1. 1 General Arrangement of the Subatomic Particles

2. 2 1896 Radioactivity was discovered by Henri Becquerel. Radioactive elements spontaneously emit alpha particles, beta particles, and gamma rays from their nuclei.

3. 3 1907 Ernest Rutherford found that alpha particles emitted by certain radioactive elements were helium nuclei.

4. 4 Rutherford in 1911 performed experiments that shot a stream of alpha particles at a gold foil. Most of the alpha particles passed through the foil with little or no deflection. He found that a few were deflected at large angles and some alpha particles even bounced back.

5. 5 Rutherford’s alpha particle scattering experiment. 5.5

6. 6 An electron with a mass of 1/1837 amu could not have deflected an alpha particle with a mass of 4 amu. Rutherford knew that like charges repel. Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume. He called this mass the nucleus.

7. 7 If a positive alpha particle approached close enough to the positive mass it was deflected. Most of the alpha particles passed through the gold foil. 1 in 8000 deflected. This led Rutherford to conclude that a gold atom was mostly empty space.

8. 8 Because alpha particles (He nucleus, 2 protons and 2 neutrons) have relatively high masses, the extent of the reflections led Rutherford to conclude that the nucleus was very heavy and dense.

9. 9 5.5 Deflection and scattering of alpha particles by positive gold nuclei. Deflection Scattering

10. 10 Rutherford was astonished at the result: “It was quite the most incredible event that ever happened to me in my life. It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”

11. 11

12. 12 Rutherford ’ s experiment showed that an atom had a dense, positively charged nucleus. Chadwick’s work in 1932 demonstrated the atom contains neutrons. Rutherford also noted that light, negatively charged electrons were present in an atom and offset the positive nuclear charge.

13. 13 Rutherford put forward a model of the atom in which a dense, positively charged nucleus is located at the atom ’ s center. The negative electrons surround the nucleus. The nucleus contains protons and neutrons

14. 14 5.6

15. 15 Nucleus contains protons and neutrons, but does not contain electrons.

16. 16 Atomic Numbers of the Elements

17. 17 The atomic number of an element is equal to the number of protons in the nucleus of that element. The atomic number of an atom determines which element the atom is.

18. 18 Every atom with an atomic number of 1 is a hydrogen atom. Every hydrogen atom contains 1 proton in its nucleus.

19. 19 Every atom with an atomic number of 6 is a carbon atom. Every carbon atom contains 6 protons in its nucleus.

20. 20 92 protons in the nucleus 92 U atomic number Every atom with an atomic number of 92 is a uranium atom.

21. 21 Isotopes of the Elements

22. 22 Atoms of the same element can have different masses. They always have the same number of protons, but they can have different numbers of neutrons in their nuclei. The difference in the number of neutrons accounts for the difference in mass. These are isotopes of the same element.

23. 23 Isotopic Notation

24. 24 Isotopic Notation C 6 6 protons 12 6 protons + 6 neutrons

25. 25 Isotopic Notation C 6 6 protons 14 6 protons + 8 neutrons

26. 26 Isotopic Notation 8 8 protons 16 8 protons + 8 neutrons O

27. 27 Isotopic Notation 8 8 protons 17 8 protons + 9 neutrons O

28. 28 Isotopic Notation 8 8 protons 18 8 protons + 10 neutrons O

29. 29 Hydrogen has three isotopes 1 proton 0 neutrons 1 proton 1 neutron 1 proton 2 neutrons

30. 30 Examples of Isotopes ElementProtonsElectronsNeutronsSymbol Hydrogen 110 1 1 H Hydrogen 111 1 2 H Hydrogen 112 1 3 H Uranium 9292 143 92 235 U Uranium 9292 146 92 238 U Chlorine 171718 17 35 Cl Chlorine 171720 17 37 Cl

31. 31 Atomic Mass

32. 32 The mass of a single atom is too small to measure on a balance. Using a mass spectrometer, the mass of the hydrogen atom was determined.

33. 33 A Modern Mass Spectrometer A mass spectrogram is recorded. From the intensity and positions of the lines on the mass spectrogram, the different isotopes and their relative amounts can be determined. Positive ions formed from sample. Electrical field at slits accelerates positive ions. Deflection of positive ions occurs at magnetic field. 5.8

34. 34 5.9 A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the abundance (%) given.

35. 35 Using a mass spectrometer, the mass of one hydrogen atom was determined to be 1.673 x 10 -24 g

36. 36 Relationship Between Mass Number and Atomic Number

37. 37 The mass number minus the atomic number equals the number of neutrons in the nucleus. mass number atomic number number of neutrons - = 62109-47= mass number atomic number

38. 38