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Atomic Structure

Atomic Structure. The Idea of the Atom. Early Models. Greek philosophers – 450 B.C. what is the smallest particle? Democritus - Particles are atomos An atom is the smallest particle of an element that retains the chemical identity of that element. Aristotle Wins.

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Atomic Structure

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  1. Atomic Structure The Idea of the Atom

  2. Early Models • Greek philosophers – 450 B.C. what is the smallest particle? • Democritus - Particles are atomos • An atom is the smallest particle of an element that retains the chemical identity of that element.

  3. Aristotle Wins • Greek society - slave based • Beneath them to work with hands – no experiments • Settled disagreements by arguments • Aristotle more famous than Democritus Aristotle wins • Didn’t believe or look into atoms until…

  4. Late 1700s • Chemists believed elements: • couldn’t be broken down by ordinary means • Combine to form compounds • Advancements in balances  better experiments

  5. Laws Discovered • Law of Conservation of mass: mass is neither created nor destroyed during chemical or physical changes • Then discovered no matter where or how a compound is made it is made of fixed proportions of elements  Law of Definite Proportions • Ex: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass

  6. Laws Discovered • Elements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2 • Law of Multiple Proportions

  7. John Dalton • 1808 –DALTON’S ATOMIC THEORY • All matter is composed of small particles called atoms. • *All atoms of a given element are identical, but they differ from those of any other element.

  8. Dalton’s Atomic Theory • *Atoms cannot be subdivided, created, or destroyed • Atoms of different elements combine in whole-# ratios to form compounds • In a chemical reaction, atoms are combined, separated, or rearranged.

  9. 1800s – The atom IS divisible • Atom has two regions: • Nucleus: center of atom • Protons: (+) charged • Neutrons: no charge • Electron cloud: region around nucleus containing electrons, (-) charged • Subatomic particles: p+, no, and e-

  10. Voltage source J.J. Thomson’s ExperimentCathode - Ray Tube - + Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)

  11. Voltage source J.J. Thomson’s Experiment + - • By adding magnets, found the moving (-) particles

  12. Thomson’s Model • Same properties no matter what element used • All atoms had ELECTRONS!! • Thomson’s Model Plum pudding: bunch of positive stuff, with the electrons able to be removed.

  13. Ernest Rutherford - 1911 • Atoms have e- = (-) charge • Atoms are neutral  need (+) to balance • Must be other particles to account for mass of atom… • Rutherford used alpha particles to test. • Alpha particle: (+) charge, (2 p+ and 2 no) • Experiment: beam of high speed alpha particles into thin gold foil

  14. Florescent Screen Lead block Uranium Gold Foil Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  15. His prediction…. Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  16. What he expected Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  17. What he happened Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  18. Results… • Most particles passed through • 1 in 8000 bounced back • Some particles completely reversed path!!!

  19. + Plum-pudding is WRONG! • Atom is mostly empty • Small dense, positive pieceat center, nucleus • Alpha particlesare deflected by nucleus if they get close. Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  20. + Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

  21. Electron Cloud Model • Current model of atom • Nucleus surrounded by negative cloud “electron cloud”

  22. Size of Atom • Atom = size of football stadium • Nucleus = smaller than a dime in center • Electron = each one smaller than Roosevelt’s eye on the dime • Atom is mostly empty space!

  23. Composition of Atom • Proton has (+) charge equal in magnitude to (-) charge of electron • Atoms neutral  # p+ = # e- • p+ and no about same mass • e- = 1836 times smaller than p+ • Nucleus has majority of mass • Nuclear Forces: short-range forces that hold nucleus together • p+-p+, p+-no, no-no

  24. Counting Atoms • Atomic Number (Z): number of p+ of each atom of the element • Identifies Elements!!

  25. Atomic Number • What is the atomic number for… • Hg • What is the element with … • 20 p+ • Neutral atoms  # p+= # e- • In neutral atoms, atomic number = # e-

  26. Isotopes • Def: atoms of same element with different # of no (different masses) • 3 H isotopes: • Protium: 1 p+ 99.9885% • Deuterium: 1 p+ 1 no 0.0115% • Tritium: 1 p+ 2 no

  27. Mass Number • Use mass number to determine isotopes • Mass #: # of p+ and n0 • Atom with 17 p+ and 20 no has mass # of… • Chlorine-37 – Hyphen Notation

  28. Writing Isotopes • Nuclear symbol: 37 Mass number Cl Elemental symbol 17 Atomic number

  29. Finding # of Neutrons • # of neutrons = mass # - atomic # • How many p+, no and e- are in… • Selenium – 79 • 27Al • Oxygen - 18 13

  30. Isotopes • Write the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.

  31. Relative Mass of Atom • Mass of Oxygen-16 = 2.656 x 10-23 g • Use relative atomic masses • Pick standard  other masses are expressed in relation to standard • Standard: Carbon –12 atom

  32. Carbon - 12 • Has mass of 12 atomic mass units (amu) • 1 amu: is exactly 1/12 the mass of a carbon-12 atom • Carbon-12: how many p+ and no? • 1 p+ 1 amu (1.007276 amu) • 1 no  1 amu (1.008665 amu) • 1 e-  0 amu (0.005486 amu)

  33. Carbon - 12 • What element is 4 x mass of Carbon –12? • What element is 1/3 mass of Carbon –12?

  34. Average Atomic Masses • Most elements are mixture of isotopes • Mass of element is average mass of isotopes • We need the % of each isotope • Average Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element.

  35. Weighted Average • Box with 100 marbles of 2 sizes: • 25% have mass of 2.00g • 75% have mass of 3.00g • What is the average mass of a marble? • 2.75g (sig figs!!)

  36. Average Atomic Mass • Or (atomic weight) found on periodic table (ROUND TO 2 DECIMAL PLACES!!) • Average Atomic Mass = (% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + … 100

  37. Average Atomic Mass • Find the atomic mass of Li if… • 7.5% is Lithium – 6 = 6.015 amu • 92.5% is Lithium – 7 = 7.016 amu

  38. Example – AAM = 16.00 amu

  39. The Mole

  40. Quantities • 1 dozen = • 12 • 1 gross = • 144 • 1 ream of paper = • 500 • In chemistry: 1 mole = 6.022 x 1023

  41. The Mole • SI unit for amount of substance (mol) • Def: the number of particles in exactly 12 g of carbon – 12. • 12 g of carbon –12 has 6.022 x 1023 atoms • Avogadro’s #, after Amadeo Avogadro • The number is HUGE!!

  42. Molar Mass • Def: mass of 1 mole of a pure substance • 1 mole Carbon –12 = 12 g • 1 atom Carbon – 12 = 12 amu • Mass of 1 mole of He atoms? • 4.00g/mol • Same as atomic mass from periodic table, different units

  43. Molar Mass So: 4.00g He, 6.94 g Li and 200.59 g Hg all have 6.022 x 1023 atoms = 1 mole!!! • Molar mass (g/mol) conversion factor!!

  44. Gram/Mole Conversions • Mass, in g, of 3.6 mol of C? • 43g • How many moles are in 23.5 g S? • 0.733 mol • p. 85 problems

  45. Conversions with Avogadro’s # • Diatomic Molecules: elements that exist as 2-atom molecules in natural state • 7 diatomic molecules • H2, O2, N2, F2, Cl2, Br2, I2 • Remember!!! HONClBrIF

  46. Conversions with Avogadro’s # • How many moles of oxygen are in 2.00x1022 molecules of oxygen? • How many atoms of sodium are in 3.80 mol of sodium?

  47. Conversions with Avogadro’s # • How many molecules of hydrogen are in 0.020 g of hydrogen? • Mass of 5.0x109 atoms of neon? • p.86 problems

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