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