Chapter 10 Nuclear Chemistry

Radioactivity Radioactivity is the process in which an unstable atomic nucleus emits charged particles and energy. Any atom containing an unstable nucleus is called a radioactive isotope, or radioisotope for short.

Radioactivity During nuclear decay, atoms of one element can change into atoms of a different element altogether. Nuclear radiation is charged particles and energy that are emitted from the nuclei of radioisotopes. Common types of nuclear radiation include alpha particles, beta particles, and gamma rays.

Alpha Decay When a uranium-238 sample decays, it emits alpha particles. An alpha particle is a positively charged particle made up of two protons and two neutrons – the same as a helium nucleus.

Alpha Decay Alpha particles are the least penetrating type of nuclear radiation. They can be stopped by a sheet of paper of clothing. Alpha particles can cause skin damage similar to a burn, but they are not a serious health hazard unless an alpha- emitting substance is inhaled or eaten. Radon gas is a dangerous natural source of alpha particles because it can be inhaled.

Beta Decay When thorium-234 decays, it releases negatively charge radiation called beta particles. A beta particle is an electron emitted by an unstable nucleus.

Beta Decay Beta particles are more penetrating than alpha particles. They pass through paper, but are stopped by a thin sheet of metal. Beta particles can damage tissues in the body more than alpha particles.

Gamma Decay When thorium-234 decays, it also releases radiation that has no charge and no mass. A gamma ray is a penetrating ray of energy emitted by an unstable nucleus. It has no charge and no mass.

Gamma Decay Gamma rays are much more penetrating than either alpha particles or beta particles. It can take several centimeters of lead or several meters of concrete to stop gamma radiation. Gamma rays can penetrate deeply into the human body, potentially exposing all organs to ionization damage.

Radioactivity You are exposed to radiation every day. Most of this is background radiation, or nuclear radiation that occurs naturally in the environment. Nuclear radiation can ionize atoms. When cells are exposed to nuclear radiation, the bonds holding together proteins and DNA molecules may break. As these molecules change, the cells may no longer function properly.

Radioactivity Radioisotopes in air, water, rocks, plants, and animals all contribute to background radiation. Cosmic rays (streams of protons and alpha particles) also contribute to background radiation. Background radiation levels are generally low enough to be safe.

Detecting Nuclear Radiation A Geiger counter uses a gas-filled tube to measure ionizing radiation. When nuclear radiation enters the tube, it ionizes the atoms of the gas. The ions produce an electric current, which can be measured.

Detecting Nuclear Radiation A film badge contains a piece of photographic film wrapped in paper. The film is developed and replaced with a new piece periodically. The exposure on the film indicates the amount of radiation exposure for the person wearing the badge.

Rates of Nuclear Decay A nuclear decay rate describes how fast nuclear changes take place in a radioactive substance. Unlike chemical reaction rates, which vary with the conditions of a reaction, nuclear decay rates are constant. Every radioisotope decays at a specific rate that can be expressed as a half-life.

Rates of Nuclear Decay A half-life is the time required for one half of a sample of a radioisotope to decay. After one half-life, half of the atoms in a radioactive sample remain After two half-lives, 1 4 of the atoms in the sample remain. After three half-lives, only 1 8 of the atoms in the sample will remain.

Rates of Nuclear Decay

Rates of Nuclear Decay In radiocarbon dating, the age of an object is determined by comparing the object’s carbon-14 levels with carbon-14 levels in the atmosphere. Carbon-14 undergoes beta-decay to form nitrogen-14

Rates of Nuclear Decay Carbon-14 can only be used to date objects that are less than 50,000 years old. For objects older than 50,000 years, scientists use other radioisotopes. Geologists use potassium-40, uranium-235, or uranium-238 to date rock formations. The older the rock, the lower the levels of the radioisotope present.

Nuclear Forces The strong nuclear force is the attractive force that binds protons and neutrons together in the nucleus. Over very short distances, the strong nuclear force is much greater than the electric forces among protons.

Nuclear Forces A. The strong nuclear forces easily overcome the electric force between the protons. B. The larger number of electric forces makes the nucleus less stable. Nuclear Forces Acting on a Proton of a Small Nucleus Nuclear Forces Acting on a Proton of a Large Nucleus

Nuclear Forces A nucleus becomes unstable, or radioactive, when the strong nuclear force can no longer overcome the repulsive electric forces among protons. The strong nuclear force does not increase with the size of the nucleus, but the electric forces do. All nuclei with more than 83 protons are radioactive.

Fission Fission is the splitting of an atomic nucleus into two smaller parts. In nuclear fission, tremendous amounts of energy can be produced from very small amounts of mass. Mass-Energy Equation E = mc2

Fission Neutron Energy

Fission In a chain reaction, neutrons released during the splitting of an initial nucleus trigger a series of nuclear fissions. A critical mass is the smallest possible mass of a fissionable material that can sustain a chain reaction. Nuclear power plants use controlled fission of uranium-235 in a fission reactor.

Nuclear Energy from Fission Unlike power plants that burn fossil fuel, nuclear power plants do not emit air pollutants. However, nuclear plants have their own safety issues. Workers in nuclear plants must wear protective clothing to reduce their exposure to nuclear radiation The fission of uranium-235 produces many radioactive isotopes with half-lives of hundreds or thousands of years. Operators of the plan could lose control of the reactor.

Fusion Fusion is a process in which the nuclei of two atoms combine to form a larger nucleus. Very high temperatures (similar to the sun) are required to start the reaction. At temperatures this high, matter exists as plasma. Plasma is a state of matter in which atoms have been stripped of their electrons.

Fusion The sun is not actually burning. The sun, and other stars, are powered by the fusion of hydrogen into helium. The light and heat given off by the sun result from nuclear fusion, not combustion.

Fusion Fusion may someday provide an efficient and clean source of energy. Scientists envision fusion reactors fueled by two hydrogen isotopes, deuterium (hydrogen-2) and tritium (hydrogen-3).

Fusion Advantages of using fusion over fission: Workers are not in as much danger from radiation Hydrogen is used, which is easily obtained from water No harmful waste products are produced Two main problems in designing a fusion reactor are the high temperatures required to start the reaction and that they must contain the plasma.