How is atomic mass calculated?• Relative atomic mass of an element is the weighted average of the masses of the isotopes of the naturally occurring element; found on periodic table• Relative abundance is a ratio of the naturally occurring isotopes of an element on Earth
Example 1: Average Atomic Mass of C Isotope Percent abundance Atomic mass (amu)• Data: 12C 98.9% 12 13C 1.1% 13• Atomic mass = (relative abundance of isotope 1)(mass of isotope 1) + (relative abundance of isotope 2)(mass of isotope 2) + (relative abundance of isotope 3)(mass of isotope 3) + ...
Example 2: Average Atomic Mass of Cl Isotope Percent abundance Atomic mass (amu)• Data: Chlorine-35 75.76% Chlorine-37 24.24%• Atomic mass = (relative abundance of isotope 1)(mass of isotope 1) + (relative abundance of isotope 2)(mass of isotope 2) + (relative abundance of isotope 3)(mass of isotope 3) + ...
Example 2: Relative abundance of Cu Isotope Rel. abundance Atomic mass (amu)• Data: Copper-63 Copper-65• n = relative abundance of one isotope• (n)(mass of isotope 1) + (1 – n)(mass of isotope 2) = average mass
Review: Nuclear Symbols• What is the mass number of 14 C ? 6 (“carbon-14”)• How many protons are in 141 I -1 ? 53• What is the symbol for uranium-238? 238 U 92
Write these nuclear symbols!• P = 47 n = 50 e = 48• P = 82 n = 80 e = 82• P = 11 n = 15 e = 17
Nuclear Chemistry• Radioactive decay occurs when an unstable nucleus releases energy by emitting radiation• Radiation refers to the penetrating rays and particles emitted by a radioactive source• Types of radiation: – Alpha (α) – Beta (β) – Gamma (γ)
Alpha (α) Radiation• Nucleus decays by emitting an alpha particle 4 – Symbols: α or He 2 – Contains two protons and two neutrons – Charge: +2• Example: 230 226 4 90 Th 88 Ra 2 He (Alpha• Blocked by: _____________ particle)
Beta (β) Radiation• Nucleus decays by emitting a beta particle• General forms: 1 0 0 1 n 1H 1 e (neutron) (proton) (β- particle) 1 1 0 1H 0 n 1e (proton) (neutron) (β +particle) 27 27 0• Example: 12 Mg 13 Al 1 e (electron, or β- particle)• Blocked by: _____________
Gamma (γ) Radiation• High-energy radiation (photons) emitted from decaying nuclei• Often accompanies alpha or beta radiation• Blocked by: _____________
Electron Capture• Nucleus decays by capturing an electron and emitting a neutron• General forms: 1 0 1 1 H e 1 0n (proton) (electron) (neutron) 26 0 26• Example: 13 Al 1e 12 Mg ve (electron) (neutrino)• Neutrinos blocked by: _____________
Review: Types of Radiation• Alpha Radiation 238 234 4 92 U 90 Th 2 He• Beta Radiation 137 137 0 55 Cs 56 Ba 1 e• _________Radiation (with _______ ray) Alpha gamma 241 237 4 95 Am 93 Np 2 He
Why do atoms emit radiation?• Unstable atoms emit one of the three types of radiation to become stable• Atoms with a number of neutrons that is equal to or greater than the number of protons tend to be stable• This is graphed and known as the band of stability
Nuclear Binding Energy• The energy liberated when a nucleus is created from other nuclei• Derived from the strong nuclear force, the force holding protons and neutrons together in the nucleus AND the force holding quarks and gluons together to make protons and neutrons
Review• Radioactive atoms have unstable nuclei because of the ratio of protons to neutrons.• Unstable nuclei eventually break down, forming a completely different type of atom – This is radioactive decay• Although radioactive isotopes decay at a constant rate, we cannot predict exactly when an individual atom will decay
Definition of Half-Life (t ½)• The time it takes to reduce the number of nuclei of an isotope in a sample by ½• After each half-life, we expect half of the existing radioactive atoms to have decayed into atoms of a new element.
Graphical Representation of Half-Life t½ t½ t½
Half-Life is Exponential Decay 100%Amount ofsample (g) 1/2 t ½ = 2 days 1/4 1/8 1/16 1/32 Days
After 24 days, 10.0 grams of thorium-234 have decayed to 5.0 grams. What is the half-life of Th-234?A. 5 daysB. 10 daysC. 12 daysD. 24 days
True or false: After 48 days, all of the initial 10.0 grams of Th-234 will have undergone radioactive decay.A. TrueB. False
Fluorine-18 has a half-life of 110 minutes. If you begin with 110 atoms of 18F, how many atoms of 18F will remain after 110 minutes?A. Exactly 110 atomsB. Approximately 110 atomsC. Exactly 55 atomsD. Approximately 55 atomsE. Impossible to predict
Half-Life Calculations• Manganese-56 has a half-life of 2.6 hours. What is the mass of manganese-56 in a 16 gram sample of the isotope after 10.4 hours? 10.4 h / 2.6 h = 4 half-lives have passed (16 g)(½)(½)(½)(½) = 1.0 g or (16 g)(½)4 = 1.0 g
Half-Life Calculations• What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 g of nitrogen- 16 in 21.6 seconds?• How many half-lives have elapsed? 100.0 g 50.0 g 25.0 g 12.5 g 3 half-lives have elapsed• 21.6 seconds / 3 half-lives = 7.20 seconds
Half-Life Calculations• The mass of cobalt-60 in a sample is found to have decreased from 0.800 g to 0.200 g in a period of 10.5 years. From this information, calculate the half-life of cobalt-60.
Fractional # of Half-Lives• If we start with 20.0 g of nitrogen-14, how much would remain after 30.0 seconds? The half-life of nitrogen-14 is 21.3 seconds.• 30.0 s / 21.3 s = 1.41 half-lives• (20.0 g)(½)1.41 = 7.53 g
Test Your Understanding• For each of the following radiation sources or processes, would a long or a short half-life desirable? – Smoke detectors – Carbon dating – Medical diagnostic imaging – Nuclear fuel rods
Fission vs. Fusion• Fission: A nuclear reaction in which a heavy isotope splits into smaller fragments, often in a chain reaction – Ex: Nuclear Reactors, Atomic Bomb• Fusion: The combining of two small nuclei to form a larger, more stable nucleus with the release of energy – Produces the most energy and is difficult to contain. – Ex: Sun, Hydrogen Bomb, Fusion Reactors– Both fission and fusion release large amounts of energy (though fusion releases more)
Nuclear Fission Nuclear Fusion Fission is the splitting of a large atom Fusion is the fusing of two or more Definition: into two or more smaller ones. lighter atoms into a larger one. Fission reaction does not normally Natural occurrence of the process: Fusion occurs in stars, such as the sun. occur in nature. Fission produces many highly Byproducts of the reaction: Few radioactive particles are produced. radioactive particles. A million times greater than that The energy released by fusion is three released in chemical reactions; but Energy Released: to four times greater than the energy lower than the energy released by released by fission. nuclear fusion. One class of nuclear weapon is a fission One class of nuclear weapon is the Nuclear weapon: bomb, also known as an atomic bomb hydrogen bomb, which uses a fission or atom bomb. reaction to "trigger" a fusion reaction. Critical mass of the substance and High density, high temperature Conditions: high-speed neutrons are required. environment is required. Takes little energy to split two atoms in Energy requirement: Extremely high energy is required. a fission reaction.http://www.diffen.com/difference/Nuclear_Fission_vs_Nuclear_Fusion
Nuclear Power Plant• Nuclear reactors use controlled fission to produce useful energy.• How does this work? – Fission produces lots of heat energy. – Coolant fluid removes heat from reactor core. – Heat generates steam. – Steam drives a turbine, creating electricity.
Controlling the Nuclear Reaction• Control rods: Keep reaction from going too fast by absorbing some of the neutrons; often made of cadmium• Moderator: slows down neutrons so that the reactor fuel (235U or 239Pu) can capture them; water and graphite are good moderators
Meltdown!• If the chain reaction goes too fast, it gets too hot and the coolant fluid cannot take the heat away fast enough, possibly leading to a meltdown.
Three Mile Island• 1979• Middletown, Pennsylvania• Partial meltdown due to stuck-open valve which allowed lots of coolant to escape
Three Mile Island generator moving to Shearon HarrisPosted: Jan. 22, 2010MIDDLETOWN, Pa. — A generator in storage for more than three decades following the accident at Three Mile Island nuclear power plant is heading to North Carolina.Officials at the Nuclear Regulatory Commission say the electrical generator from the damaged Unit 2 reactor at TMI will be used at Progress Energy Inc.s Shearon Harris nuclear plant in southwest Wake County.NRC spokesman Neil Sheehan said Thursday that preliminary work is under way to move the generator. It will be transported in two parts, weighing a combined 670 tons.Progress Energy spokeswoman Julia Milstead said the generator is coming from the non-nuclear side of TMI. The generator has been refurbished, and the parts were extensively tested to ensure they werent contaminated.The Raleigh-based utility will save money by using the older generator instead of buying a new one, meaning the cost savings can be passed on to its customers, Milstead said.The generator will be shipped to Shearon Harris by rail and will not impact traffic on any area roads, she said.TMIs Unit 2 reactor has been shut down since a partial meltdown in 1979.
What do you do with spent fuel? • Nuclear waste (leftover fuel and fission products) must be stored in “holding tanks” filled with water for a few years. • Eventually, they are taken to a more permanent storage facility.
Chernobyl• 1986• Ukraine (formerly USSR)• Full meltdown due to a safety test gone awry• Soviet-designed RBMK reactor had serious design flaws• No containment dome• Last Chernobyl reactor taken offline in 2000• Other RMBK reactors still in use in Eastern Europe
Lyudmilla Ignatenko Wife of fireman Vasily Ignatenko