Rutherford & Discovery of the Nucleus

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Presentation transcript:

Rutherford & Discovery of the Nucleus Topic 15.3

Outcomes You will explain, qualitatively, the significance of the results of Rutherford’s scattering experiment, in terms of scientists’ understanding of the relative size and mass of the nucleus and the atom You will explain, qualitatively, how emission of EMR by an accelerating charged particle invalidates the classical model of the atom

Review: Thomson & Millikan Thomson and Millikan’s work, charge-to-mass ratio and elementary charge, respectively, led to a new model of the atom… the “plum pudding” model Electrons were embedded in a positively charge fluid which accounted for most of the atoms mass and volume

Rutherford’s Scattering Experiment Rutherford shot alpha particles at a piece of gold foil He hypothesized that the alpha particles would travel through the foil and on to the screen (similarly to shooting bullets at a Kleenex) However, some particles were deflected backwards along their incident path

Rutherford’s Conclusion I shall call it… A NUCLEUS!!! Meaning “kernel” or “inner part” That the mass was not evenly distributed throughout the atom The positive particles were densely packed – would have to be to create enough electrostatic force to cause the alpha particles to deflect backwards He called this “densely packed” area THE NUCLEUS!

Rutherford’s Atomic Model The “positive fluid” was contained in the nucleus, while the electrons were external to the nucleus It was also determined that the nucleus was very small relative to the atom, so much of the gold foil was empty space

How small is an atom? https://www.youtube.com/watch?v=yQP4UJhNn0I

The Problem with Rutherford’s Model According to the Law of Charges…what is wrong with Rutherford’s model? Scientist then predicted that the electrons must have enough kinetic energy to counterbalance the electrostatic forces between the protons and electrons Think of Earth and the Sun and gravitational forces This was called the planetary model of the atom

The Problem with the Planetary Model Circular motion of electron = centripetal acceleration An accelerating charge = EMR If EMR is being emitted, then the electron is losing energy If the electron is losing kinetic energy it is losing speed If the electron is losing speed is should eventually crash into the nucleus According to Maxwell’s Theory… the planetary model could not work Empirical evidence supported the Rutherford planetary model, and yet atoms do not collapse.

TO BE CONTINUED!

Homework p. 770 #1, 2, & 4