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Science Dept.

Chemistry CHE 100 Notes
E. The Atom: the History of Discovery

1. The Electron, e^-
   1. Sir William Crookes (1832-1911) in 1879 discovered cathode rays

   2. J.J. Thomson (1856-1940) in 1897 measured the ratio of the charge of an electron over the mass of an electron, e^-/m_e = 1.76 x 10⁸ coulombs/gram

   3. Millikan (1868-1953) in 1909 measured the charge on an electron, -1.60 x 10^-19 coulombs

   4. Knowing the ratio, e/m, and the charge, e, calculate the mass of the electron

2. The Proton, p⁺
   1. Goldstein in 1886 - "anode rays"

   2. J. J. Thomson
      measured the ratio of the charge of a proton, p⁺, over the mass of a proton, m_p, = p⁺/m_p

   3. For the hydrogen nucleus, p⁺/m_p = 9.57 x 10⁴ coulombs/gram

   4. Knowing the ratio of positive charge to mass, and the charge, calculate the mass of the proton

3. Radioactivity
   1. Becquerel - discoverd radioactivity

   2. Curies - discovered Po and Ra

   3. Rutherford - discovered and named alpha, beta and gamma rays

   4. Rutherford - definition of radioactivity

   5. Nobel Prizes

4. Rutherford - discovers the Nuclear Atom
   1. Bombarded Au foil with alpha particles

   2. Gold Foil Experiment lead to three conclusions:
      1. Most of the matter of an atom is concentrated in a small volume

      2. The positive charge of an atom is also concetrated in a small volume of the atom

      3. Matter is made up mostly of empty space

   3. Geiger and his counter

   4. Marsden and his help

   5. Chadwick (1891-1974) in 1932 discoverd the neutron, n^o, that Rutherford predicted

5. The Bohr Atom
   1. The Simple Notion of Planetary Electons

   2. Related the line spectra to orbital jumps

6. The Wave Mechanical Model of the Atom
   1. de Broglie: the Wave Nature of Matter

   2. Schrodinger: the Wave Mechanical Model of the Atom

7. Energy Levels or Atomic Orbitals
   1. The Principal Energy Levels od 1, 2, 3, 4, 5, 6, 7

   2. Maximum number of electrons in any Principle Energy Level is = 2n²

   3. Therefore:
      1. in Level #1 there are 2 electrons
      2. in Level #2 there are 8 electrons
      3. in Level #3 there are 18 electrons
      4. in Level #4 there are 32 electrons

8. Arrangement of Electrons in Sublevels
   1. In Principle Energy Levels there may be s, p, d, and f sublevels
      1. In the s sublevel there are 2 electrons
      2. In the p sublevel there are 6 electrons
      3. In the d sublevel there are 10 electrons
      4. In the f sublevel there are 14 electrons

   2. The "Stacking of Electrons"
      1. 1s²
      2. 2s²2p⁶
      3. 3s²3p⁶3d¹⁰
      4. 4s²4p⁶4d¹⁰4f¹⁴
      5. 5s²5p⁶5d¹⁰5f¹⁴

   3. RULES of INFILLING: Draw diagonals through the sublevels from the upper right to the lower left
      1. The first diagonal goes through the 1s²
      2. The second diagonal goes through the 2s²
      3. The third diagonal goes through the 2p⁶ and the 3s²
      4. The fourth diagonal goes through the 3p⁶ and the 4s²; N.B. it bypasses the 3d¹⁰!
      5. The Fifth diagonal goes through the 3d¹⁰, the 4p⁶ and the 5s²
      6. These diagonals follow the rules of infilling of the electons in an atom

   4. Some Examples
      1. ₁₉K: 1S² 2S² 2P⁶ 3S² 3P⁶ 4S¹
         This element tends to lose one electron, the 4s¹,
         with the 3S² 3P⁶ left as the outer electrons - a stable outer octet.

      2. ₁₈Ar: 1S² 2S² 2P⁶ 3S² 3P⁶
         This element is an inert gas with a stable outer octet.

      3. ₃₅Br: 1S² 2S² 2P⁶ 3S² 3P⁶ 4S² 3D¹⁰ 4P⁵
         This element tends to gain one electron to fill the 4P level.
         The electron gained creates a stable outer octet of 4S² 4P⁶.

      4. ₃₆Kr: 1S² 2S² 2P⁶ 3S² 3P⁶ 4S² 3D¹⁰ 4P⁶
         This element is an inert gas with a stable outer octet.

9. Electron Dot Formulas - also known as Lewis Structures
   1. Electron Dot Formulas show only the electrons in the outermost principal energy level

   2. For example: the element Na has 11 electrons, but only the outermost electron is shown: Na^.
      The symbol Na will stand for the nucleus and the 10 innermost electrons

   3. For the A elements, write the symbol and place as many dots as the column number

10. How About a Quick Quiz?

 
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