Isotopes

1. Isotopes Pisgah High School M. Jones Rev. 022609

2. Isotopes … …have the same number of protons and electrons but different numbers of neutrons. Therefore, isotopes of the same element have different masses.

3. Isotopes … …have different mass numbers but the same atomic number. Atomic number - the number of protons in the nucleus of an atom Mass number - the number of protons added to the number of neutrons

4. Mass number Symbol of Element Atomic number Symbols for Isotopes A is the symbol for mass number A E Z Z is the symbol for atomic number

5. Mass number Symbol of Element Atomic number Symbols for Isotopes 92 235 U

6. Mass number Symbol of Element Atomic number Symbols for Isotopes U 235 92

7. Symbol of Element Mass number Symbols for Isotopes Find U in the periodic table. Z = 92 U-235 How do you know the atomic number?

8. Determining the Atomic Number and Mass Number • The atomic number is the number of protons. • The mass number is the sum of the protons + neutrons. Consider an atom of aluminum with 13 protons and 15 neutrons. What is Z and A?

9. Determining the Atomic Number and Mass Number Answer: Atomic number: Z = 13 Mass number: A = 28 A = #p+ + #n0 13 + 15 = 28

10. Finding the number of Protons, Neutrons, and Electrons The atomic number is the number of protons in the nucleus. The number of electrons in a neutral atom equals the number of protons.

11. Finding the number of Protons, Neutrons, and Electrons The number of neutrons is the difference between the mass number and the atomic number. # of neutrons = A - Z

12. Finding the number of Protons, Neutrons, and Electrons U-235 Look at the Periodic Table and find the element by using the symbol Z = 92 protons = 92 electrons = 92 A = 235 protons + neutrons = 235

13. Finding the number of Protons, Neutrons, and Electrons U-235 235 prot.& neut. - 92 protons 143 neutrons Z = 92 protons = 92 electrons = 92 A = 235 protons + neutrons = 235

14. Finding the number of Protons, Neutrons, and Electrons Q. Find the number of neutrons in the Ba-137 isotope. • In the Ba-137 isotope … • … Z = 56 and A = 137 • 137 – 56 = 81 neutrons

15. Finding the number of Protons, Neutrons, and Electrons Fill in the following table.

16. Finding the number of Protons, Neutrons, and Electrons

17. Finding the number of Protons, Neutrons, and Electrons

18. Finding the number of Protons, Neutrons, and Electrons

19. Finding the number of Protons, Neutrons, and Electrons

20. Finding the number of Protons, Neutrons, and Electrons

21. Finding the number of Protons, Neutrons, and Electrons

22. Finding the number of Protons, Neutrons, and Electrons

23. Alike: Number of protons and electrons Atomic number Chemical properties Different: Number of neutrons Mass Number Atomic weight of the isotopes How are isotopes of the same element alike and different?

24. Which of the following is the same for the three isotopes of magnesium? • The atomic number of 12 • The number of protons and electrons • The number of neutrons • The atomic weight of 24.986 AMU • The reaction with hydrochloric acid • The speed of gaseous Mg atoms

25. Which of the following is the same for the three isotopes of magnesium? • The atomic number of 12 Same All three isotopes of magnesium have the same atomic number.

26. Which of the following is the same for the three isotopes of magnesium? 2. The number of protons and electrons Same All isotopes of the same element have the same number of protons in the nucleus, and electrons outside the nucleus.

27. Which of the following is the same for the three isotopes of magnesium? 3. The number of neutrons Not the same The number of neutrons varies with the isotope. Different isotopes have different numbers of neutrons.

28. Which of the following is the same for the three isotopes of magnesium? 4. Atomic weight of 24.986 AMU Not the same Mg-24  23.985 AMU Mg-25  24.986 AMU Mg-26  25.983 AMU

29. Which of the following is the same for the three isotopes of magnesium? 5. The reaction with HCl Same All isotopes of the same element react the same chemically. The number and arrangement of electrons is the same for each isotope.

30. Which of the following is the same for the three isotopes of magnesium? 6. The speed of gaseous Mg atoms Not the same The speeds of atoms depend on mass. Heavier atoms move more slowly, and lighter atoms move faster.

31. How did knowing that the speeds of gaseous isotopes are different, allow the United States to win World War II?

32. Who were the two guys responsible for winning World War II? Fat Man, and … Little Boy Atomic bombs dropped on Hiroshima and Nagasaki

33. Manhattan Project Oak Ridge, TN Gaseous Diffusion Most naturally occurring uranium is U-238 Less than 1% of naturally occurring uranium is U-235 To sustain a nuclear chain reaction, uranium must be at least 4% U-235.

34. To make either bomb-grade uranium, or fuel for a nuclear reactor you need enriched uranium. Enriched uranium has a higher percentage of U-235. 12% for a reactor, 90% for a bomb.

35. The gaseous diffusion plant at Oak Ridge was used to make enriched uranium. The uranium oxide is reacted with fluorine to make UF6 which is a gas. The UF6 is introduced into a container with a porous barrier.

36. The molecules of UF6 containing U-235 diffuse through the porous barrier slightly faster. Produces a slightly higher percentage of U-235 in the UF6. Then the process is repeated many times.

37. The K-25 building at Oak Ridge

38. Atomic Weights

39. Calculating Average Atomic Weight Atomic mass (atomic weight) is the weighted average of the masses of all the isotopes of the element.

40. Calculating Average Atomic Weight A weighted average puts more weight or importance on one value that another. Example: Compute the average age of everyone in this room.

41. Calculating Average Atomic Weight Suppose there were two isotopes of the element Q. Q-30 95% Q-32 5% Determine the average atomic mass.

42. 3010 Calculating Average Atomic Weight Is the average atomic weight 31? ( 30 + 32 ) / 2 = 31 Assume 100 atoms of Q: Much closer to 30 95 x 30 amu = 2850 5 x 32 amu = 160 / 100 = 30.1 amu

43. Calculating Average Atomic Weight You can compute the atomic weight by knowing … … the mass (M) and … the relative abundance (RA) of each naturally occurring isotope.

44. Calculating Average Atomic Weight The mass is the actual mass of the isotope in amu. The relative abundance is the percentage of that isotope divided by 100.

45. Calculating Average Atomic Weight Example: Suppose that 48.6% of the atoms of zinc in the world were Zn-64. Then the relative abundance of zinc-64 would be 0.486.

46. Calculating Average Atomic Weight Average atomic weight = (M1 * RA1) + (M2 * RA2) + Mass of each isotope times its relative abundance (M3 * RA3) …

47. Calculating Average Atomic Weight Calculate the average atomic weight for Cu-63 and Cu-65.

48. Calculating Average Atomic Weight Calculate the average atomic weight for Cu-63 and Cu-65. AW = (M1 * RA1) + (M2 *RA2) AW = (62.9298 * .6909) + (64.9278 * .3091) AW = 63.55 AMU

49. Calculate the Average Atomic Weight of Chlorine Stop Do the calculations, then continue.

50. Calculate the Average Atomic Weight of Chlorine AW = (34.969 * 0.7577) + (36.966 * 0.2423) AW = 35.45 AMU