Nuclear Chemistry

How are chemical reactions and nuclear reactions different? Chemical Reactions Nuclear Reactions

Chemical Reactions 1. Occur when bonds are broken and formed. 1.Atoms remain unchanged though they may be rearranged. 1.Involve only valence electrons. 1.Associated with small energy changes. 1.Reaction rate is influenced by temp., pressure, concentration, and catalysts.

Nuclear Reactions 1.Occur when nuclei emit particles and/or rays. 1.Atoms are often converted into atoms of another element. 1.May involve protons, neutrons, and electrons. 1.Associated with large energy changes. 1.Reaction rate is not normally affected by temp., pressure, or catalysts.

► Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. ► The decomposition process is called radioactivity. ► The energy and particles which are released during the decomposition process are called radiation. ► When unstable nuclei decompose in nature, the process is referred to as natural radioactivity. ► When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

There are three major types of natural radioactivity: There are three major types of natural radioactivity: Types of Radioactivity AlphaBetaGamma

Alpha Radiation ► Alpha radiation consists of a stream of positively charged particles, called alpha particles, which have an atomic mass of 4 and a charge of +2 (a helium nucleus). ► When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. ► For example: U -> 4 2 He Th ► The helium nucleus is the alpha particle.

Beta Radiation ► Beta radiation is a stream of electrons, called beta particles. ► When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. ► For example: Th -> 0 -1 e Pa ► The electron is the beta particle.

Gamma Radiation ► Gamma rays are high-energy photons with a very short wavelength ( to 0.1 nm). ► The emission of gamma radiation results from an energy change within the atomic nucleus. ► Gamma emission changes neither the atomic number nor the atomic mass. ► Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

Bombardment Reactions ► Alpha, beta, and gamma radiation also accompany induced radioactivity. ► Radioactive isotopes are prepared in the lab using bombardment reactions to convert a stable nucleus into one which is radioactive. ► Positron (particle with the same mass as an electron, but a charge of +1 instead of -1) emission isn't observed in natural radioactivity, but it is a common mode of decay in induced radioactivity. ► Bombardment reactions can be used to produce very heavy elements, including many which don't occur in nature.

How to shield against radiation ► In general, the shorter the EM wavelength, the thicker and denser the shield material must be. ► Ultraviolet (UV) can be blocked simply by a sufficiently opaque sheet of plastic. We are all familiar with tinted sunglasses that promise to block some 97% of solar UV rays. Not much additional protection is required in space. ► X-rays and gamma rays are another matter. Where intense x-rays and gamma rays occur, it requires several inches or centimeters of lead and/or concrete to provide adequate shielding.

Blocking Alpha particles ► Alpha particles are very large particles. As such they don't penetrate very deeply into many things. ► In fact, alpha particles will not even penetrate the epidermal (dead) layer of skin, and so present no special hazard to humans. ► A sheet of reasonably thick paper will block all alpha particles.

Blocking Beta particles ► Beta particles are very small and can penetrate centimeters into the body. ► But luckily they're too small to cause much damage if they hit anything. ► But there's a special problem here. When beta particles hit large atoms, the impact causes those atoms to give off x-rays. Metal atoms are usually quite heavy, and so are especially susceptible to this kind of re-radiation which is known by its German name "Bremsstrahlung". In fact, this is how x-rays are produced intentionally for medical applications.

► The best materials to shield against beta particles have lots of hydrogen atoms in them. ► Hydrogen atoms are light, and so absorb the particles without giving off x-rays. ► Plain old water works very well. In fact, 4 inches (10 centimeters) of water will block almost all background beta particles. ► But water is impractical for shielding in space, so high-density polyethylene (HPDE, chemical formula CH 2 CH 2...) is frequently used instead. This also effectively blocks protons.

Blocking Gamma rays ► Gamma rays are a form of electromagnetic radiation, so they have no mass, carry energy and travel the speed of light. ► Where intense gamma rays occur, it requires several inches or centimeters of lead and/or concrete to provide adequate shielding.

Radiation Source Dose (rems) ► Chest x-ray ► Chest x-ray ► Mammogram ► Mammogram ► Cosmic rays0.031 annually ► Cosmic rays0.031 annually ► Human body annually ► Human body annually ► Household radon annually ► Household radon annually ► Cross-country airplane flight ► Cross-country airplane flight ► Are there any legal limits for radiation exposure? ► Worker Category Legal Limit ► Worker Category Legal Limit ► 18-year old male 5 rem/year ► 18-year old male 5 rem/year ► Pregnant woman0.500 rem