚s laboratory. Rutherford responded, “Why not let him see whether any -particles can be scattered through a large angle?” When this experiment was done, Marsden found that a small fraction (perhaps 1 in 20,000) of the -particles were scattered through angles larger than . Many years later, reflecting on his reaction to these results, Rutherford said: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford Experiment

Rutherford Experimenth

Observations: Conflicts with Thomson՚s Model

  • Most alpha particles passed straight through the gold foil.
  • Some deflected slightly, suggesting interactions with other positively charged particles within the atom.
  • Few alpha particles scattered at large angles (> 90 degrees) ,
  • Very few even bounced back toward the source.
Illustration: Observations: Conflicts with Thomson՚s Model

“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

  • Most alpha particles passed straight through the gold foil, which implied that atoms are mostly composed of open space. Some alpha particles were deflected slightly, suggesting interactions with other positively charged particles within the atom. Still other alpha particles were scattered at large angles, while a very few even bounced back toward the source. (Rutherford famously said later, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” )
  • Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion. The negative electrons that balanced electrically the positive nuclear charge were regarded as traveling in circular orbits about the nucleus. The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun. Most of this planetary atom was open space and offered no resistance to the passage of the alpha particles.

Rutherford Model

  • The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil.
  • Negatively charged electrons surround the nucleus.
  • He went back to the planetary model of atom with electrons revolving in orbits.
  • Electrons are held by a electrostatic force of attraction by nucleus.
Illustration: Rutherford Model

Rutherford Model Limitations

Classical mechanics predicts that the electron will release electromagnetic radiation (loose energy) while orbiting a nucleus.

Illustration: Rutherford Model Limitations
  • In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Given this experimental data, Rutherford naturally considered a planetary model of the atom, the Rutherford model of 1911.
  • This had electrons orbiting a solar nucleus, but involved a technical difficulty: the laws of classical mechanics (i.e.. , the Larmor formula) predict that the electron will release electromagnetic radiation while orbiting a nucleus. Because the electron would lose energy, it would rapidly spiral inwards, collapsing into the nucleus on a timescale of around 16 picoseconds. Rutherford՚s atom model is disastrous because it predicts that all atoms are unstable

Bohr՚s Model (1913)

  • Electron can revolve in certain stable orbits around the nucleus without radiating any energy.
  • These stable orbits called stationary orbits are attained at certain discrete distances from the nucleus.
  • The electron cannot have any other orbit in between the discrete ones.
  • In stationary orbits angular momentum of the revolving electron is an integer multiple of the reduced Planck constant
Illustration: Bohr՚s Model (1913)

Superseded

Rutherford՚s model

Explains

Stability of electrons in orbits

  • In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Given this experimental data, Rutherford naturally considered a planetary model of the atom, the Rutherford model of 1911.
  • This had electrons orbiting a solar nucleus, but involved a technical difficulty: the laws of classical mechanics (i.e.. , the Larmor formula) predict that the electron will release electromagnetic radiation while orbiting a nucleus.
  • Because the electron would lose energy, it would rapidly spiral inwards, collapsing into the nucleus on a timescale of around 16 picoseconds. Rutherford՚s atom model is disastrous because it predicts that all atoms are unstable

Mayank