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Nuclear Chemistry. Basic Concepts. Basic Concepts. What part of an atom is involved in chemical reactions?. Valence electrons. What is the difference between chemical reactions and nuclear reactions?.
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Nuclear Chemistry Basic Concepts
Basic Concepts • What part of an atom is involved in chemical reactions? Valence electrons. • What is the difference between chemical reactions and nuclear reactions? Chemical reactions: the nucleus remains unchanged, involves the transfer or sharing of valence electrons. Nuclear reactions involve changes in the nuclei of atoms.
Radioactivity • Nuclear reactions are those in which the nuclei of unstable isotopes, called radioisotopes, gain stability by undergoing changes. • Unlike chemical reactions, nuclear reactions are not affected by changes in temperature, pressure or the presence of catalysts.
What types of radiation exist? • The three most common types of radiation emitted by unstable nuclei are • Alpha • Beta • Gamma • Gamma rays are particularly harmful because they penetrate body tissues.
Alpha decay • Symbol is a or is written as . • An alpha particle is a helium nucleus - two protons and two neutrons. It therefore has considerable mass compared to a beta particle or gamma ray. • Atomic number decreases by 2, mass number decreases by 4. • Stopped by a sheet of paper.
Beta decay • Symbol is b or is written as . • A beta particle is an electron therefore it has negligible mass. • It is emitted from the nucleus along with a neutrino. In the process, a neutron becomes a proton. • Atomic number increases by one, mass number is unchanged. • It can be stopped by 6 mm of aluminum foil.
Gamma decay • Symbol is g. • Gamma rays have no mass and no electrical charge. • Gamma radiation is electromagnetic radiation. It is like visible light, but much more energetic and has high penetrating power. • Mass number and atomic number do not change. • Stopped incompletely by several feet of concrete or lead.
Transmutation • The conversion of an atom of one element to an atom of another element is called transmutation. • Radioactive decay is one way in which transmutation occurs. • Transmutation can also occur when high energy particles bombard the nucleus of an atom. The high-energy particles may be protons, neutrons or alpha particles.
Rutherford’s Artificial Transmutation • Ernest Rutherford carried out the first artificial transmutation when he bombarded nitrogen gas with alpha particles. • What particles were formed? • This initial reaction was unstable and quickly decomposed to a stable isotope of oxygen. What was the other particle?
Artificial Transmutation proton
Half-Life of a Radioisotope The time for the radiation level to fall (decay) to one-half its initial value decay curve 8 mg 4 mg 2 mg 1 mg initial 1 half-life 2 3
Half-Life • Every radioisotope decays at a characteristic rate A half-life is the time required for one-half of the nuclei in a radioisotope to decay.
Examples of Half-Life Isotope Half life C-15 2.4 sec Ra-224 3.6 days Ra-223 12 days I-125 60 days C-14 5700 years U-235 710 000 000 years
Learning Check The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?
Solution t1/2 =13 hrs 26 hours = 2 x t1/2 Amount initial = 64mg Amount remaining = 64 mg x ½ x ½ = 16 mg
Nuclear Fission • Nuclear fission occurs when fissionable isotopes are bombarded with neutrons. • As the isotopes split, they release more neutrons, which split more nuclei in a chain reaction.
Nuclear Fission Fission large nuclei break up 235U + 1n 139Ba + 94Kr + 3 1n + 92 0 56 36 0 Energy
Fission Nuclear power plants use neutron moderation and neutron absorption to control the fission reaction used to produce electricity.
Nuclear Fusion • In nuclear fusion, small nuclei fuse to make heavier nuclei. • The sun’s energy is released when hydrogen nuclei fuse to make helium. This occurs in other stars too. Energy
Learning Check Indicate if each of the following are • Fission (2) fusion • Nucleus splits • Large amounts of energy released • Small nuclei form larger nuclei • Hydrogen nuclei react Energy
Solution Indicate if each of the following are • Fission (2) fusion • 1 Nucleus splits • 1 + 2 Large amounts of energy released • 2 Small nuclei form larger nuclei • 2 Hydrogen nuclei react
