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Chapter 3: Atomic Structure

Chapter 3: Atomic Structure. Scientists. Democritus- Matter composed of atoms (indivisible) (~450B.C.) Lavoisier – conservation of mass Proust – law of constant composition Dalton – modern atomic theory (KNOW the 4 postulates). Law of Constant Composition.

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Chapter 3: Atomic Structure

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  1. Chapter 3: Atomic Structure

  2. Scientists • Democritus- Matter composed of atoms (indivisible) (~450B.C.) • Lavoisier – conservation of mass • Proust – law of constant composition • Dalton – modern atomic theory (KNOW the 4 postulates)

  3. Law of Constant Composition • Faraday – Atoms contain charged particles • Thomson – atoms are divisible, he discovered electrons • Millikan – found the charge and mass of electrons

  4. Scientists • Becquerel –discovered radioactivity • Marie Curie – isolated radioactive elements. • Rutherford – demonstrated existence of neutrons and the nucleus

  5. Rutherford • Modern atom • Nucleus - central charge concentrated into a very small volume in comparison to the rest of the atom • tiny electrons circling around the nucleus like planets around the sun.

  6. Modern Atomic Theory • Atoms are composed of three fundamental particles • Protons p+ Neutrons no Electrons e- • The nucleus is made of protons and neutrons, (positively charged) • Electrons orbit the nucleus in and electron cloud (negatively charged) • An atom is neutral, the # p+ = # e-

  7. Chart

  8. Atomic Number • Atoms identity comes from the number of protons in the nucleus • In a chemical reaction, atoms gain/lose electrons and become an ion. • Ion is a charged particle. This can be + or – depending on whether an electron is gained or lost.

  9. Calculating charges and writing ions. • If an electron is gained, the charge becomes negative. • If an electron is lost, the charge becomes positive. • Ex. Magnesium • Number of protons = 12 • Number of electrons = 10 • Charge of ion = 2+ or Mg+2

  10. Isotopes • Isotopes -atoms of the same element (same #p+) but with different number of neutrons. • All elements have isotopes. • Isotopes of elements are almost indistinguishable (they exhibit the same properties)

  11. The mass number is used to identify isotopes. • Mass number = the sum of the p+ and n0 • Mass number → 37 • Cl • Atomic number →17

  12. More Examples • Other examples • Cl-35 C-12 C-14 • 35 12 14 • Cl C C • 17 6 6

  13. Even MORE examples • Ions • 56 16 27 • Fe+2 O2- Al+3 • 26 8 13

  14. Mass of atoms- • Masses measured in amu’s • AMU = atomic mass unit = 1/12 the weight of a carbon-12 atom • Atomic Mass = atomic weight = average atomic mass

  15. Calculation of average atomic mass • = Average mass of an element’s atoms • Lithium – 6 7.42% = 6 x 0.0742 = 0.4452 • Lithium – 7 92.58% =7 x .9258 = + 6.4806 • 6.9258 amu

  16. You try it! • Neon – 20 90.92% = • Neon – 21 0.26% = • Neon – 22 8.82% = • 20.179 amu

  17. Changes in the nucleus • Nuclear Reactions – Change the composition of the nucleus. • Atoms undergo nuclear decay and produce new elements! • Why are some atoms radioactive?

  18. Changes in the Nucleus • What governs nuclear stability? • part of reason is the # of p+ and # no • strong nuclear force • force which holds the nucleus together

  19. Pattern of stable nuclei “belt of stability” – as atomic number increases, you need more neutrons to keep the atom stable • All atoms with an atomic number greater than 83 are radioactive

  20. Radioactive Decay • Radioactive Decay – emission of radiation • 3 types: • Alpha: • High-energy alpha particles • 2p+ and 2 n0. • 4 •  • 2 • Weak, stopped by paper or clothing • Mass number 4

  21. Radioactive Decay • Beta: • High speed electrons • 0 0 •  e • -1 -1 • Mass number = 0 • Can pass through clothing, some damage to skin

  22. Radioactive Decay • Gamma: • Most dangerous • Consists of radiation waves • Only stopped by heavy dense material like lead/concrete • 0 •  • 0

  23. Writing nuclear equations: • Atomic mass • Chemical symbol • Atomic Number

  24. Alpha particles When a nucleus emits an alpha particle, the mass decreases by 4 amu’s and the number decreases by 2 amu’s. • 226 222 4 •  Ra → Rn +  • 88 86 2

  25. Beta particles • When a nucleus emits a beta particle, the mass of the atom is practically unchanged, but the atomic number increases by one unit. 131 131 0 •  I → Xe +  • 53 54 -1

  26. Gamma Rays When a nucleus emits a gamma ray, both the atomic number and atomic mass remain the same. • 113 113 0 •  In → In +  • 49 49 0

  27. Application of Nuclear Chemistry • Use of half life + Radioactive Dating • Nuclear Bombardment – Reactions • Create radioactive isotopes used in medicine • Power Generation • Fission – Limerick Generating Plant • Fusion – “research”

  28. Radioisotope – an isotope that is radioactive. • Half-life – The amount of time it takes for ½ of a sample of a radioactive isotope to decay. (1/2 of the radioactive atoms) • Ex. 90Sr = 28.8 yrs

  29. Radiocarbon Dating • uses carbon-14 • carbon-14 is radioactive • half-life is 5370 yrs • Produced naturally from reaction between N-14 and cosmic rays • Rate of production carbon-14 = rate of decay of carbon-14

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