Chapter 19 Radioactivity and Nuclear Chemistry
Nuclear Chemistry • Nuclei that are unstable and spontaneously decompose are said to be radioactive. • Nuclear chemistry is the study of nuclear reactions and their uses in chemistry. • Nuclear energy is produced from the radioactive nuclei and accounts for 20% of our electrical generation in the US.
Radioactivity • Atomic number = number of protons in the nucleus • Mass number = number of protons and neutrons in the nucleus • Protons and Neutrons collectively called nucleons.
Radioactivity • Isotopes = atoms with the same atomic number, but have different mass numbers. • Each element can have one or more isotopes that occur naturally. • Example) Uranium has three isotopes: U-234, U-235, U-238 • Each isotope has an abundance. • U-238 = 99.3%, U-235 = 0.7%, and U-234 = trace
Radioactivity • Most isotopes are stable and only a few are unstable. • Unstable isotopes are called radioisotopes. • A nuclear equation is used to describe a decay process. Daughter Nuclei Parent Nuclei
Types of Decay • There are a few common types of particles found in nuclear reactions. • Alpha = a helium nuclei (He-4). This is a massive particle, but relatively low energy. • Beta = an electron. The electron comes from the neutron changing it into a proton. Light mass, but higher energy. • Gamma = release of a photon of energy. Very light mass with very high energy.
Types of Decay • Positron = a positively charged electron. Converts a proton into a neutron. Like the beta particle except charge is plus one. • Neutrons can be generated in some reactions. These have a moderate mass and energy. • Electron capture (EC) = the capture of an inner shell electron by the nucleus. Converts a proton into a neutron.
Balancing Nuclear Decays • The sum of all the mass numbers must be equal. • The sum of all the atomic numbers must be equal. • The atomic number identifies the nuclide or particle. • LEP #1
Patterns of Nuclear Stability • Stability of nuclei depends on many factors and no one factor allows us to predict stability. • The Strong Force holds the nucleons together. • So powerful that it can hold together like charges (protons). • The Weak Force describes the nuclear decay processes.
Neutron / Proton Numbers • For Atomic number of 20 or less, a 1:1 ratio is preferred. Odd number elements may have exactly one more neutron than protons. • For Atomic numbers greater than 20, more and more neutrons are required. Hg-200 has a 1.5:1 ratio. • For Atomic numbers greater than 83, all are radioactive.
Band of Stability • Shows stable isotopes. • To the left and less than 83 protons – will decay via Beta particle. • To the right and less than 83 protons – will either decay via positron or do an EC. • Greater than 83 will decay via an alpha particle.
Very Heavy Isotopes • Many very large nuclei will decay via a series of alpha and beta emissions – this is called a decay series.
Magic Numbers • Certain numbers of protons and neutrons have a special stability. • These are referred to as “Magic” numbers. • protons = 2, 8, 20, 28, 50, or 82 • neutrons = same as above, but also 126 • When a nucleus has a magic number of both protons and neutrons, then nucleus is particularly stable.
Even, Odd • Evens are favored over odds. • Even protons and neutrons = 157 stable isotopes. • Even, Odd = 53. • Odd, Even = 50 • Odd, Odd = only 5! • obvious one is N-14 • LEP #2
Nuclear Transmutations • A nuclear reaction can be induced by colliding two nuclei together. • Rutherford was first to do this in 1919. • Used to produce the very largest man-made isotopes. • LEP #3 • LEP #4
Rates of Decay • Radioactive decay follows first order kinetics. • Same as Ch. 14
Carbon Dating • Carbon-14 is produced in the upper atmosphere from the nuclear reaction of N-14 with a solar neutron. • The C-14 is radioactive and undergoes decay with a half-life of 5730 years. • Assumption is made that C-14 levels have been stable for the past 50,000 years.
Carbon Dating • Thus, all living things – both plants and animals – have a steady state amount of C-14 until death. • After death, the C-14 slowly decays and can be compared to levels in living things. • Can provide ages for between 100 – 50,000 years old. • LEP #5
Other Dating Techniques • Rocks can be aged by comparison of their U-238 to Pb-206 masses. • Rock must contain U-238 at formation ,but be free of any lead. • In time, U-238 decays back to Pb-206 (see decay series earlier). • Calculations using these techniques. • LEP #6
Detection • Photographic material – incorporated into a badge that a worker wears. • Geiger tube – contains Argon gas inside of a tube that has a positively charged wire.
E=mc2 • The energy associated with a nuclear reaction is due to the loss of mass which is converted to energy. • Even a very tiny loss of mass can produce a huge quantity of energy. • Say, for example, 1 x 10-6 kg is lost. Then, E = (1 x 10-6kg)(3.00 x 108 m/s)2 = 9 x 1010 J
E = mc2 • The energy produced is many orders of magnitude larger than ordinary exothermic reactions. • Example: The decay of one mole of U-238 produces 50,000 times more energy than the combustion of one mole of CH4. • This is why nuclear energy is so attractive! • LEP #7
Nuclear Binding Energies • Scientists in the 1930’s discovered that the mass of every nuclei after hydrogen is always LESS than the sum of the individual masses of the protons and neutrons that make them up. • Example: Mass of He-4 = 4.0015amu Mass of 2p + 2n = 4.03188amu
Nuclear Binding Energies • Missing mass is called the mass defect. For He-4 = 4.03188amu – 4.00150amu = 0.03038amu • This is then converted to an energy per nucleon. (0.0000308kg) x (3 x 108 m/s)2 = 2.772 x 1012 J 2.772 x 1012 J / 6.02 x 1023 atoms/mole = 4.60 x 10-12 J 4.06 x 10-12 J / 4 nucleons = 1.15 x 10-12 J/nucleon LEP #8
Nuclear Fission • Nuclear Fission – the process of splitting larger nuclei into smaller ones. • U-235, U-233, and Pu-239 will undergo fission when the nucleus is struck by a slow moving neutron. • The heavier nuclei does not split the same way – rather a whole variety of nuclear reactions result.
Nuclear Fission • All fission reactions produce two smaller nuclei and several neutrons. • One possible reaction for U-235 is: • Note that the 3 neutrons produced can strike another U-235 nuclei and split it as well.
Chain Reactions • Because each U-235 that splits generates two or more neutrons, the possibility of a chain reaction occurs. • Critical Mass – the minimum amount of U-235 necessary to maintain the chain reaction. This means that exactly one neutron will continue the reaction each time. • Supercritical Mass – exceeds the critical mass. Results in an uncontrolled chain reaction.
Nuclear Weapons • A nuclear weapon (bomb) can be constructed if you have two or more sub-critical masses of U-235, which when combined would produce a critical mass. • The critical mass of U-235 is about 1 kilogram. • Problem: Naturally occurring Uranium contains only 0.7% U-235. • Solution: Must separate the U-235 from the other isotopes.
Enrichment • Enrichment is the process by which the quantity of U-235 present in a sample is increased by removing the other undesirable isotopes. • This is NOT easy to do! • U.S. used gaseous diffusion of UF6 back in the early 1940’s to obtain enough U-235. • Current methods involving using centrifuges.
Weapon Design • Problem: sub-critical masses need to be kept separate until the weapon is deployed. Then, they must be combined to produce the super-critical mass. • Solution: Implosion of sub-critical masses forces them together. • First design was relatively simple.
Nuclear Reactors • The energy of a nuclear reaction can be captured in a nuclear reactor. • Uranium ore is enriched to about 3% U-235 and converted to UO2. The UO2 pellets are then encased in either Zr or stainless steel tubes and referred to as fuel rods. • Rods composed of Cd or B, which are good absorbers of neutrons are also constructed and are referred to as control rods.
Fast Breeder Reactor • A proven, yet unused method to make more nuclear fuel than it consumes. • Uses “fast” neutrons and heats a liquid metal like sodium.
Nuclear Wastes • Fission products accumulate as the reactor operates. • Fuel rods must be replaced or reprocessed periodically. • Every year about 1/3 of the fuel rods are replaced or repacked. • When replaced, the spent fuel rods are still highly radioactive and are stored on site in large water pools.
Nuclear Wastes • One of the side products is Pu-239 – another fissionable isotope. • Pu-239 can be separated from the other wastes and is easily weaponized. • US foreign policy.
Fusion • The Sun and other stars use a different type of nuclear reaction called fusion. • Fusion occurs when two or more smaller nuclei are squeezed together to make a larger isotope. • The net reaction on the Sun is: • 4 H He + 2 e+ • This requires very high temperatures and pressures – of the type found only in stars.
Fusion • Fusion on this planet can be achieved in a special reactor. • The lowest energy reaction for fusion is: • There is enough deuterium (H-2) and tritium (H-3) present in the world’s oceans to supply us with fusion energy forever. • Why is this not feasible? • LEP #9
Fusion • A novel process for fusion has been proposed by Dr. Robert Bussard (deceased). • Uses a Boron-11 and H-1 collision to generate three alpha particles. • Research efforts can be followed at: http://focusfusion.org/
Biological Effects of Radiation • We all receive some radiation whether we want it or not. • Background radiation comes from many sources including: • Food – K-40 • Air – Rn-222 • Ground – U-238 • Also are exposed to man-made sources like X-rays, nuclear medicine, air travel, and cigarettes. • Total background average is about 360mrem.
Ionizing Radiation • When molecules absorb radiation it can lose an electron. • For example, when radiation strikes a water molecule: H2O + radiation H2O+ + 1e- • That ion then reacts with a second water molecule: H2O+ + H2O H3O+ + OH
Free Radical • The OH has an odd number of electrons and is called a free radical. • Any free radical is highly reactive and can cause biomolecules to form free radicals. • Free radicals can also interfere with electron transfer reactions.
Radiation Doses • Two factors are combined: • rad = radiation absorbed dose = 1 x 10-2 J/kg of body tissue • RBE = multiplier that depends on the particle • RBE = 1 for beta and gamma, RBE = 10 for alpha • rem = rad x RBE