Nuclear Chemistry

1. Nuclear Chemistry M. Jones Pisgah High School Last revision: 100211

2. Nuclear chemistry studies • Atomic theory • Radioactivity • Isotopes • Half-life • Decay equations • Energy, fission and fusion

3. Atomic Theory

4. Atomic Theory Atoms are the smallest particles of elements. Atoms were first proposed by Democritus over 2000 years ago. The idea of atoms was reintroduced in 1803 by John Dalton.

5. Dalton’s Atomic Theory • Atoms are tiny, discrete particles • Atoms are indestructible • Atoms of the same element have the same mass and properties • Atoms combine in simple whole-number ratios • Atoms in different ratios produce different compounds.

6. Dalton’s Atomic Theory • Atoms are tiny, discrete particles • Atoms are indestructible • Atoms of the same element have the same mass and properties • Atoms combine in simple whole-number ratios • Atoms in different ratios produce different compounds. We know that parts of Dalton’s atomic theory are no longer valid in today’s modern Quantum Mechanical model of the atom.

7. Dalton’s Atomic Theory • Atoms are tiny, discrete particles • Atoms are indestructible • Atoms of the same element have the same mass and properties We know that atoms are made up of smaller particles, and that there are slight differences between atoms of the same element - isotopes.

8. William Crookes Used spectroscopy to discover thallium and used vacuums to measure its mass. Invented the radiometer. Improved vacuum systems. Used by Edison to make light bulbs.

9. William Crookes What we now call the cathode ray tube. The Crookes’ Tube

10. William Crookes Used the cathode ray tube to to study electric fields in a vacuum and discovered rays, … which were called “cathode rays” by Goldstein, since they came from the cathode, or negative electrode.

11. William Crookes The shadow of the Maltese cross indicates that cathode rays travel in straight lines and can be stopped by a solid object.

12. William Crookes He found that the cathode rays could be deflected by a magnet. This suggested that the cathode rays might be a stream of electrically charged particles.

13. Cathode Ray Tube Direction of cathode rays Cathode Anode + High voltage

14. Cathode Ray Tube Magnet Direction of cathode rays Cathode Anode + High voltage

15. Cathode Ray Tube Used by J. J. Thomson … to discover the electron. Cathode Anode + High voltage

16. J.J. Thomson and Cathode Rays • Attracted to positive electrode • Thought might be atoms • Had same charge to mass ratio regardless of metal in the cathode • The particle was much less massive than the lightest element – H • Particle must be common to all matter, a subatomic particle

17. J.J. Thomson and Cathode Rays In 1897 J. J. Thomson found that cathode rays are a basic building block of matter. He had discovered the electron.

18. The term “electron” comes from George Stoney’s term for the “minimum electrical charge”. J.J. Thomson and Cathode Rays Thomson concluded that this particle was the carrier of the minimum electrical charge and so the particle was later called an “electron”.

19. J.J. Thomson and Cathode Rays Even though Crookes and others observed cathode rays, Thomson is credited with the discovery of the electron because he recognized that it was a fundamental particle of nature as well as a sub-atomic particle.

20. J.J. Thomson and Cathode Rays Measured the charge to mass ratio, and found … … that if this “minimum charge” was equal to the charge on a hydrogen ion, then the mass of the electron would be 1/1837th the mass of a hydrogen atom.

21. J.J. Thomson and Cathode Rays If that were the case, then the electron would be much smaller than the smallest atom ..… showing for the first time that matter is made up of particles smaller than atoms. Thomson tried to measure the fundamental charge on the electron.

22. Robert A. Millikan Robert A. Millikan, an American physicist, set out to determine the charge on an electron. From 1909 through 1910, he performed what is now called the “Oil Drop Experiment”.

23. Robert A. Millikan Atomizer High Voltage Telescope Cast iron pot

24. Robert A. Millikan Atomizer Parallel charged plates High Voltage Oil Drop Telescope Cast iron pot

25. Robert A. Millikan Radiation stripped electrons from the oil droplets. The charged droplets fell between two electrically charged plates. By adjusting the voltage, he could change the rate of fall or rise of a single oil drop. After observing hundreds of drops, he calculated the charge on a single electron.

26. Robert A. Millikan Charges on drops are multiples of 1.602 x 10-19 coulombs.

27. Robert A. Millikan The fundamental charge on an electron is 1.602 x 10-19 coulombs. With J. J. Thomson’s charge to mass ratio, and Millikan’s charge on the electron, we are able to compute the mass of an electron: 9.109 x 10-28 gram

28. Ernest Rutherford He is to the atom what Darwin is to evolution, Newton to mechanics, Faraday to electricity and Einstein to relativity. John Campbell http://www.rutherford.org.nz/biography.htm

29. Ernest Rutherford He moved from New Zealand to Cambridge University in England (1895) where he pioneered the detection of electromagnetic waves, but was lured away by J.J. Thomson on work that would lead to the discovery of the electron. The invention of radio communications went to Marconi, instead. He later switched to working with radioactivity (1896) and discovered alpha and beta rays. He went to Montreal to teach at McGill University (1898) where he continued his work on radioactivity with Frederick Soddy, and others (1898-1907). He moved back to back to England to teach at Manchester (1907). He received the Nobel prize in chemistry in 1908 for his work on radioactivity in Canada.

30. Ernest Rutherford In 1907, he and a student, Hans Geiger, developed what would later become the “Geiger counter”. While at McGill, Rutherford discovered that after alpha rays passed through a thin film of mica, the image formed on a photographic plate was “fuzzy”. He and Geiger began a project to investigate the scattering of alpha particles by thin films. Rutherford later gave Ernest Marsden, an undergraduate, his own research project which was to look for evidence of the backscatter of alphas (1909). To their surprise, Marsden found that some alpha particles were scattered backwards from thin films of lead, platinum, tin, silver, copper, iron, aluminum, and gold.

31. Ernest Rutherford Rutherford remarked that it was like firing a navel gun at a piece of tissue paper and the shell bouncing back and hitting you. By 1910, Hans Geiger had finished his research on the forward scattering of alpha particles but he could not reconcile it with Marsden’s observations of the backscatter of alphas. The problem was passed on to Rutherford, who came up with the answer, and the astounding results were published in 1911.

32. Ernest Rutherford Rutherford had discovered a new piece to the atomic puzzle, the nucleus. According to Rutherford, the positively charged alpha particles were encountering a tiny, positively charged particle within the atoms of the metal and were being repelled. The atoms themselves appeared to mostly empty space. It was the repulsion of two positively charged particles which caused the scattering observed by Geiger and Marsden. Rutherford had found that atoms are mostly empty space with a small, dense, positively charged nucleus.

33. Alpha scattering Apparatus for investigating alpha scattering. What some textbook authors call the “gold foil experiment.”

34. + Alpha scattering a source Most of the alpha particles pass through undeflected.

35. + Alpha scattering a source Some positive alpha particles are repelled by the small, dense, positively charged nucleus.

36. + Alpha scattering a source Some positive alpha particles are repelled by the small, dense, positively charged nucleus.

37. Alpha scattering Alpha particles are repelled by a small, dense, positively charged nucleus. Almost all the mass of an atom is in the nucleus. Atoms are mostly empty space. Electrons are located outside the nucleus. Published results in 1911.

38. Ernest Rutherford Rutherford, during the First World War, worked on developing SONAR and submarine detection, but still found time to tinker with alpha radiation. In 1917 he bombarded nitrogen gas with alpha particles and discovered that oxygen and hydrogen were produced. Rutherford had resorted to alchemy and accomplished the first transmutation of one element into another. He had also indirectly discovered the proton. N + a O + H

39. 7 protons 1 proton 2 protons 8 protons 9 protons 9 protons Ernest Rutherford We now know… N + a O + H

40. Ernest Rutherford Rutherford concluded that the nucleus must contain the positively charged protons in a number equal to the negative charge from the electrons, but this did not account for all of the mass of the atom. He, along with James Chadwick, rejected the idea that there must be additional protons and electrons in the nucleus, and concluded that there must be a neutral particle in the nucleus that accounted for the additional mass. In 1932, Chadwick confirmed the existence of the neutron.

41. Radioactivity

42. Demonstrations with radioactivity Investigate the properties of Alpha, Beta and Gamma Radiation

43. Wire (+ side of circuit) Metal shield (- side) Low pressure Ar gas Mica window (fragile) Geiger-Mueller Tube Counter 2435

44. Geiger-Mueller Tube Rays leave the source Some hit the GM tube Most do nothing One ray may cause a discharge… Source and the detector clicks

45. Geiger-Mueller Tube • Filled with low pressure argon gas • About 1% efficiency • About 1 in 100 rays causes an electric spark between the case and the wire • Each spark registers as a count or click on the counter

46. Radioactivity • Alpha particles • Beta particles • Gamma rays • a • b • g • helium nuclei • electrons • high energy electromagnetic energy - similar to light, but higher in energy.

47. Radioactivity Alpha particles An unstable nucleus splits to form a more stable nucleus an an alpha particle. An alpha particle is the nucleus of a helium atom. Two protons and two neutrons. Has a +2 charge.

48. Radioactivity Beta particles Ejected from the nucleus when a neutron decays. A beta particle is identical to an electron Has a -1 charge.

49. Radioactivity Gamma rays Emitted by an unstable nucleus as it becomes more stable Electromagnetic energy with short wavelengths and high energy. Has no charge.

50. Radioactivity - comes from the natural decay of unstable atoms. - can be detected by photographic film, scintillation detector or a Geiger counter. - is “ionizing radiation”. Causes cell damage and mutations – cancer. - is protected against by shielding and distance.