Chapter 25

 Marie Curie was a Polish scientist whose research led to many discoveries about radiation and radioactive elements. In 1934 she died from leukemia caused by her long-term exposure to radiation. You will learn about the various types of radiation and their effects. 25.1

 Marie Curie ( ) and Pierre Curie ( ) were able to show that rays emitted by uranium atoms caused fogging in photographic plates.  Marie Curie named the process by which materials give off such rays radioactivity.  The penetrating rays and particles emitted by a radioactive source are called radiation. 25.1

 Nuclear reactions differ from chemical reactions in a number of important ways.  In chemical reactions, atoms tend to attain stable electron configurations by losing or sharing electrons.  In nuclear reactions, the nuclei of unstable isotopes, called radioisotopes, gain stability by undergoing changes.  In the periodic table, radioactive elements have mass numbers in parentheses.  Reference table N 25.1

◦ An unstable nucleus releases energy by emitting radiation during the process of radioactive decay. 25.1

 Types of Radiation ◦ What are the three main types of nuclear radiation? 25.1

 The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.  Reference table O 25.1

◦ Alpha Radiation  Alpha radiation consists of helium nuclei that have been emitted from a radioactive source. These emitted particles, called alpha particles, contain two protons and two neutrons and have a double positive charge.  Note: In a nuclear decay equation, mass and charge are conserved! 25.1

◦ Beta Radiation  An electron resulting from the breaking apart of a neutron in an atom is called a beta particle. 25.1

 Carbon-14 emits a beta particle as it undergoes radioactive decay to form nitrogen

◦ Gamma Radiation  A high-energy photon emitted by a radioisotope is called a gamma ray. The high-energy photons are electromagnetic radiation. 25.1

 Alpha particles are the least penetrating. Gamma rays are the most penetrating. 25.1

 Radon-222 is a radioactive isotope that is present naturally in the soil in some areas. It has a constant rate of decay. You will learn about decay rates of radioactive substances. 25.2

 Radioactive substances decay at a constant rate regardless of factors such as temperature and concentration.  Radioactive decay is a random event.  Decay continues until a stable isotope is formed. ◦ Some radioisotopes have a long decay series.

25.2 Stable Isotope

 The time it takes for half the atoms in a sample to decay is called the half-life.  Each isotope has its own unique half-life.  The shorter the half-life, the less stable it is. Reference table N

 If a 100 g sample has a half-life of 5 days: ◦ After 5 days ½ (50g) will remain undecayed ◦ After 10 days ¼ (25g) will remain undecayed ◦ After 15 days 1/8 (12.5g) will remain undecayed ◦ After 20 days 1/16 (6.25g) will remain undecayed ◦ And so on…

 The ratio of Carbon-14 to stable carbon in the remains of an organism changes in a predictable way that enables the archaeologist to obtain an estimate of its age. 25.2

 Radioactive decay is measured with a Geiger counter. ◦ A Geiger counter records individual decay events.  Graphing decay allows determination of the half-life.

25.2

 Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.

 The conversion of an atom of one element to an atom of another element is called transmutation.  What are two ways that transmutation can occur?

 Transmutation can occur naturally by radioactive decay. ◦ a single nucleus undergoes decay  Transmutation can also occur artificially when particles bombard the nucleus of an atom. ◦ at least two reactants produce the target material

 A nucleus is bombarded with high-energy particles to bring about a change.  Two types: ◦ Bombarding with charged particles: protons or alpha particles.  Particle accelerators use magnetic or electrostatic fields to accelerate particles towards the target.  Cern video ◦ Bombarding with neutrons.  Neutrons are obtained as by-products of nuclear reactors.

 The first artificial transmutation reaction involved bombarding nitrogen gas with alpha particles.

 The elements in the periodic table with atomic numbers above 92, the atomic number of uranium, are called the transuranium elements. ◦ All transuranium elements undergo transmutation. ◦ None of the transuranium elements occur in nature, and all of them are radioactive.

 The sun is not actually burning. If the energy given off by the sun were the product of a combustion reaction, the sun would have burned out approximately 2000 years after it was formed, long before today. You will learn how energy is produced in the sun. 25.3

 Nuclear Fission ◦ What happens in a nuclear chain reaction? 25.3

 When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments. 25.3

◦ In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms. 25.3

 Nuclear Fission 25.3

A Nuclear Power Plant

 Nuclear Waste ◦ Why are spent fuel rods from a nuclear reaction stored in water? 25.3

 Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels. 25.3

 Nuclear Fusion ◦ How do fission reactions and fusion reactions differ? 25.3

 Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons. 25.3

◦ Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split. 25.3

 The use of controlled fusion as an energy source on Earth is appealing.  The potential fuels are inexpensive and readily available.  The problems with fusion lie in achieving the high temperatures necessary to start the reaction and in containing the reaction once it has started. 25.3

 In a smoke detector, radiation from the Americum nuclei ionizes the nitrogen and oxygen in smoke-free air, allowing a current to flow. When smoke particles get in the way, a drop in current is detected by an electronic circuit, causing it to sound an alarm. You will learn about some of the other practical uses of radiation. 25.4

 C-14 is used to date fossils.  U-238 is used to date rocks.

 Radioisotopes are used to trace chemical pathways in biological systems.

 Radioisotopes are used to determine minute thicknesses such as plastic wrap and aluminum foil.  Gamma radiation is used to irradiate foods to kill bacteria and make the foods last longer.

 Radioisotopes with short half-lives that are quickly eliminated from the body are used as tracers in medical diagnoses and treatments. ◦ I-131 is used to detect and treat thyroid disorders. ◦ Co-60 is used in cancer treatments. ◦ Tc-99 is used to detect tumors.

 This scanned image of a thyroid gland shows where radioactive iodine-131 has been absorbed. 25.4

 In medical treatments radiation can destroy normal cells as well as cancerous cells.  Radiation leaking from a nuclear power plant can cause mutations that could be inherited.  Spent fuel rods are a disposal concern. They are still radioactive and potentially dangerous.