2. The Nucleus Chapter 8

3. The Atom and Nucleus Atoms are the smallest particles of ordinary matter. Every atom has a central core called the nucleus. The nucleus contains protons and neutrons that provide nearly all the atom’s mass. Moving around the nucleus are the much lighter electrons. In a neutral atom, the number of protons and electrons are equal. http://www.metacafe.com/watch/yt- 2x3F08_8B80/lec_1_mit_5_111_principles_of_chemical_science_fall_2005/

4. The Atom and Nucleus The chief properties (except mass) of atoms, molecules, solids, and liquids can be traced to the behavior of atomic electrons. The nucleus is important, too.  The continuing evolution of the universe is powered by energy from nuclear reactions and transformations as well as the heat from the Earth’s interior. All the energy at our command has a nuclear origin, except for the energy of the tides (gravity).

5. The Atom and Nucleus JJ THOMSON - in 1898, he suggested that atoms were simply positively charged lumps of matter with electrons embedded in them, like raisins in a fruitcake. His idea was taken seriously since he had played an important part in discovering electrons.

6. The Atom and Nucleus ERNEST RUTHERFORD – (1871-1937) developed the model of the atom that had a positive charge concentrated in a central nucleus with the electrons some distance away. Rutherford wanted to know what was inside an atom, so he probed the atom using Alpha Particles.  Alpha particles are emitted by certain substances. (discussed later in this chapter)  Alpha particles are 8000 times heavier than an electron and each one has a +2e charge.

7. Rutherford Model of the Atom Rutherford experiment:  Alpha-emitting substance was placed behind a lead screen with a small hole in it, so that a narrow beam of alpha particles was produced.  The beam was aimed at a thin gold foil.  A zinc sulfide screen, which emits a flash of visible light when struck with an alpha particle, was set on the other side of the foil.

8. Results of Rutherford Experiment Rutherford expected the alpha particles to go right through the foil with little or no deflection.  Follows from Thomson model which the electric charge inside an atom is uniformly spread thru its volume. What he found was that, although most of the alpha particles were not deflected much, a few were scattered through very large angles.  Some even backward

9. Conclusions from the Rutherford Experiment Since alpha particles are relatively heavy and since those in the experiment were traveling at high speeds, strong forces HAD to be exerted upon them to cause such large deflections. Rutherford found that the only way to explain the deflections was to picture an atom with a tiny nucleus in which positive charge existed and nearly all the mass existed; And the electrons were some distance away from the nucleus. In other words, AN ATOM IS MOSTLY EMPTY SPACE.  That’s why most alpha particle go right through a thin foil.  However, when an alpha particle gets close to a nucleus, the strong electric field there causes the alpha particle to be deflected.  The electrons, being so light, have little effect on the alpha particles.

10. IF ALL THE ELECTRONS AND NUCLEI IN OUR BODIES COULD BE PACKED CLOSELY TOGETHER, WE WOULD BE NO LARGER THAN SPECKS JUST VISIBLE WITH A MICROSCOPE!

11. Nuclear Structure Proton  Positively charged particle  +e charge  Mass 1836 times that of an electron. Neutron  Particle with no charge  Mass 1839 times that of an electron.

12. Nuclear Structure Elements are the simplest substances in the bulk matter around us. Over 100 elements are known:  11 are gases  5 are liquids (Cs, Ga, Hg, Fr, Br)  Remaining are solids at room temperature and atmospheric pressure Most of the solid elements are metals.

13. Nuclear Structure Atomic number – in a neutral atom of any element, the number of protons or electrons (since p=e). (small blue number in upper right corner of each element box on periodic table)  The atomic number is an element’s most basic property since this determines how many electrons its atoms have and how they are arranged, which in turn determines the physical and chemical behavior of the element.

14. Nuclear Structure Mass number – Also called nucleon number is the number of protons and neutrons in an atom. Atomic mass is a weighted average of the atomic masses of the different isotopes of an element. Units are the atomic mass unit (amu) (this is the number located at bottom of each element box on periodic table)  Carbon-12 (98.89%) & Carbon-13 (1.11%)  98.89/100 x 12.000 amu + 1.11/100 x 13.003354 amu = 12.011 amu The number of neutrons of an element is determined by taking the mass number and subtracting the atomic number.

15. What is the difference between atomic mass and mass number? Mass number and atomic mass are easily confused. Let’s clear up the difference. Concentrate on the 2 nd word of each: number and mass Mass number is the count of the number of nucleons in an isotope and requires no units because it is simply a count. Atomic mass is a measure of the total mass of an atom whose units are the amu (atomic mass units).

16. What is the difference between atomic mass number and mass number? What is the atomic mass of oxygen?  15.9994 amu What is the mass number of oxygen?  16

17. Isotopes All atoms of a given element have the same number of protons in their nuclei, but not necessarily the same number of neutrons. Isotopes of an element have atoms with the same atomic number but different atomic masses.  Their number of neutrons varies. All elements have isotopes.

18. Isotopes of Hydrogen About 1 in every 7000 hydrogen atoms is a deuterium atom. Only about 2 kg of tritium of natural origin is present of the Earth, nearly all in the oceans. Tritium decays to a helium isotope. Nuclear reactions in the atmosphere caused by cosmic rays from space continually replenish the Earth’s tritium.

19. Nuclide A nucleus with a particular composition is called a nuclide. Symbols for nuclides are represented as follows:  X is chemical symbol of element  A is mass number  Z is atomic number

20. Radioactivity In 1896, Henri Becquerel accidentally discovered radioactivity.  Uranium Subsequent to Becquerel’s discovery, Pierre and Marie Curie found two other radioactive elements called polonium and radium.  Radium turned out to be thousands of times more radioactive than uranium.

21. Radioactivity Chemical reactions, heating, nor cooling change the ability of a radioactive material to emit radiation. Radioactivity must be associated with atomic nuclei because these are the only parts of atoms not affected by such treatments. The radioactivity of an element is due to the radioactivity of one or more of its isotopes. A nucleus is said to decay when it emits an alpha or beta particle or a gamma ray.

22. Radioactivity Decay How unstable nuclei change into stable ones Early experimenters found that a magnetic field splits the radiation from a radioactive material such a radium into three parts: Alpha Particles were deflected one way  helium nuclei (probes used in Rutherford experiment)  Positive charge  2 protons, 2 neutrons Beta Particles were deflected another way  Electrons  Negatively charged Gamma Rays were not deflected  High energy electromagnetic waves

23. Alpha and Beta Particles and Gamma Rays Tam7s6_7

24. Why Does a Nucleus Decay? A nucleus decays because it is UNSTABLE. Nuclei are unstable for several reasons:  Nucleus has excess energy  Nucleus is too large to be stable  Nucleus has too many protons relative to number of neutrons Larger nuclei need more neutrons than protons in order to overcome the electrical repulsion of the protons  Nucleus has too many neutrons relative to number of protons

25. 5 Kinds of Radioactive Decay Gamma Decay Alpha Decay Beta Decay Electron Capture Positron Emission

26. Alpha Particles and Alpha Decay  Alpha Particles – radiation is deflected as though it consists of positively charged particles.  Alpha particles are, in fact, the nuclei of helium atoms.  The nuclei contain 2 protons and 2 neutrons.  Alpha particles are least penetrating.  Alpha decay occurs in nuclei too large to be stable.

27. Alpha Decay 238/92U238/92U → 234/90Th + 4/2He 2+ [1]234/90Th4/2He 2+ [1] 99% of He on the Earth comes from Alpha Decay of Uraneum Deposits

28. Beta Particles and Beta Decay  Beta Particles – radiation is deflected as though it consists of negatively charged particles and are, in fact, electrons.  Beta particles are more penetrating than an alpha, but less penetrating than a gamma ray.  Beta decay occurs when one of the neutrons in a nucleus with too many neutrons spontaneously turns into a proton with the emission of an electron.

29. Electron Emission ( neutron to proton ratio too high) Carbon has 6 protons and 8 Neutrons Nitrogen has 7 protons and 7 Neutrons One of the Carbon neutrons changed to a proton And Emitted an electron plus a neutrino The change involves the conversion of a neutron into a proton with the emission of an electron and an antineutrino (n → p+e − +ν¯)

31. Gamma Rays and Gamma Decay  Gamma Rays – are em waves whose frequencies are higher than those of x-rays.  They are not affected by a magnetic field.  In gamma decay, a gamma ray is emitted by a nucleus that has more than its normal amount of energy.  The composition of the nucleus does not change in gamma decay, unlike alpha and beta decay.

32. Positron Emission  In a nucleus with too few neutrons for stability, one of the protons may become a neutron with the emission of a positron.  A positron is an electron that has a positive charge rather than a negative one.

33. Electron Capture  In a nucleus with too few neutrons for stability, one of the electrons in the atom is absorbed by one of the protons to form a neutron in a process called electron capture. (The reverse of Beta decay)  The Electron comes from one of the atoms orbiting electrons

34. 5 Kinds of Radioactive Decay

35. Half-life The half-life of a radionuclide is the period of time needed for half of any initial amount of the nuclide to decay.

36. Radiation Hazards  All ionizing radiation is harmful to living tissue, although to varying degrees.  Radiation hazards are easy to underestimate because there is usually a delay between an exposure and possible health consequences.  Radiation dosage is measured in sieverts (Sv) where 1 Sv is the amount of any radiation that has the same biological effects as those produced when 1 kg of body tissue absorbs 1 joule of x-rays or gamma rays.

37. Radiation Hazards  The largest portion of human exposure to radiation is from radon, which is naturally occurring in the environment.  Medical x-rays and nuclear medicines are second. Sources of radiation exposure (U.S.)

38. Nuclear Energy  The atomic nucleus is the energy source of the reactors that produce some of the world’s electricity.  It is also the energy source of destructive weapons.  Nearly all the energy that keeps the sun and stars shining comes from the nucleus.

39. Units of Mass and Energy  The atomic mass unit (u) is the mass unit in atomic physics since kg are much too large.  The atomic mass unit = 1 u = 1.66 x 10 -27 kg  1u is approximately equal to the mass of hydrogen (1.008u)  The electronvolt (eV) is the energy unit used in atomic physics which is the energy gained by an electron accelerated by a potential difference of 1 volt.  One electronvolt = 1 eV = 1.60 x 10 -19 J  The energy equivalent (E=mc 2 ) of a rest mass of 1u is 931 MeV.

40. Binding Energy Binding energy is the missing energy that keeps a nucleus together. When a nucleus forms, energy is given off due to the action of the forces that hold the neutrons and protons together. The resulting nucleus therefore has LESS mass than the total mass of the particles before interacting. The Binding Energy is the energy equivalent of the missing mass of a nucleus. Greater binding energy means more energy must be supplied to break up the nucleus. Without binding energy, no nuclei more complex than the single proton of hydrogen would be stable.

41. Binding Energy Since the energy equivalent of 1u of mass is 931 MeV, the energy that corresponds to the missing deuterium mass of 0.0024 u is: Missing Energy = (0.0024u)(931MeV/u) = 2.2 MeV

42. Binding Energy of Deuterium

43. Binding Energy Nuclear binding energies are very high.  2.2MeV for deuterium to 1640MeV for Larger nuclei are all unstable and decay radioactively. To understand how large binding energies are, we can compare them in terms of kJ/kg.  A typical B.E. is 800 billion kJ/kg.  Boiling water has a heat of vaporization of 2260 kJ/kg.  Burning gasoline 4.7 x 10 4 kJ/kg.

44. Binding Energy per Nucleon- -the most important graph in science- Binding energy per nucleon – For a given nucleus, the binding energy per nucleon is found by dividing the total binding energy of the nucleus by the number of nucleons (protons and neutrons) it contains. The binding energy for deuterium is:  2.2MeV / 2=1.1MeV/nucleon

45. Binding Energy The greater the binding energy per nucleon, the more stable the nucleus. The graph has its maximum of 8.8 MeV/nucleon when the number of nucleons is 56. That nucleus corresponds to an iron isotope This is the most stable nucleus of all, since the most energy is needed to pull a nucleon away from it. Larger and smaller nuclei are less stable.

46. Binding Energy Per Nucleon This figure is the key to energy production in the universe.

47. The Most Important Graph in Science Two conclusions can be made from figure 7-14.  If a heavy nucleus is split into 2 medium size ones, each of the new nuclei will have more binding energy per nucleon (and hence less mass per nucleon) than the original nucleus did. The extra energy will be given off. Problem:  If 235-U is broken into 2 smaller nuclei, the difference in b.e. per nucleon is about 0.8 MeV. 235-U contains 235 nucleons. The total energy given off can be calculated as: (0.8 MeV/nucleon)x(235 nucleons) = 188 MeV This is nuclear fission and involves a hundred million time more energy per atom than burning coal or oil.

48. The Most Important Graph in Science Two conclusions can be made from figure 7-14.  The other notable conclusion from Fig. 7-14 is that joining two light nuclei together into a single nucleus of medium size also means more binding energy per nucleon in the new nucleus. If 2 deuterium nuclei combine to form a helium nucleus, over 23 MeV is released! This is nuclear fusion and is a very effective way to obtain energy.  Nuclear fusion is the main energy source of the sun and other stars.

49. More About Fission and Fusion Nuclear Fission – Splitting of a nucleus Splitting a nucleus is not easy. When splitting a heavy nucleus you must NOT use more energy to split than you get back from the process. In 1939, a discovery was made: a nucleus of 235-U undergoes fission when struck by a NEUTRON.  What actually occurs is the neutron is absorbed by the 235- U nucleus and becomes 236-U.  236-U is so unstable that it almost immediately splits into 2 pieces.

50. Nuclear Fission

51. Nuclear Fission and Chain Reactions When a nucleus breaks apart, 2 or 3 neutrons are set free. A chain reaction can occur when at least one neutron produced by each fission, on the average, leads to another fission and does not either escape or become absorbed without producing a fission.  If exactly one neutron per fission causes another fission, energy is released at a steady rate.  This is what occurs in nuclear reactors.  Chain reactions were first demonstrated by Enrico Fermi in 1942.

52. Chain Reaction (Nuclear Reactor)

53. Nuclear Fission and Chain Reactions If more than one neutron from each fission causes another fission, the chain reaction speeds up and an explosion occurs. An atomic bomb makes use of this effect.

54. How Nuclear Reactors Works Reactors need 235 U for the fissionable isotope in the production of electricity.  Each fission of a 235 U liberates an average of 2.5 neutrons. However, natural uranium contains only 0.7% 235 U. The rest is 238 U, an isotope that captures the rapidly moving neutrons emitted during 235 U fission but usually does not undergo fission afterward.  The neutrons absorbed by 238 U are wasted. Did you know for every gram of uranium that undergoes fission in a reactor, 2.6 tons of coal must be burned?

55. How Nuclear Reactors Work Since the neutrons absorbed by 238 U are wasted, and since 99.3% of natural U is 238 U, too many disappear for a chain reaction to occur in a solid lump of natural U. Slow neutrons are more likely to induce fission in 235 U than fast ones. To get around this problem of fast moving neutrons, they are slowed down to induce fission in 235 U.

56. Nuclear Reactors To slow down fission neutrons, the uranium fuel in a reactor is mixed with a moderator, a substance whose nuclei absorb energy from fast neutrons that collide with them. Hydrogen is widely used as a moderator in the form of water. Also, enriched uranium is used in nuclear reactors where 235 U content has been increased to about 3%.

57. Nuclear Fusion Fusion – joining of 2 nuclei Fusion is the sun and stars main energy source. The fusion of small nuclei to form larger ones can give out even more energy per kg of starting material than fission. It is possible that fusion will become the ultimate source of energy.  Safe, almost nonpolluting, and the oceans supplying limitless fuel.

58. Nuclear Fusion Three conditions must be met by a successful fusion reactor:  High temperature 100 million °C or more so that the nuclei are moving fast enough to collide despite the repulsion of their positive electric charges.  High concentration of nuclei To ensure collisions are frequent  Reacting nuclei must remain together long enough to give off more energy than the reactor’s operation uses.

59. Nuclear Fusion The fusion reaction that is the basis of current research is: deuteriumtritium helium neutron energy Most of the energy given off is carried by the neutron that is emitted. About 0.015 percent of the waters of the world is deuterium- no scarcity. A gallon of seawater has the potential for fusion energy equivalent to the chemical energy in 600 gallons of gasoline.

60. Elementary Particles The electrons, protons, and neutrons of which atoms are composed are considered elementary particles because they cannot be broken down into anything else.  A great many other elementary particles besides electrons, protons, and neutrons exist. Although protons and neutrons consist of still smaller particles called quarks, the quarks in a nucleon stick together too tightly to permit the nucleon from splitting apart, so nucleons are regarded as elementary.

61. Elementary Particles

62. Antiparticles The same but different…. Nearly all elementary particles have antiparticles. The antiparticle of a given particle has the same mass as the particle and behaves similarly, but its electric charge is opposite in sign.  Antiproton p - is the antiparticle of the proton p +  Positron e + is the antiparticle of the electron e -  Neutrons also have antiparticles that have properties other than charge that are different.

63. Antiparticles Antiparticles are hard to find because when a particle and its antiparticle come together they destroy each other in a process called annihilation.  The loss of mass reappears as energy in the form of gamma rays when electrons and positrons are annihilated.  Unstable particles of other types may be produced instead of gamma rays when protons and antiprotons (or neutrons and antineutrons) are annihilated.

64. Annihilation

65. The Reverse of Annihilation Energy becomes matter and electric charge being created where none existed before in the process of Pair Production.  A particle and its anti-particle materialize when a high-energy gamma ray passes near an atomic nucleus.  The antiparticles formed in pair production exist for only a short time before they meet their particle counterparts in ordinary matter and are annihilated.

66. Fundamental Interactions Only 4 Give Rise to All Physical Processes Elementary particles interact with each other in only four ways. In order of decreasing strength, these interactions are:  Strong  Electromagnetic  Weak  Gravitational

67. Fundamental Interactions Strong interaction- which holds protons and neutrons together to form atomic nuclei despite the mutual repulsion of the protons. Electromagnetic interaction– which gives rise to electric and magnetic forces between charged particles.  Electromagnetic forces act on electrons, unlike the strong forces.

68. Fundamental Interactions Weak interaction – which affects all particles and, by causing beta decay, helps determine the compositions of atomic nuclei. Gravitational interaction – which is responsible for the attractive force one mass exerts on another.

69. 4 Fundamental Interactions

70. Unifying the Interactions In the 1960’s, Weinberg and Salam indicated that the weak and electromagnetic interactions are really different aspects of the same basic phenomenon. A more recent development links the electroweak and strong interactions. Although a grand unified theory is still far from its final form, it seems to be on the right track.

71. Developing a Single Theory of the Universe A GOAL OF PHYSICS:

72. Leptons and Hadrons All elementary particles fall into two broad categories that depend on their response to the strong interaction:  Leptons- are not affected by this interaction and seem to be point particles with no size or internal structure. The electron and neutrino (p.279) are leptons.  Hadrons- are subject to the strong interaction and have definite sizes and internal structures. The proton and neutron are hadrons. Several hundred other particles are also included in the hadron family, most with very short lifetimes (less than a billionth of a second for some).

73. Quarks The particles that make up hadrons are quarks. Only six kinds of quarks are needed to account for all known hadrons. A quark’s electric charge is less than Quarks do not seem to be able to exist outside of hadrons.  No quark has ever been found by itself.

74. The End