The History of the Atom… A Summary

1. The History of the Atom…A Summary Miss Maroldo

2. Early ideas of the Structure of the Atom Leucippus, a philosopher of the 5th century BCE, was the first believed to consider that matter was composed of smaller particles. His philosophy was called atomism. These smaller particles, he believed, could be divided into two categories: full and solid, or empty and void. Democritus, a supposed student of Leucippus, continued the idea of atomism. He called the small particles that composed all matter atoms (meaning indivisible). He said that all atoms were firm and solid, but not the same. Atoms would be similar to the objects they made, i.e. “the atoms of a needle would be sharp” (J. Nguyen) Aristotle, a philosopher of the 3rd century BCE, rejected the ideas of Leucippus and Democritus and maintained the a popular idea of the day; that all matter is composed of four “elements,” earth, wind, water and fire. Image courtesy of: http://www.sixsquare.com/blog/images/0805/airearthfirewater2.jpg Thank you J. Nguyen, J. Lawrence, K. Porter and M. Jones

3. John Dalton Continued the earlier theories of particulate matter with his atomic theory of matter: 1. Everything is formed of atoms. 2. All atoms of the same element are identical, but differ from any other element. 3. Atoms can be neither created nor destroyed 4. A given compound always forms in the same who number ratios of atoms. Much of Dalton’s contributions to atomic theory stemmed from his work studying meteorology.

4. J.J. Thomson J.J. Thomson, a school teacher, conducted a series of experiments with a cathode ray tube beginning in 1896. Check out this video: http://online.cctt.org/physicslab/content/Phy2HON/lessonnotes/modern/electronbeams.asp He found that the mysterious glowing ray inside the cathode ray tube was responsive to the presence of a magnetic. Thomson concluded that the ray must be composed of negatively charged particles that he called electrons. Although he was unsuccessful at determining the mass of an electron, he did determine the charge ratio of the electron to be 1.76 x 108 coulombs/gram.

5. J.J. Thomson Conn… After discovering the electron, Thompson theorized that an atom must be composed of electrons floating in a sea of positive charge as shown below. This model is often referred to as the plumb-pudding model. Courtesy of: http://en.wikipedia.org/wiki/Image:Plum_pudding_atom.svg

6. R.A. Millikan Millikan succeeded in measuring the charge of an electron in 1909. How you ask??? He sprayed oil droplets into the apparatus shown. An x-ray was used to give the droplets a charge. As the droplets fell between charged plates, he measured the how the different charges on the plates affected the rate at which the droplets fell. Courtesy of: http://www68.pair.com/willisb/millikan/experiment.html From this data, Milikan measured that the charge of each droplet was a multiple of 1.60 x 10-19 coulombs. This meant the each electron had a charge of 1.60 x 10-19 coulombs. Using Thomson’s mass-to-charge ratio, Millikan determined the mass of an electron to be 9.11x10-28 grams (2000 times smaller than a hydrogen atom!)

7. Rutherford Rutherford’s gold-foil experiment lead him and his team to discover the nuclear atom in 1909. When Rutherford shot a beam of alpha particles (which have a positive charge) at a piece of thin gold foil, he found that while most particles passed straight through, others seemed to bounce off. Courtesy of:http://www.dlt.ncssm.edu/TIGER/diagrams/structure/GoldFoilExperiment.jpg

8. Rutherford Conn… Earlier models of the atom hypothesized the atom to be composed of electrons simply floating in a sea of positive charge. Had this been true, all of the alpha particles would have passed right through as shown in diagram a. Because some alpha particle were deflected, he concluded that there must be a concentration of positive charge in the center of the atom (diagram b). He called this this nucleus. Picture Courtesy of: http://library.tedankara.k12.tr/chemistry/vol2/the%20structure%20of%20the%20atom/z15.jpg Check out this web site:http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

9. Max Plank Recall that at this point in history, the atom consists of electrons somehow moving around a nucleus of positive charge. Plank suggested that energy emitted or absorbed by any object is restricted to particular sizes, which he referred to as quanta, as is expressed in his equation E = hv. The equation shows us that the energy emitted by an object is directly related to the wavelength is emits. What is quantized energy? It is not continuous (like an escalator at the mall). Quantized energy occurs in specific steps (like taking the stairs at the mall). The size of the steps are unique to the object absorbing or emitting the energy.

10. Max Plank Applying Plank’s theory to atoms tells us that the energies absorbed or emitted by atoms are quantized, which means that their values are restricted to certain quantities and the energy is not continuous. Although we now recognize Plank’s discovery as laying the foundation for advancing the model of the atom, his discovery did not attract much attention at first. Even Plank himself was uncomfortable with his proposed concept and did not recognize its importance at this time.

11. Albert Einstein After studying the work of Max Plank, Einstein saw a new way of thinking about light. He used Plank’s theory to explain a puzzling phenomena, the photoelectric effect. In the photoelectric effect, electrons are ejected from the surface of a metal upon exposure to light. However, it was known that only certain frequencies of light would induce the photoelectric effect in certain metals. Red light, for example, was unable to eject electrons from potassium metal, however, violet light, even faint violet light, released electrons easily.

12. The Photoelectric Effect Courtesy of: http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod2/pelec.gif Notice above, that only certain wavelengths of light (or certain energies of light) can eject electrons from the surface of this metal.

13. Albert Einstein Conn… Einstein extrapolated Plank’s idea and said that light must consist of quanta of energy that behave like tiny particles of light. He called these quanta of light photons, which carry the amount of energy given by Plank’s equation E = hv. When a photon strikes the surface of a metal, it transfers its energy to an electron in a metal atom. The electron can only absorb the entire photon or none of the photon, but not part of the photon. Nor can it absorb several photons to gain the energy of more than one photon. Therefore, if the energy of the photon is not great enough, the electron cannot escape the metal atom. Lastly, if light has the ability to eject electrons from metals, then light must have the properties not just of waves, but as particles as well.

14. Why does green and violet light eject electrons from the metal? Courtesy of: http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod2/pelec.gif Green and violet light have greater energy than red light, and “result in a great amount of energy per photon (LeMay).” (Recall, the energy of visible light increases through the rainbow, ROYGBIV.)

15. Niels Bohr The fact that electrons can only absorb or emit energy in chunks, or quanta, laid the foundation for theorizing just how electrons are arranged in an atom. This and the explanation of another puzzling phenomena lead Niels Bohr to propose the first model of the atom that provided the first illustration the arrangement of electrons in an atom.

16. Line Spectra Early in the century it was observed that samples of elements emit light when they are vaporized in an intense flame. The light that is seen results from the atoms of the element absorbing the energy of the flame and releasing the absorbed energy in the form of light. When this light is passed through a prism, a resulting line spectra of specific wavelengths results. This line spectra is unique for each element.

17. Line Spectra Notice that the color flame produced from the vaporization of sodium is yellow. If the yellow light is separated into its component wavelengths with a prism, the resulting spectrum is shown at left. Courtesy of: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Flametest--Na.swn.jpg/200px-Flametest--Na.swn.jpg Courtesy of: ://www.luc.edu/faculty/spavko1/JCE/line-spectra/SODIUM.GIF http

18. Niels Bohr Conn… Bohr was the first to see the connection between the characteristic line spectra of each element and atomic structure. Using Rutherford’s nuclear atom and Plank’s idea of quantization, Bohr created a model of the atom that explained line spectra. His model of the atom still represents the simplified atom today.

19. The Bohr Model of the Atom Courtesy of: http://upload.wikimedia.org/wikipedia/commons/9/9b/Bohratommodel.png Bohr proposed that the electrons of an atom must be arranged in certain orbits that correspond to the amount of energy of that electron (these energies are represented by the letter “n” and increase as the distance from the nucleus increases.) Here, Hydrogen is represented, since on proton is present and one electron orbits the nucleus.

20. How does this Explain Line Spectra? When an electron absorbs a photon with high enough energy, it becomes excited and jumps to the energy level that corresponds to the new energy of the electron. When the electron releases the absorbed energy and returns to its original position, light is released. Because the energy levels of the electrons are quantized, only certain photons will allow the electron to absorb enough energy to jump to n=2, n=3 and so on. Therefore, only certain wavelengths will be released upon return to the original energy level. Although Bohr’s model worked well for the hydrogen atom, it could not explain spectra of elements with more than one electron.

21. Louis de Broglie As you recall, Einstein stated that if light waves could eject electrons from the surface of a metal, then light must have some particulate properties. de Broglie wondered that if light could behave as particles, could particles behave as waves??? de Broglie theorized that all matter exhibits properties of both particles and waves. He derived this mathematical relationship and showed that the wavelength of an object was directly related to its velocity and inversely related to its mass.

22. Louis de Broglie Conn… What does this have to do with the model of the atom??? de Broglie showed that electrons, while traditionally thought of as minute particles, could also behave like waves. This laid the foundation for Werner Heisenberg who proposed the Heisenberg Uncertainty Principle which states that the position and momentum of a moving object cannot both be known precisely at the same time. You will learn shortly that we now describe not only the arrangement of electrons around the nucleus in terms energy, but also of the probability of finding the electron in certain locations around the nucleus.

23. Becquerel Becquerel became interested in a predecessor's discovery of a mysterious energy called X-rays. These rays were found induce fluorescence in certain minerals and had the ability to pass through low-density shields. Since X-rays could induce fluorescence in certain minerals, Becquerel wanted to see if a fluorescent mineral was capable of emitting X-rays, specifically, uranium.

24. Henri Becquerel Conn… After researching for some time, Bacquerel becomes frustrated with his work. He wraps the uranium sample in black paper to prevent light exposure and throws it into his drawer, where it would remain for several days in the dark. Just by chance, Becquerel threw the sample on top of a photographic plate and just by chance, he decided to develop the plate after several days (even though the plate had been unexposed to light, or so he thought…)

25. Becquerel Conn… What do you know??? The photographic plate showed exposure! This lead Becquerel to the discovery of the phenomena of radioactivity and the first radioactive element: Uranium.

26. Marie Curie Marie and her husband, colleagues of Becquerel, are credited with the isolation of two more radioactive isotopes: radium and polonium. “As scientists studied radioactivity, they made an important observation: Radioactivity accompanies fundamental changes in an atom…Studying the nature of these changes gave scientists further clues about the substructure of the atom.” (LeMay)

27. Works Cited LeMay, H., Beall, H., Robblee, K., & Brower, D. (1996). Chemistry: Connections to Our Changing World. Upper Saddle River, NJ: Prentice Hall.