Unit D Conclusion

Unit D Conclusion

In Module 7 you studied the early works that led to the discovery of the cathode ray, which served as a vehicle for investigations into the nature of the particles that produced it. You also explored J.J. Thomson’s work with the cathode ray, most importantly his determination of the charge-to-mass ratio of the particles in cathode rays, which was a ratio thousands of times larger than that for other common particles like the hydrogen ion. The concepts and theories used in Thomson’s original experiment are now commonly applied in mass spectrometer technology.

By showing that cathode rays were deflected by electric and magnetic fields, Thomson proved that they were streams of negatively charged elementary particles—the electrons. He then developed the “raisin-bun model” of the atom, which featured electrons plopped into a bread of positively charged matter. Millikan’s now-famous oil drop experiment determined the actual charge of that particle. It also confirmed that the atom was divisible into smaller parts.

Rutherford’s scattering experiment further developed the atomic model. A high percentage of the large positive particles directed at the thin foil passed through the foil, indicating that the atom was mostly empty space (and not a bun). Some particles, however, were deflected. Rutherford concluded that the atom included a small, positively charged region that could deflect the incoming particles by electrostatic repulsion. His discoveries lead to the planetary model of the atom.

The planetary model presented some difficulty with classical physics and required significant revisions, which were accomplished courtesy of Niels Bohr. Bohr recognized that, according to Maxwell’s theory, if electrons (which are charged particles) are experiencing centripetal acceleration, these accelerating particles should continuously emit electromagnetic radiation (EMR). According to conservation of energy, however, this emission of energy should result in a decrease in the electron’s kinetic energy and its eventual spiral into the nucleus.

Bohr’s Semi-Classical Model describes electrons orbiting the nucleus in certain stable states (energy levels) with specific energies and radii. This quantization of the energy explained patterns in the EMR spectrum of certain elements leading to the identification of atoms and elements on distant objects, such as the Sun.

Finally, you explored the Quantum Mechanical Model of the atom, which described the electron position using a probability distribution—a distribution that indicates where the electron is most likely to be found at any given time.

Studying all of these theories about the composition and structure of the atom in Module 7 helped you understand that theories are constantly changing and that scientific models are human inventions created to try to understand and explain physical phenomena. As science continues to uncover secrets of the atom, new models will evolve. This is very important to remember as we try to explain things we cannot see in Module 8.

In Module 8 you explored how an ionizing smoke detector depends on the unstable americium-241 nucleus in order to detect smoke and save lives. Since some nuclei are unstable, they can decay, resulting in different elements. This natural change from one element to another is called transmutation, and it can produce alpha and beta particles.

You learned how both alpha and beta decay produce significant amounts of energy observed in the kinetic energy of the emitted particles and the production of high-frequency gamma radiation, both of which can ionize gas in a smoke detector when smoke particles are absent.

Next, you investigated how the unstable nuclei decays and how the amount of radioactivity emitted reduces over time. The rate of decay is described by the half-life of the radioactive isotope. You then learned how it is possible to accurately determine the age of a sample, given the half-life of the radioactive isotope, the remaining amount of parent nuclei, and the original amount of the parent nuclei.

You then explored fission and fusion, which are also very powerful. You discovered that both fission and fusion nuclear reactions are processes that lead to an increase in the binding energy per nucleon, increasing the stability of the resulting nuclei. In both cases, the total, final binding energy is much larger than the initial binding energy, producing a more stable nucleus and releasing a large amount of energy that can power modern civilization.

You then compared the product of hydrogen fusion (helium), which is stable and safe, to the unstable, dangerous daughter nuclei produced in a fission reaction. Both reactions, however, have positive and negative aspects.

Next, you learned about particle accelerators and how they are being used to create collisions at energies never before seen on Earth. From these collisions the content of the particles can be probed and studied, which is very exciting to the world of subatomic physics.

Lastly, you explored the subatomic world and the standard model and discovered it is composed of strange ideas and particles (some that still need to be verified!), such as antimatter, mediating particles, and the quarks that make up protons and neutrons. You learned that the standard model changes as more evidence is collected and that the interaction of theory and observation lead to more discoveries. Together, they both work toward a grand unified theory that will connect the fundamental forces of the universe and the particles that mediate, create, and sustain them.