In this lesson you explored the following questions:

  • What does the Rutherford Scattering Experiment suggest about the nature of the nucleus, and how did it lead to the planetary model of the atom?

  • What is the Bohr model of the atom, and how is the concept of stationary states and energy quantization used to explain how a gas absorbs and emits only certain wavelengths of electromagnetic radiation?

Rutherford's Scattering Experiment was significant because he expected to verify the J. J. Thomson model of the atom and have the alpha particles travel straight through the gold foil.  Instead, Rutherford was surprised to discover that some of the alpha particles were significantly deflected by something in the atom.  His observations led him to develop a new model of the atom with a small, dense centre with most of the mass and a positive charge that was named the nucleus.  To balance the charge, the electrons orbited around the nucleus like planets orbiting around the sun.  He maintained the net neutral electrical charge and the electrons as discovered by Thomson, but the discovery of the nucleus was a new advance.

Bohr quickly realized that the Rutherford model with the orbiting electrons was flawed.  For electrons to orbit (move in a circle), they must constantly accelerate.  According the Maxwell's electromagnetic theory, this means that the electrons would constantly be emitting EMR and losing energy.  As a result, the electrons would slow down and spiral into the nucleus.  Clearly, this wasn't happening, so changes to the model were needed.  Bohr used spectroscopy to examine the patterns of specific wavelengths of EMR absorbed and emitted by gaseous elements.  From these observations he determined that the electrons were in specific orbitals and could only gain and release specific amounts of energy-quantized energy.  Bohr established that electrons in the atom have quantized orbitals and that the electrons absorb specific energy photons to go up orbital levels and release specific energy photons when they drop down orbital levels.  However, Bohr could not explain why there were orbital levels at those specific energies.  Bohr's model is considered semi-classical because it incorporates the quantum but does not include the wave-duality of the electron, which was discovered later.

The quantum mechanical model links the Bohr model with de Broglie wavelengths for electrons.  The stationary orbitals are stable because the circumference of the orbital is a whole number multiple of the wavelength of the electron.  If the orbital circumference is not a whole number multiple, then the electron destructively interferes with itself and the orbital is not stable.  Due to Heisenberg's uncertainty principle, predicting exactly where an electron is located is not possible.  The probability cloud of where an electron is, however, can be calculated.  Quantum theory, while not able to predict exactly where an electron is, does allows physicists to calculate the probability of it being in a specific location.

Theories about the composition and structure of the atom are constantly changing, resulting in new scientific models.  Remember, models are human inventions; they are tools developed to help explain physical phenomena.  As such, models are not a literal representation of the world.  Rather, they illustrate a way of looking at the world and a way of understanding certain phenomena.  As science continues to uncover secrets of the atom, new models will evolve.

 

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