Reading #5: Moving from Rutherford to Bohr
Rutherford's Model
Rutherford revised the model of the atom based on evidence from the Geiger-Marsden gold foil experiments. One of the major revisions was the addition of a positively charged nucleus. Even the idea of the nucleus was changed throughout Rutherford's career and study. The movement of the negatively charged particles around the nucleus was also hypothesized by Rutherford.
Results Based on the Gold Foil Experiments
Image from: http://www.universetoday.com/38326/plum-pudding-model/
Rutherford's Models of Atoms
Image from: http://nobeliefs.com/atom.htm
Problems with the Rutherford Model
What do you know about opposite charges? Scientists realized that opposite charges should attract. This would mean that negatively charged particles moving around a positively charged nucleus would be attracted toward each other. If they are attracted, shouldn't the negative and positive particles crash into each other? The diagram below shows the possible spiral path of the moving, negatively charged particles toward the nucleus. Can you hypothesize why the particles do not get pulled together?
Diagram of Expected Spiral of Negative Particles into Positive Nucleus
Image from: http://nobeliefs.com/atom.htm
Niels Bohr
Niels Bohr was born in Denmark in 1885. His father was a physiology professor and his mother was the daughter of a wealthy politician. Niels' father was Lutheran and his mother was Jewish, but the different religions were not an issue as neither parent practiced their religions.
Education was important to Niels' parents. They pushed and provided for the best possible experiences for their children. Niels was a good student in most subjects, but did not enjoy writing. Bohr became interested in completing experiments throughout his schooling, including college. However, he avoided writing lab reports until encouraged to do so by his father.
After earning his doctorate degree, Bohr had the opportunity to work in J. J. Thomson's laboratory, but did not get along with Thomson. During Bohr's time in Thomson's lab, Bohr was introduced to Rutherford, who influenced Bohr's work on the behavior of the negatively charged particles in atoms and Bohr's general attitude and approach toward his work.
Bohr's Study of Atoms
Niels Bohr knew that classical physics could not be used to explain the behavior of particles in atoms, so he started looking at the work of Max Planck and Albert Einstein that focused on quantum physics. Bohr took the step of introducing a new way to think about and explain atoms.
Bohr knew that heat or light waves given off by an object were not steady, continuous streams, but were packets of energy, which Einstein called photons and had a speed, frequency, and wavelength. Bohr had a hard time applying this information to atoms until he learned about the work of mathematician Johann Balmer that developed a formula for predicting wavelengths and colors of light given off by hot hydrogen.
Bohr used Balmer's work to help explain that negative particles farther from the nucleus had higher energy and negative particles closer to the nucleus had lower energy. The amount of energy determines the paths of the negatively charged particles around the positively charged nucleus. Bohr's work also helped explain how matter could absorb and emit light. Negatively charged particles could gain energy and change to orbits farther from the nucleus by absorbing light or fall to orbits closer to the nucleus when giving off light.
Bohr Model of the Atom Demonstrating Atom Emitting Light (Photon)
Image from: http://www.famousscientists.org/niels-bohr/
Revising the Model of the Atom
Bohr used quantum physics to demonstrate that negatively charged particles could have fixed orbits based on the energy of the particles. The movement of negatively charged particles was placed in defined orbits of fixed energy. Review the ideas of Niels Bohr and look for patterns in and evidence for his predictions of the atomic model, then answer the questions below.
Questions:
1. Niels Bohr changed the perspective used when studying atoms from classical physics to quantum physics. What evidence did Bohr use to support his model?
2. Using the information from Democritus, Dalton, Thomson, Rutherford, and Bohr, develop a revised model of the atom.
3. How did your model change from the one based on Rutherford's information?
4. We have used the terms negatively charged particles and positively charged particles when talking about models of the atom. Do you know other terms used to describe these particles?
References: http://www.famousscientists.org/niels-bohr/
http://nobeliefs.com/atom.htm