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Transcript
The Nucleus What goes on in the nucleus of an atom has almost nothing to do with the atom’s chemistry , and vice versa. The energies available in the nucleus are much greater than those available among electrons. All nuclei are composed of protons and neutrons ◦ Exception is ordinary hydrogen with just a proton The atomic number, Z, equals the number of protons in the nucleus The neutron number, N, is the number of neutrons in the nucleus The mass number, A, is the number of nucleons in the nucleus ◦ A=Z+N ◦ Nucleon is a generic term used to refer to either a proton or a neutron ◦ The mass number is not the same as the mass Symbol: A ◦ X is the chemical symbol of the element Z Example: X ◦ Mass number is 27 Atomic number is 13 Contains 13 protons Contains 14 (27 – 13) neutrons 27 The Z may be omitted since the element can be 13 to determine Z used Al The nuclei of all atoms of a particular element must contain the same number of protons They may contain varying numbers of neutrons ◦ Isotopes of an element have the same Z but differing N and A values ◦ Example: 11 6 C 12 6 C 13 6 C 14 6 C The proton has a single positive charge, +e The electron has a single negative charge, -e The neutron has no charge ◦ Makes it difficult to detect e = 1.602 177 33 x 10-19 C It is convenient to use unified mass units, u, to express masses ◦ 1 u = 1.660 559 x 10-27 kg ◦ Based on definition that the mass of one atom of C-12 is exactly 12 u Mass can also be expressed in MeV/c2 ◦ From ER = m c2 ◦ 1 u = 931.494 MeV/c2 Since the time of Rutherford, many other experiments have concluded: ◦ Most nuclei are approximately spherical ◦ Average radius is 1 3 -15 roxA10 m ror= 1.2 A is the number of nucleons The volume of the nucleus (assumed to be spherical) is directly proportional to the total number of nucleons This suggests that all nuclei have nearly the same density Nucleons combine to form a nucleus as though they were tightly packed spheres An atom has nine protons and eight neutrons in the nucleus and 10 electrons in orbit. 1. What element is? 2. What is its mass number? 3. What is it’s atomic number? 4. What is its electric charge? 5. How is possible that the number of protons and electrons is different? How many protons ,neutrons , and electrons are contained in the atom 56Fe when it has a charge of +2? Atomic number is 26. There are very large repulsive electrostatic forces between protons ◦ These forces should cause the nucleus to fly apart The nuclei are stable because of the presence of another, short-range force, called the nuclear force ◦ This is an attractive force that acts between all nuclear particles ◦ The nuclear attractive force is stronger than the Coulomb repulsive force at the short ranges within the nucleus Light nuclei are most stable if N = Z Heavy nuclei are most stable when N > Z ◦ As the number of protons increase, the Coulomb force increases and so more nucleons are needed to keep the nucleus stable No nuclei are stable when Z > 83 Mass and Energy are equivalent. C is speed of light = 3X 108 m/s , It is a very large number. A given amount of mass can be converted into energy. The total energy of the bound system (the nucleus) is less than the combined energy of the separated nucleons ◦ This difference in energy is called the binding energy of the nucleus It can be thought of as the amount of energy you need to add to the nucleus to break it apart into separated protons and neutrons Except for light nuclei, the binding energy is about 8 MeV per nucleon The curve peaks in the vicinity of A = 60 ◦ Nuclei with mass numbers greater than or less than 60 are not as strongly bound as those near the middle of the periodic table The curve is slowly varying at A > 40 ◦ This suggests that the nuclear force saturates ◦ A particular nucleon can interact with only a limited number of other nucleons Radioactivity is the spontaneous emission of radiation Experiments suggested that radioactivity was the result of the decay, or disintegration, of unstable nuclei Three types of radiation can be emitted ◦ Alpha particles The particles are 4He nuclei ◦ Beta particles The particles are either electrons or positrons A positron is the antiparticle of the electron It is similar to the electron except its charge is +e ◦ Gamma rays The “rays” are high energy photons A radioactive beam is directed into a region with a magnetic field The gamma particles carry no charge and they are not deflected The alpha particles are deflected upward The beta particles are deflected downward ◦ A positron would be deflected upward Alpha particles ◦ Barely penetrate a piece of paper Beta particles ◦ Can penetrate a few mm of aluminum Gamma rays ◦ Can penetrate several cm of lead The decay curve follows the equation ◦ N = No e- λt The half-life is also a useful parameter ◦ The half-life is defined as the time it takes for half of any given number of radioactive nuclei to decay T1 2 ln 2 0.693 The unit of activity, R, is the Curie, Ci ◦ 1 Ci = 3.7 x 1010 decays/second The SI unit of activity is the Becquerel, Bq ◦ 1 Bq = 1 decay / second Therefore, 1 Ci = 3.7 x 1010 Bq The most commonly used units of activity are the mCi and the µCi When a nucleus emits an alpha particle it loses two protons and two neutrons ◦ N decreases by 2 ◦ Z decreases by 2 ◦ A decreases by 4 Symbolically ◦ X is called the parent A nucleus A 4 4 Z X Z 2Y 2 He ◦ Y is called the daughter nucleus Decay of 226 88 Ra 226 222 86 Ra Rn He 4 2 Half life for this decay is 1600 years Excess mass is converted into kinetic energy Momentum of the two particles is equal and opposite When one element changes into another element, the process is called spontaneous decay or transmutation The sum of the mass numbers, A, must be the same on both sides of the equation The sum of the atomic numbers, Z, must be the same on both sides of the equation Conservation of mass-energy and conservation of momentum must hold During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is changed by one Symbolically A Z X Y e A Z X Y e A Z 1 A Z 1 The emission of the electron is from the nucleus ◦ The nucleus contains protons and neutrons ◦ The process occurs when a neutron is transformed into a proton and an electron ◦ Energy must be conserved To account for this “missing” energy, in 1930 Pauli proposed the existence of another particle Enrico Fermi later named this particle the Properties of the neutrino neutrino ◦ Zero electrical charge ◦ Mass much smaller than the electron, recent experiments indicate definitely some mass ◦ Spin of ½ ◦ Very weak interaction with matter Symbolically A Z XZA1Y e A symbol A the neutrino ◦ is the for Z X Z 1Y e ◦ is the symbol for the antineutrino To summarize, in beta decay, the following pairs of particles are emitted ◦ An electron and an antineutrino ◦ A positron and a neutrino Gamma rays are given off when an excited nucleus “falls” to a lower energy state ◦ Similar to the process of electron “jumps” to lower energy states and giving off photons ◦ The photons are called gamma rays, very high energy relative to light The excited nuclear states result from “jumps” made by a proton or neutron The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission Example of a decay sequence ◦ The first decay is a beta emission ◦ The second step is a gamma emission 12 5 B C * e 12 6 ◦ The C* indicates 12 12 the Carbon nucleus is in an 6 C* 6 C excited state ◦ Gamma emission doesn’t change either A or Z Carbon Dating Smoke detectors ◦ Beta decay of 14C is used to date organic samples ◦ The ratio of 14C to 12C is used ◦ Ionization-type smoke detectors use a radioactive source to ionize the air in a chamber ◦ A voltage and current are maintained ◦ When smoke enters the chamber, the current is decreased and the alarm sounds Radon detection ◦ Radon is an inert, gaseous element associated with the decay of radium ◦ It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building ◦ May also come from the ground itself Classification of nuclei ◦ Unstable nuclei found in nature Give rise to natural radioactivity ◦ Nuclei produced in the laboratory through nuclear reactions Exhibit artificial radioactivity Three series of natural radioactivity exist ◦ Uranium ◦ Actinium ◦ Thorium See table 29.2 Series starts with 232Th Processes through a series of alpha and beta decays Ends with a stable isotope of lead, 208Pb Structure of nuclei can be changed by bombarding them with energetic particles ◦ The changes are called nuclear reactions As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation Alpha particle colliding with nitrogen: Balancing the equation allows for the identification of X 4 14 1 2 He 7 N X 1H So the reaction is X 4 2 He 17 8 14 7 X N 17 8 O 17 8 1 1 O H Radiation absorbed by matter can cause damage The degree and type of damage depend on many factors ◦ Type and energy of the radiation ◦ Properties of the absorbing matter Radiation damage in biological organisms is primarily due to ionization effects in cells ◦ Ionization disrupts the normal functioning of the cell Roentgen [R] ◦ That amount of ionizing radiation that will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditions ◦ That amount of radiation that deposits 8.76 x 103 J of energy into 1 kg of air Rad (Radiation Absorbed Dose) ◦ That amount of radiation that deposits 10-2 J of energy into 1 kg of absorbing material RBE (Relative Biological Effectiveness) ◦ The number of rad of x-radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being used ◦ Accounts for type of particle which the rad itself does not Rem (Roentgen Equivalent in Man) ◦ Defined as the product of the dose in rad and the RBE factor Dose in rem = dose in rad X RBE Natural sources – rocks and soil, cosmic rays Upper limit suggested by US government Occupational ◦ Background radiation ◦ About 0.13 rem/yr ◦ 0.50 rem/yr ◦ Excludes background and medical exposures ◦ 5 rem/yr for whole-body radiation ◦ Certain body parts can withstand higher levels ◦ Ingestion or inhalation is most dangerous Sterilization ◦ Radiation has been used to sterilize medical equipment ◦ Used to destroy bacteria, worms and insects in food ◦ Bone, cartilage, and skin used in graphs is often irradiated before grafting to reduce the chances of infection Tracing ◦ Radioactive particles can be used to trace chemicals participating in various reactions Example, 131I to test thyroid action MRI ◦ Magnetic Resonance Imaging ◦ When a nucleus having a magnetic moment is placed in an external magnetic field, its moment processes about the magnetic field with a frequency that is proportional to the field ◦ Transitions between energy states can be detected electronically Nuclear fission is a nuclear reaction in which a heavy nucleus (such as uranium) splits into two lighter nuclei. Chain Reactions. http://library.thinkquest.org/17940/texts/fissi on/fission.html A typical fission reaction releases energy of about 200,000,000 ev . Explosion of a TNT molecule releases only 30ev. The combined mass of the fission fragments and neutrons produced in fission is less than the mass of the original uranium nucleus. The tiny amount of missing mass converted to the awesome amount of energy. If a U-238 split into two identical pieces and then each piece underwent an alpha decay , what would the two final pieces be? When two light nuclei combine to form a heavier nucleus. All stars generate their energy through fusion processes. Proton-Proton Cycle http://hyperphysics.phyastr.gsu.edu/hbase/astro/procyc.html#c1