Unit 2: Atomic Structure & Nuclear Chemistry

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. Millikenin 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) • Discoverednucleus of the atom thru his experiments with gold foiland positive alpha particles(helium atoms that have lost both electrons) • As particles moved thru the foil, they stayed in a straight lineOR bounced straight back or at a large angle

7. Gold Foil Experimental Set-Up

8. Passing in a straight line indicated that there was nothing to block the movement of the particle, so most of the overall structure of the atom was empty space, which would be filled occasionally by the motion of electrons.

9. Objectives #1-3: The Structure of the Atom • Bouncing straight back or off to the side would happen becauselike 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 somethingelse contributing to the mass, but WHAT???? • Neutral particle was discovered and termed the neutron.

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

13. V. Calculation of Average Atomic Mass • Average atomic masses can be determined from the massesof the various isotopes that naturallyoccur for an element and their percent of abundancein 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 I. 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 • X Z • Isotope Notation: element name - A Mass Number Element Symbol Atomic Number

18. Examples: 23Na carbon-14 11 • Calculate the number of : Sodium Carbon 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.

20. Mass Defect

21. Objectives #4-8: The Nucleus/Radioactive Decay Stability of Nuclei • This “loss” of mass is converted into energy,called 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. The energy equivalent to this mass can be found using Einstein’s equation: E = m c 2 Speed of light = 3 x 10 8 m/s Binding Energy Speed of light Mass defect

24. Objectives #4-8: The Nucleus/Radioactive Decay • The binding energy of the elements (and therefore the stabilityof nuclei) steadily increasesup to the element iron and then slowlydecreases. • Nuclei that have a neutron/proton ratio of approximately 1:1tend to be stableand are notradioactive; these include atoms with an atomic number of 83 or less and are found within the “band of stability”.

25. Band of Stability

26. 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 decaywhich will continue until the atom is stable.

27. Objectives #4-8: The Nucleus/Radioactive Decay • 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 unchangingin their composition, however, some atomic nuclei are radioactive and are referred to as radioisotopes.

28. 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 radioactivedecay. Changes can also be man-made(production of plutonium); these changes are referred to as transmutations.

29. Objectives #4-8: The Nucleus/Radioactive Decay • Stability depends on the ratioof 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.

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

31. Radiation Demo: What patterns do you notice?

32. Common Nuclear Equation Symbols Alpha a Beta b Gamma g Positron Emission b+

33. 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 alphaparticle, from an unstablenucleus. • Alpha decay occurs when a nucleus has too many protonsand neutrons. • This release reducesthe atomicnumberby 2and the mass numberby 4.

34. The alpha particle is rather largefor a radioactive particle and therefore it haspoorpenetrating power; your skincan block alpha particles

35. Alpha Decay

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

37. Predict the products… 222 Ra(alpha decay) 88 4218 He+Rn 286

38. 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) 110 n  p+e 01-1 • Beta decay occurs when a nucleus has too many neutrons.

39. Objectives #4-8: The Nucleus/Radioactive Decay • This release increases the atomicnumber by 1as a new proton is created; the mass number is not changed as the loss of the neutron is balanced by the gain of the proton. • The beta particle is much smaller than the alpha particle and therefore it has a greaterpenetrating power; a piece of metal foil can block beta particles.

40. Objectives #4-8: The Nucleus/Radioactive Decay • Beta Radiation Examples: 14 C  (beta decay) 6 014 e+N -17

41. Predict the products… 131 I ---› (beta decay) 53 0 131 e + Xe -1 54

42. 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

43. Gamma Decay

44. Example: 14 0 14 C g + C 60 6

45. Objectives #4-8 :The Nucleus & Radioactive Decay • Gamma radiation has no mass or chargeand therefore has highenergy and high penetrating 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

46. 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.

47. Electron Capture

48. Electron Capture… 1 0 1 p + e  n 1 -1 0

49. Electron Capture… 201 0 Hg + e  80 -1 201 Au 79

50. Predict the products… 243 0 Am + e  95 -1 243 Pu 94