Nuclear Chemistry Chapter 24
Radioactivity Radioisotopes are isotopes that have an unstable nucleus. They emit radiation to attain more stable atomic configurations in a process called radioactive decay. Radioactivity is the property by which an atomic nucleus gives off alpha, beta, or gamma radiation. Marie Curie named the process. In 1898, Marie & Pierre Curie identified 2 new elements, polonium & radium. The penetrating rays and particles emitted by a radioactive source are called radiation.
Radioactivity (cont) The presence of too many or too few neutrons, relative to the number of protons, leads to an unstable nucleus. The types of radiation are alpha (α), beta (β), or gamma (γ). An unstable nucleus loses energy by emitting radiation during the process of radioactive decay. Spontaneous and does not require any input of energy.
Alpha radiation A type of radiation called alpha radiation consists of helium nuclei that have been emitted from a radioactive source. These emitted particles, called alpha particles, contain 2 protons and 2 neutrons and have a double positive charge.
Alpha Radiation (cont) Because of their large mass and charge, alpha particles do not tend to travel very far and are not very penetrating. They are easily stopped by a piece of paper or the surface of skin. Radioisotopes that emit alpha particles are dangerous when ingested.
Alpha radiation occurs when an unstable nucleus emits a particle composed of 2 protons and 2 neutrons. The atom giving up the alpha particle has its atomic number reduced by two. Of course, this results in the atom becoming a different element. For example, Rn undergoes alpha decay to Po.
Beta Particles A beta particle is essentially an electron that s emitted from the nucleus. A neutron is converted (decayed) into a proton & electron so the atomic number increases by 1 and the electron leaves the nucleus. Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay allows the # of neutrons to be decreased by one & the # of protons to be increased by one, thus lowering the neutron/proton ratio.
Beta radiation occurs when an unstable nucleus emits an electron. As the emission occurs, a neutron turns into a proton.
Gamma Radiation Gamma radiation is similar to x-rays high energy, short wavelength emissions (photons). The symbol is γ, the Greek letter gamma. It commonly accompanies alpha and beta emission, but it s usually not shown in a balanced nuclear reaction. Some isotopes, such as Cobalt-60, give off large amounts of gamma radiation. Co-60 is used in the radiation treatment of cancer the gamma rays focus on the tumor, thus destroying it.
Gamma radiation occurs when an unstable nucleus emits electromagnetic radiation. The radiation has no mass, and so its emission does not change the element. However, gamma radiation often accompanies alpha and beta emission, which do change the element's identity.
Half Life A half-life (t 1/2 ) is the time required for one-half of the nuclei of a radioisotope sample to decay to products. Half-lives may be as short as a fraction of a second or as long as billions of years. This is an example of exponential decay. If you want to find times or amounts that are not associated with a simple multiple of a halflife, you can use this equation: ln(n 0 /N) = (.6963/t 1/2 )t ln=natural log, N 0 =amnt iso start, N=amnt iso left t=time, t 1/2 =half-life
Half-Life Problem The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?
Man-Made Radioactive Decay on Earth Fission Fusion Occurs naturally in space Powers the sun Supernovas allow atoms to fuse into heavier elements, this is how the other elements came into existence
Fission Nuclear fission occurs when scientists bombard a large isotope with a neutron. This collision causes the larger isotope to break apart into two or more elements. These reactions release a lot of energy. You can calculate the amount of energy produced during a nuclear reaction using an equation developed by Einstein: E=mc 2
Chain Reactions A chain reaction is a continuing cascade of nuclear fissions. This chain reaction depends on the release of more neutrons then were used during the nuclear reaction. Isotopes that produce an excess of neutrons in their fission support a chain reaction - fissionable. There are only two main fissionable isotopes used during nuclear reactions uranium-235 & plutonium-239.
Chain Reactions (cont) The minimum amount of fissionable material needed to ensure that a chain reaction occurs is called the critical mass. Anything less than this amount is subcritical.
Chain Reaction Figure
Atomic Bombs Because of the tremendous amount of energy released in a fission chain reaction, the military implications of nuclear reactions were immediately realized. The first atomic bomb was dropped on Hiroshima, Japan, on August 6, 1945. In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is subcritical. When it s time for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass. The chain reaction is uncontrolled, releasing a tremendous amount of energy almost instantaneously.
Mushroom Cloud
Nuclear Power Plants If the neutrons can be controlled, then the energy can be released in a controlled way. Nuclear power plants produce heat through controlled nuclear fission chain reactions. The fissionable isotope is contained in fuel rods in the reactor core. All the fuel rods together comprise the critical mass. Control rods, commonly made of boron and cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay.
Nuclear Power Plants (cont) In the U.S., there are approximately 100 nuclear reactors, producing a little more than 20% of the country s electricity. Advantages No fossil fuels are burned. No combustion products (CO 2, SO 2, etc) to pollute the air and water. Disadvantages Cost - expensive to build and operate. Limited supply of fissionable Uranium-235. Accidents (Three Mile Island & Chernobyl) Disposal of nuclear wastes
A nuclear power plant. Heat produced in the reactor core is transferred by coolant circulating in a closed loop to a steam generator, and the steam then drives a turbine to generate electricity.
Diagram of a nuclear power plant
Three Mile Island
Chernobyl
Nuclear Fusion Fusion is when lighter nuclei are fused into a heavier nucleus. Fusion powers the sun. Four isotopes of hydrogen-1 are fused into a helium-4 with the release of a tremendous amount of energy. On Earth, H-2 (deuterium) & H-3 (tritium) are used.
Nuclear Fusion (cont) The first demonstration of nuclear fusion the hydrogen bomb was conducted by the military. A hydrogen bomb is approximately 1,000 times as powerful as an ordinary atomic bomb. The goal of scientists has been the controlled release of energy from a fusion reaction. If the energy can be released slowly, it can be used to produce electricity. It will provide an unlimited supply of energy that has no wastes to deal with or contaminants to harm the atmosphere. The 3 problems are: temperature, time, containment
Nuclear Fusion (cont) Temperature Hydrogen isotopes must be heated to 40,000,000 K (hotter than the sun). Electrons are gone all that s left is positively charged plasma. Time The plasma needs to be held together for about one second at 40,000,000 K. Containment Everything is a gas ceramics vaporize. A magnetic field could be used but the plasma leaks from those as well.