Chapter 18 The Nucleolus: A Chemist’s View
Topics • Nuclear stability and radioactive decay • The kinetics of radioactivity • Nuclear transformations • Detection and use of radioactivity • Thermodynamic stability of the nucleus • Nuclear fission and nuclear fusion • Effects of radiation
IntroductionNuclear Reactions vs Chemical Reactions • Chemical reactions: Changes in the outer electronic structure of atoms or molecules • Nuclear reactions: study of changes in structure of nuclei and subsequent changes in chemistry. • Radioactive nuclei: spontaneously change structure and emit radiation. • Differences between nuclear and chemical reactions: • Much larger release in energy in nuclear reaction. • Isotopes show identical chemical reactions but different nuclear reactions. • Nuclear reactions not sensitive to chemical environment. • Nuclear reaction produces different elements. • Rate of nuclear reaction not dependent upon temperature.
Representation ofatomicnuclei Mass number- A Atomic number- Z Isotopes
Nucleus components • Nucleon: any nuclear particle, e.g. protons, p, and neutrons, n. Nuclide Isotopes: atoms that have identical atomic numbers but different mass numbers Nuclide: is a term used to identify an individual atom. Each individual atom is called nuclide
Radioactivity • Radioactivity is a nuclear reaction in which an unstable nucleus decomposes spontaneously • Natural radioactivity Natural unstable nuclei decompose more stable nuclei • Artificialradioactivity Synthetic unstable nuclei decompose more stable nuclei Decay Daughter nuclei Parent nuclei
18.1 Nuclear stability and radioactive decay • Nuclear stability • Thermodynamic stability: the potential energy of a nucleus as compared with sum of the potential energies of its components protons and neutrons • Kinetic stability: it describes the probability that a nucleus will undergo decomposition to form a different nucleus- a process called radioactive decay • Stability depends upon a balance between repulsive forces (between protons) and strong attraction forces between nuclei
Nuclear Stability • The stability of a nucleus depends mainly on A, the mass number andZ, the atomic number. Up to the mass number 30 or 40, a nucleus has approximately the same number of neutrons and protons to be stable. • Bigger nuclei must have more neutrons than protons.As Z gets bigger, repulsive forces get bigger. • When nucleus gets big enough, no neutron is enough to keep it stable. After, Z= 82, no nuclei is stable. Such unstable nuclei are radioactive, which means they undergo radiations in order to become stable.
Nuclear Stability • A nucleus having very much protons compared to neutrons will never be stable • This does not mean that a nucleus with many neutrons and little protons will be stable. • To understand this we may look at this graph,
Empirical rules for predicting stability of nuclei • Neutron-to-proton ratiovaries with atomic number • Light isotopes (small atomic number) have aNeutron-to-proton ratio almost =1(almost stable) • Nuclei are held together by strong attractive forces; but electrostatic repulsion causes large atoms (>83 protons) to be unstable.
Nuclides with even number of nucleons (p +n) are more stable than those with odd number • Certain number of protons or neutrons appear to be particularly stable. The magic numbers are: 2, 8, 20, 28,50, 82, 126 • These numbers are in parallel to those produce chemical stability: 2, 10, 18, 36, 54 and 86 (Noble gas configuration)
Typesofradioactivedecay radiation = attracted towards negatively charged plate Þ Positivelycharged radiation = attracted towards positively charged plate ÞNegativelycharged =1e- radiation = not attracted to either plateÞ Neutral.When emitted it does not change atomic or mass numbers Very high energy photons; very short wavelength . Positron is a positive electron Positron emission is equivalent to a fall of e-1 in nucleus
NUCLEAR REACTIONS • Radioactivity: nucleus unstable and spontaneously disintegrates. • Nuclear Bombardment: causes nuclei to disintegrate due to bombardment with very energetic particles. • Particles in nuclear reactions:
Balancing nuclear equations Protactinium • Total Nucleon Number (TOP VALUES) =Total number of protons and neutrons • Total electric charge (BOTTOM VALUES) • Are kept the same.
+ + • Nuclear reaction is written maintaining mass and charge balance. • E.g.
Examples of adioactive decay • Beta emission: Converts neutron into a proton by emission of energetic electron; atomic # increases: E.g. Determine product for following reaction: • Alpha emission: emits He particle. E.g. Determine product:
Positron emission: Converts proton to neutron: • E.g. Determine product of • Gamma emission: no change in mass or charge but usually part of some other decay process. • E.g. Electron capture: electron from electron orbitals captured to convert proton to neutron. E.g. Determine product
More examples of radioactive decay Alpha production (): helium nucleus, Beta production ():
Examples of radioactive decay Gamma ray production (): Positron production: Electron capture: (inner-orbital electron is captured by the nucleus)
18.2 The kinetics of radioactive decay • Nuclear decay is a first order reaction • Rateamount of radioactive isotope present • For a radioactive nuclides, the rate of decay, that is the negative change in the number of nuclides per unit time is directly proportional to the number of nuclides N That is This is a first order process # of nuclides remaining at time t Original # of nuclides
Half-Life The time required for the number of nuclides to reach half the original value (N0/2).
Examples of Half-Life Isotope Half life C-15 2.4 sec Ra-224 3.6 days Ra-223 12 days I-125 60 days C-14 5700 years U-235 710 000 000 years
Examples 1. The half-life of Cobalt-60 is 5.26 years how much of the original amount would be left after 21.04 years? 2. Tritium decays by beta emission with a half-life of 12.3 years. How much of the original amount would be left after 30 years? 3. If a 1.0 g sample of tritium is stored for 5.0 years, what mass of that isotope remains? k = 0.563/year.
18.3 Nuclear Transformation • The change of one element into another • Bombard nuclei with nuclear particles to convert element to another one to become more stable through radioactivity is transmutation. Rutherford Irene Curie
Nuclear transformation can occur by alpha or beta radiation, or • some other nuclear reactions such as nuclear bombardment • Nuclear transformation is achieved mostly using particle accelerator • Accelerators are needed when positive ions are used as the • bombarding particles • The particle is accelerated to a very high velocity thus it can • overcome the repulsion and can penetrate the target nucleus • Neutrons are also used often as bombarding particles • Neutrons are uncharged, thus they are not repelled and readily • absorbed by many nuclides • Using neutron and positive ion bombardment made possible to • extend the periodic table • Since 1940, elements with atomic numbers 93 through • 112 have been synthesized • These elements are called transuranium elements
Schematic diagram of a cyclotron Positive ion Nucleus
4. Detection and uses of radiation • Geiger counters detect charged particles produced from interaction of gas with particles emitted from radioactive material. The device detects the current flow • Scintillation counters detect particles from radioactive material by measuring intensity of light when these particles hit substances such as ZnS. • Units: 1 curie (Ci) = 3.7x1010 disintigrations×s-1
A representation of a Geiger-Müller counter. High energy particles produced from radioactive decay produce ions when they travel through matter Ar(g) Ar+(g) + e-
Dating by radioactivity Carbon-14 Dating Carbon-14 is formed naturally at a fairly constant rate by bombardment of atmospheric nitrogen by cosmic rays (high energy neutrons). 147N + 10n 146C + 11 H and then over time C-14 decays 146C 147N + 0-1e
Age of organic material • As long the plant or animal lives the C-14/C-12 ratio in its molecules remains the same as in the atmosphere (1/1012) because of the continuous uptake of carbon. • When the plant/animal dies, C-14 decays and the ratio decreases • t1/2 for C-14 = 5730 yr • If C-14/C-12 found in the old wood is ½ of that in a currently living plant, then its age is 5730 yr. • This assumes that the current C-14/C-12 ratio is the same as that in the ancient plant
Age of rocks/Age of earth • U-238 present in certain rocks slowly decays to Pb-206 • Pb-206 was not present originally • As time progresses the amount of U-238 decreases and Pb-206 increases • By measuring the ratio of Pb-206 / U-238 scientists can determine the age of a rock • The oldest rocks can then be used to determine the minimum age of the earth • It is assumed that • Pb-206 was not present originally • All of the decay products are retained
Medical applications of radioactivity • Radioactive nuclides can be introduced into organisms in food or drugs where their paths can betraced by monitoring their radioactivity • Radioactive tracers provide sensitive methods for: • learning about biological systems, • detection of disease, • monitoring the action and effectiveness of drugs, • early detection of pregnancy,