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CH. 25 NUCLEAR CHEMISTRY
25.1 Nuclear Radiation
Radioactivity
⦿ Spontaneous emission of rays or particles from an element
⦿ Nuclear reaction● Atoms become more stable● Radioisotopes become more stable when
nucleus changes and energy is emitted● Not affected by changes in temperature,
pressure, or catalysts● Happen at a given rate which cannot be
changed
Types of Radiation
⦿ Alpha⦿ Beta⦿ Gamma
Alpha Radiation
⦿ Radioactive source emits helium nuclei (alpha particles) composed of two protons and two neutrons
⦿ Stopped by paper or skin⦿ Harmful if ingested
Beta Radiation⦿ Radioactive source emits beta particles
● Neutron broken into a proton and an electron● Proton stays in nucleus● Electron escapes (beta particle)
⦿ Less charge and mass than alpha particle
⦿ Stopped by aluminum foil or wood
Gamma Radiation
⦿ Radiation source emits high-energy photons
⦿ No mass or charge⦿ Often emitted with alpha or beta
particles
⦿ Stopped by several meters of concrete or several centimeters of lead
Practice Completing Nuclear Reactions⦿
Lesson Check
⦿ What factors do not affect nuclear reactions, but do affect chemical reactions?
⦿ What part of an atom changes during radioactive decay?
⦿ How is the atomic number of a nucleus changed by alpha decay? By beta decay? By gamma decay?
Lesson Check
⦿ What two items must be equal for a nuclear equation to be balanced?
⦿ When polonium-210 decays by alpha radiation, what isotope is formed?
25.2 Nuclear Transformations
Nuclear Stability and Decay⦿ Nuclear force: attractive force that acts
between particles in the nucleus⦿ Band of stability shown below
Nuclear Stability and Decay⦿ Neutron to proton ratio determines type
of decay that occurs (to stabilize the nucleus)● Beta emission
Nuclear Stability and Decay
● Electron capture
● Positron emission
● Alpha emission
Half-Life⦿ t1/2: time required for ½ of the nuclei of a
sample of radioactive isotope to decay to products
Radiocarbon Dating
⦿ Compare carbon-14 to carbon-12 in a sample
⦿ Carbon-14’s half-life = 5730 years⦿ Carbon-14 is produced by high-energy
cosmic rays in CO2 in upper atmosphere⦿ Carbon-14 ratio is constant during an
organism’s life, decreases after death as it radioactively decays
Exponential Decay Function
⦿ Equation used to determine amount of an isotope remaining after a given number of half-lives
⦿ A = A0 x (1/2)n
● A: amount remaining● A0: initial amount● n: number of half-lives
Practice
⦿ Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 2.00 x 10-12 g of carbon-14.● How long is three half-lives?● How many grams of the isotope remain at
the end of three half-lives?
Practice
⦿ Manganese-56 is a beta emitter with a half-life of 2.6 h. What is the mass of manganese-56 in a 1.0-mg sample of the isotope at the end of 10.4 h?
⦿ Thorium-234 has a half-life of 24.1 days. Will all the thorium atoms in a sample decay in 48.2 days? Explain.
Transmutation Reactions⦿ Conversion of one
element into another element
⦿ Occurs by radioactive decay or when particles bombard the nucleus of an atom
Transuranium Elements
⦿ Atomic numbers greater than 92⦿ Radioactive, undergo transmutation⦿ Made in nuclear reactors and nuclear
accelerators● Bombard uranium with particles (neutrons)
Lesson Check
⦿ How much of a sample of a radioisotope remains after one half-live? After two half-lives? After 3 half-lives?
⦿ How can transmutation occur in a stable isotope
⦿ A radioisotope has a half-life of 4 days. How much of a 20-g sample of this radioisotope remains at the end of 4 days? 8 days?
Lesson Check
⦿ Complete the following nuclear equations.
25.3 Fission and Fusion
Nuclear Fission⦿ Nucleus splits into smaller fragments⦿ In a chain-reaction, emitted neutrons
collide with fissionable atoms which continues the reaction● Uncontrolled chain reactions release energy
in fractions of a second (nuclear bomb)● Controlled chain reactions release energy
more slowly (nuclear reactor to produce energy)
Nuclear Fission
⦿ Neutron moderation: slows neutrons down enough to be captured by nucleus● Water and graphite are good moderators
⦿ Neutron Absorption: decreases number of slow-moving neutrons to control reaction● Control-rods of cadmium absorb extra
neutrons to prevent reaction occurring too quickly; prevents meltdown of the reactor core
Nuclear Reactor
Nuclear Fission
⦿ Nuclear Waste: spent fuel rods are kept in holding tanks (water cools rods and blocks radiation) and may be moved to off-site storage facilities● Some radioactive products have long
half-lives (plutonium-239 = 20,000 years)
Nuclear Fusion
⦿ Nuclei combine to form a more massive nucleus
⦿ Fusion reactions release more energy than fission reactions
⦿ Occur only at very high temperatures (>40,000,000°C)
⦿ How elements are produced in stars
Nuclear Fusion
⦿ Fusion reaction can occur in heat produced by a fission reaction● Useful as a bomb (H-bomb)● Not controllable for generating power
25.4 Radiation in Your Life
Detecting Radiation
⦿ Ionizing radiation: enough energy to knock electrons off atoms to produce ions● Detectable with Geiger counters, scintillation
counters, film, film badges
Detecting Radiation
⦿ Geiger Counters● Gas-filled metal tube w/ central electrode
and power supply● When radiation ionizes the gas it can
conduct● Current produces audible clicks from a
speaker● First developed in 1930s● Uses: detect cosmic rays (astronomy), find
radioactive minerals (geology), find leaks in hospitals
Detecting Radiation
⦿ Scintillation Counters● Phosphor-coated surface produces bright
flashes when struck by ionizing radiation● Number of flashes and energy detected
electronically● More sensitive than Geiger counters● Uses: track path of radioisotopes through
body, monitor transport of radioactive materials across borders and through airports
Detecting Radiation
⦿ Film badges● Film darkens when exposed to radiation● Sealed film in plastic or metal container with
filter than lets in some radiation● Worn by people who work with/near ionizing
radiation to monitor degree of radiation exposure
Using Radiation
⦿ Analyzing matter: neutron activation analysis can detect trace amounts of elements in a sample● Art museums: detect forgeries● Crime labs: analyze gunpowder residues
⦿ Using tracers: test effects of herbicides, pesticides, and fertilizers by monitoring plants, treated with substance, animals that ate the plants, or water and soil in the area
Using Radiation
⦿ Diagnosing Medical Problems● Radioactive isotopes introduced that will be
absorbed by affected area● Scan to analyze uptake of the isotope
● Iodine-131 for thyroid● Technetium-99m for brains tumorsand liver disorders● Phosphorus-32 for skin cancers
Using Radiation
⦿ Treating Diseases● Treat some cancers: fast-growing tumors are
more susceptible to damage by high-energy radiation
● Cobalt-60, cesium-137● Radioisotope salts sealed in gold containers
“seeded” into tumors emit beta and gamma rays to kill tumor but can’t travel
● Radiation therapy: radioisotopes of gold, iodine, or phosphorus
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