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Unit 2: Atomic Structure & Nuclear Chemistry

Unit 2: Atomic Structure & Nuclear Chemistry. Chemistry I Honors. Objectives #1-3: The Structure of the Atom. Contributions of J.J Thomson (1856-1940, English) Discovered the electro n through experiments with cathode ray tube (CRT) Determined the mass of electron to be

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Unit 2: Atomic Structure & Nuclear Chemistry

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  1. Unit 2: Atomic Structure & Nuclear Chemistry Chemistry I Honors

  2. Objectives #1-3: The Structure of the Atom • Contributions of J.J Thomson (1856-1940, English) • Discovered the electron through experiments with cathode ray tube (CRT) • Determined the mass of electron to be 9.11 X 10-28 grams or 1/1840 of an atomic mass unit (a.m.u.)

  3. Cathode Ray Tube

  4. Objectives #1-3: The Structure of the Atom • Concluded that electron charge is negative based on the movement of electrons in the CRT when positive and negative plates are placed near the tube • Charge and mass of electron confirmed by U.S. physicist R. Milliken in 1909 • Developed the plum pudding model for an atom:

  5. Objectives #1-3: The Structure of the Atom • Atom composed of a sea of negative and positive charges evenly distributed thru the atom

  6. Objectives #1-3: The Structure of the Atom II. Contributions of Ernest Rutherford (1871-1937, English) • Discovered nucleus of the atom thru his experiments with gold foil and positive alpha particles (helium atoms that have lost both electrons) • As particles moved thru the foil, they stayed in a straight line OR bounced straight back or at a large angle

  7. Gold Foil Experimental Set-Up

  8. Rutherford’s Gold Foil Experiment

  9. Objectives #1-3: The Structure of the Atom • Bouncing straight back or off to the side would happen because like charges repel, so he concluded that all atoms contain a central dense nucleus composed of positive protons • This model of the atom is known as the nuclear atom

  10. Gold Foil Experiment Results:Thomson Atom vs. Rutherford Atom

  11. Objectives # 1-3: The Structure of the Atom III. Contributions of James Chadwick (1891 - 1974), English • Mass of nucleus was always greater than predicted, so there must be something else contributing to the Mass, but WHAT???? • Neutral particle was discovered and termed the neutron

  12. Objectives #1-3: The Structure of the Atom Major Subatomic Particles In Atom:

  13. IV. Calculation of Average Atomic Mass • Average atomic masses can be determined from the masses of the various isotopes that naturally occur for an element and their percent of abundance in nature

  14. Examples: • Naturally occurring chlorine is 75.78 % chlorine-35 with a mass of 34.969 amu and 24.22 % chlorine-37 with a mass of 36.966 amu. Determine the average atomic mass for chlorine. Average atomic mass = (.7578) (34.969) + (.2422) (36.966) = 26.50 + 8.953 = 35.45 amu

  15. Three isotopes of silicon occur in nature: silicon-28 92.23 % 27.97693 amu silicon-29 4.68 % 28.97649 amu silicon-30 3.09 % 29.97377 amu • Calculate the average atomic mass for silicon: (.9223) (27.97693 amu) + (.0468) (28.97649 amu) + (.0309) (29.97377 amu) = 25.803 amu + 1.356 amu + .9262 amu = 28.0852 amu

  16. Objectives #4-8: The Nucleus/Radioactive Decay Distinguishing Among Nuclides • A nuclide is identified by the number of protons and neutrons in its nucleus • The identity of an element is determined by the number of protons in one atom of that element • Atoms are identified by numbersthat describe fundamental characteristics of that atom

  17. Objectives #4-8: The Nucleus/Radioactive Decay • Atomic number = number of protons, (also indicates number of electrons) • Mass Number = number of protons + neutrons • Atomic Notation: A • E Z • Isotope Notation: element name - A Mass Number Element Symbol Atomic Number

  18. Examples: 23Na carbon-14 11 • Calculate the number of : Protons Neutrons Electrons

  19. Objectives #4-8: The Nucleus/Radioactive Decay • The difference between the actual mass of a nuclide and the calculated combined mass of its protons, electrons, and neutrons is called the mass defect. • Quantified by Einstein’s equation: E = (m) (c2)

  20. Mass Defect

  21. Objectives #4-8: The Nucleus/Radioactive Decay Stability of Nuclei • This “loss” of mass is converted into energycalled binding energy in order to hold together the nucleus of the nuclide (remember the nucleus contains positive particles that would tend to repeleach other)

  22. Graph of Binding Energy

  23. Objectives #4-8: The Nucleus/Radioactive Decay • The binding energy of the elements (and therefore the stability of nuclei) steadily increasesup to the element iron and then slowly decreases • Nuclei that have a neutron/proton ratio of approximately 1:1tend to be stableand are not radioactive; these include atoms with an atomic number of 83 or less and are found within the “band of stability”

  24. Band of Stability

  25. Objectives #4-8: The Nucleus/Radioactive Decay • An isotope of an atom could be radioactive if the neutron/proton ratio is greater than 1:1 Example: carbon-12 has a ratio of 1:1 – not radioactive carbon-14 has a ratio of 1.3:1 – is radioactive • Nuclei found outsidethe band of stability will undergo spontaneous radioactive decay which will continue until the atom is stable

  26. Objectives #4-8: The Nucleus/Radioactive Decay • Stability of nuclei is greatest when nucleons are in pairs • Nucleons are theorized to be arranged in energy levels (like electrons) according to the nuclear shell model III. Nuclear Radiation • Most atoms are very stable and unchanging in their nuclear composition, however, some atomic nuclei are radioactive and are referred to as radioisotopes.

  27. Objectives #4-8: The Nucleus/Radioactive Decay • These nuclei are unstableand will break into smaller, more stable nuclei • Changes can be spontaneous and naturally occurring for some atoms (smoke detectors); these changes are referred to as radioactive decay or can be man-made(production of plutonium); these types of changes are referred to as transmutations

  28. Objectives #4-8: The Nucleus/Radioactive Decay • Stability depends on the ratio of neutronsto protonsand the size of the nucleus. • During these nuclear changes, aka nuclear decay, energy (radiation) in the form of raysand particles of varying penetrating power are given off, and the unstable nuclei changes into a stablenuclei of a different atom.

  29. Examples of Radioactive Decay: Alpha Decay, Beta Decay, Electron Capture

  30. Objectives #4-8: The Nucleus/Radioactive Decay The three main types of radiation released are: alpha, beta, and gamma • Alpha radiation involves the release of a highly energetic helium nucleus, (aka alpha particle), from an unstablenucleus. • Alpha decay occurs when a nucleus has too many protonsand neutrons. • This release reducesthe atomic number by 2and the mass numberby 4.

  31. The alpha particle is rather largefor a radioactive particle and therefore it hasapoor penetrating power; your skincan block alpha particles

  32. Alpha Decay

  33. Alpha Radiation Example: • 240Pu236U+4He 9492 2

  34. 222 Ra(alpha decay) 88 4 218 He + Rn 286

  35. Objectives #4-8: The Nucleus/Radioactive Decay • Beta radiation involves the formationof a highly energetic electron, aka betaparticle, from the breakdownof a neutron: neutron  proton + beta particle (b) 1 1 0 n  p+e 01-1 • Beta decay occurs when a nucleus has too many neutrons.

  36. Objectives #4-8: The Nucleus/Radioactive Decay • This release increases the atomicnumber by 1 as a new proton is created; the mass number is notchangedas the loss of the neutron is balanced by the gain of the proton • The betaparticle is much smaller than the alpha particle and therefore it has a greaterpenetrating power; a piece of metal foil can block beta particles

  37. Beta Decay

  38. Objectives #4-8: The Nucleus/Radioactive Decay • Beta Radiation Examples: 14 C  (beta decay) 6 0 14 e+N -17

  39. Objectives #4-8: The Nucleus & Radioactive Decay *Gamma radiation is highenergy electromagneticradiation (energy that travels as waves) produced from an unstable nucleus; this form of radiation often accompanies releases of alpha and betaradiation

  40. Objectives #4-8 :The Nucleus & Radioactive Decay • Gamma radiation has no mass or chargeand therefore has highenergy and highpenetrating power; several sheets of leadare required to block gamma radiation • Massnumber and atomicnumber of unstable nucleus remains unchanged from gamma decay, BUT will change if accompanied by alpha or beta decay

  41. Gamma Decay

  42. Objectives #4-8: The Nucleus & Radioactive Decay • If a nucleus has too many protons, it will become more stable by the process of electron capture or positron emission (both of which will use up some of the extra protons) • Free electrons would be available due to emission of electrons produced during betadecay of another isotope

  43. Objectives #4-8: The Nucleus & Radioactive Decay • Positrons are produced when protons decay to form neutrons and a specialized electron with a positive charge (antimatter) Examples: 1 1 0 pn + e 10 +1

  44. The processes of electron capture and positron emission will decreasethe number of protonsand increase the number of neutrons.

  45. Positron Formation

  46. 22 Na  (positron formation) 11 0 22 e + Ne +1 10

  47. Electron Capture

  48. 201 0 Hg + e  80 -1 201 Au 79

  49. Matter / Antimatter Annihilation Reaction

  50. 00 e + e  (annihilation rx.) +1 -1 0 g(energy) 0

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