Electron Arrangements within atoms

1. Electron Arrangements within atoms The Work of Niels Bohr

2. In review • The work of Crookes & Thomson had identified the electron • Ernest Rutherford in attempting to prove Thomson’s ‘plum pudding’ model had identified the nucleus

3. Rutherford’s Experiment • Rutherford not only identified the nucleus and thus the proton… • He recognized that the mass of the nucleus couldn’t be explained by protons alone. • He theorized that additional, non-charged particles must exist in the nucleus • the ‘neutron was finally discovered in the 1930’s

4. Niels Bohr • The next great advancement in atomic theory came as a result of the work done by Danish scientist, Neils Bohr. • Scientists wondered that if the nucleus were positive and the electron was negative, why did they not attract one another and crash together?

5. Niels Bohr’s Theory • Bohr used the information gathered from cathode ray tube experiments to develop his theory. • Visible light is broken down by a prism into the spectrum we call a rainbow

6. Bohr’s experiment • When the light from a gas discharge tube is examined in a similar way (through a prism) in a device called a spectroscope, a different spectrum is displayed for each type of gas. • Bohr’s theory was that this was due to the fact that electrons existed in special energy levels, each line in the spectrum representing a separate level.

7. Bohr’s Theory • Bohr’s model suggests that as electrons are energized by electricity in the gas discharge tube they jump to a higher level. • When the electrons fall back to their normal state, they release the energy they absorbed as light. • This, he stated, is what makes the gas glow in the first place (a phenomenon that had never been explained although thoroughly studied)

8. hyperlink http://library.advanced.org/16468/excit-a2.htm this link found in the site “the Amazing world of Electrons and Photons” shows a video clip of gas discharge tubes doing their thing…check it out

9. The Bohr-Rutherford Model • Bohr explains the fact that electrons do not crash into the nucleus for the same reason planets do not crash onto the sun…they are moving in ‘orbits’ • due to the small size of atoms the speed of these orbits is very fast

10. Bohr-Rutherford Model • In this model the nucleus is composed of tightly packed neutrons and protons • Electrons circle the nucleus in energy levels called electron shells. • The first electron shell can hold a maximum of 2 electrons • the second electron shell can hold a maximum of 8 electrons, the third can hold 8, and so on.

11. In addition to explaining the energy levels found in the spectral analysis of gas discharge tubes…. Bohr’s work also helps to explain the large gaps in the periodic table. • An element’s position on the table is explained by the number of electrons it has in its outermost shell

12. Bohr -Rutherford and the periodic table • For example, all of the Alkali metals (group 1) have 1 electron in their outer shell • all of the Halogens (group 7A,)have 7 electrons in their last shell • All of the noble gases (group 8) have 8 electrons in their last shell