Nuclear Chemistry

Radioactivity Nuclear Reactions – reactions in which the nuclei of unstable isotopes (radioisotopes) gain stability by undergoing changes

Radioactivity Henri Becquerel noticed that uranium ore exposed photographic film Marie & Pierre Curie named the process radioactivity The particles and rays emitted by a radioactive source are called radiation

Radioactivity Radioactivity disproves Dalton’s theory of indivisible atoms The stability of a nucleus depends on the relative proportion of neutrons to protons in the nucleus as well as the overall size of the nucleus An unstable nucleus loses energy by emitting radiation during the process of radioactive decay

Types of Radiation Alpha Radiation – consists of helium nuclei emitted from a radioactive source (alpha particles). In nuclear equations, an alpha particle is written as

Types of Radiation Beta Radiation – fast moving electrons formed by the decomposition of a neutron in an atom (beta particles) In nuclear equations, beta particles are written as

Types of Radiation Gamma Radiation – high energy electromagnetic radiation given off by a radioisotope Often emitted along with alpha or beta radiation by the nuclei of disintegrating radioactive atoms

Types of Radiation Property Alpha Rad Beta Rad Gamma Rad Composition Alpha Particle Beta Particle Gamma Ray Symbol Charge 2+ 1- Mass 4 1/1837 Shielding Paper, clothing Metal foil Lead, concrete

Nuclear Stability and Decay 1500 nuclei are known. Of those, only 264 are stable and do not decay with time In elements of low atomic number (less than 20), stable nuclei have roughly equal n0 and p+ In elements of high atomic number, stable nuclei have more n0 than p+

Nuclear Stability and Decay The stable nuclei on a neutron vs proton plot are located in a region called the band of stability

Nuclear Stability and Decay A nucleus may be unstable for several reasons: Nucleus has too many neutrons relative to number of protons. Decay occurs by turning a neutron into a proton and emitting a beta particle (electron) from the nucleus Result: increase in p+ and decrease in n0

Nuclear Stability and Decay Nucleus has too few neutrons relative to the number of protons. Increase stability by converting a proton to a neutron by the nucleus capturing an electron

Nuclear Stability and Decay Positron – a particle with the mass of an electron but with a positive charge. It may be emitted as a proton changes to a neutron

Nuclear Stability and Decay All nuclei with atomic number 83 and higher are radioactive because they have too many n0 and p+ to be stable Most emit alpha particles

Half-Life Every radioisotope has a characteristic rate of decay measured by its half-life Half-life – time required for one-half of the nuclei of a radioactive sample to decay to products

Transmutation Reactions Transmutation – conversion of an atom of one element to an atom of another element. Radioactive decay Bombarding the nucleus with high-energy particles (p+, n0, or α)

Transmutation Reactions The elements with atomic numbers greater than 92 are called the transuranium elements None occur in nature, and all are radioactive Synthesized in nuclear reactors

Nuclear Fission When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments Fission reactions generate additional neutrons, which lead to a chain reaction

Nuclear Fusion Fusion occurs when nuclei combine to produce a nucleus of greater mass Requires very high temperatures to start the reaction. At these temperatures, matter exists as plasma, a high-energy state in which ions exist in a gaslike form