Chapter19 Nuclear Chemistry Lecture Outline Common Student Misconceptions • This is new territory for students who have not taken advanced courses in physics. Radioactivity • When nuclei change spontaneously, emitting energy, they are said to be radioactive . • Nuclear chemistry is the study of nuclear reactions and their uses. • Nucleons are particles in the nucleus: • p + : proton • n 0 : neutron. • Mass number: the number of p + + n 0 . • The mass number is the total number of nucleons in the nucleus. • Atomic number: the number of p + . • Isotopes have the same number of p + but different numbers of n 0 • Different isotopes of the same element are distinguished by their mass numbers. • Different isotopes have different natural abundances. • A radionuclide is a radioactive nucleus. • Atoms containing these nuclei are called radioisotopes . Nuclear Equations • Most nuclei are stable. • Radionuclides are unstable and spontaneously emit particles and/or electromagnetic radiation. • Example: Uranium-238 is radioactive. • It emits alpha particles . • These are helium-4 particles. • When a nucleus spontaneously decomposes in this manner, we say it has decayed ( radioactive decay ). • In nuclear equations, the total number of nucleons is conserved: • We can represent the uranium-238 decay by the following nuclear equation: • The total number of protons and neutrons before a nuclear reaction must be the same as the total number of nucleons after reaction. Types of Radioactive Decay • There are three types of radiation which we will consider: • α -Radiation is the loss of ( alpha particles ) from the nucleus. • β -Radiation is the loss of an electron from the nucleus. • These high-speed electrons are called beta particles . • γ -Radiation is the loss of high-energy photons from the nucleus. • Nucleons can undergo two other types of decay: • Positron emission . • A positron is a particle with the same mass as an electron but an opposite sign. • Electron capture . • The nucleus captures an electron from the electron cloud surrounding the nucleus. • Representations: He Th+ U 4 2 234 90 238 92 → He 4 2
e p+ n 0 1 - 1 1 1 0 → e n+ p 0 1 1 0 1 1 → n e p 1 0 0 1 1 1 → + - α 4 2 • In nuclear chemistry to ensure conservation of nucleons we write all particles with their atomic and mass numbers: 2 4 He and represent α -radiation. • Nucleons can undergo decay: • ( β -emission) • (positron annihilation) • (positron or β + -emission) • (electron capture) Patterns of Nuclear Stability Neutron-to-Proton Ratio • The proton has high mass and high charge. • Therefore the proton-proton repulsion is large. • In the nucleus the protons are very close to each other. • The cohesive forces in the nucleus are called strong nuclear forces .