Nuclear Chemistry Chapter 25

Learning Objectives Students will understand The criteria for nuclear stability and the processes by which unstable nuclides decay Issues of safety and health with respect to radioactivity Uses of radioactive isotopes in science and medicine

Learning Objectives Students will be able to Write equations for the decay of radioactive elements and the synthesis of new isotopes Carry out calculations based on equations for the first-order rate of decomposition of unstable isotopes

25.1 Natural Radioactivity occurs when a nucleus undergoes decomposition to form a different nucleus and additional particles when a radioactive particle is stopped by a material, or when radiation is absorbed, the energy associated with the radiation is transferred to the material damage caused by radiation is related to the energy absorbed

Alpha Radiation An alpha particle, α particle, is a helium nucleus: 4He Least penetrating, can be stopped by several sheets of ordinary paper or clothing Heavy radioactive nuclides, isotopes, undergo decay to form alpha particles.

Practice Problem Write a balanced equation for the decay of uranium-235 by α-particle production.

Beta Radiation A beta particle, β particle, is an electron represented in a nuclear reaction by the symbol: 0-1e The beta particle has a mass number 0. Its mass is very small Can penetrate several millimeters of living bone or tissue, but about 0.5 cm of lead will stop the particles

Gamma-Ray Production A gamma ray, γ, is a high-energy photon often produced during nuclear decay. Gamma rays are unaffected by electric and magnetic fields. The most penetrating and can pass completely through the human body, thick layers of lead or concrete are required to shield the body A nucleus with excess energy can relax to its ground state by emitting a gamma ray: 235U  4He + 231Th + 0γ

25.2 Nuclear Reactions and Radioactive Decay Nuclear reactions occur when an isotope of one element changes into an isotope of a different element.

Practice Problem Six αand four βparticles are emitted in the thorium-232 radioactive decay series before a stable isotope is reached. The first three steps are α, β, βemission. What is the final product in this series?

Other Types of Decay A positron is an emitted particle which has the same mass as an electron but opposite charge. (antimatter to an electron) results in a decrease in the atomic number Electron capture occurs when a nucleus captures an inner orbital electron. 7Be + 0e  7Li

Practice Problems Write a balanced equation for the decay of carbon-14 by β-particle production. Write a balanced equation for the decay of bismuth-205 by positron production.

Alpha 4He, 4α +2 Beta 0-1e, 0-1β -1 Gamma γ Name Symbols Charge Mass (g/particle) Alpha 4He, 4α +2 6.65 x 10-24 Beta 0-1e, 0-1β -1 9.11 x 10-28 Gamma γ

25.4 Rates of Nuclear Decay Half Life is the time required for half of the sample to decay to products (independent of the amount of sample for first-order decomposition) t1/2 = 0.693/k

Practice Problem Tritium (3H), has a half-life of 12.3 years. Starting with 1.5 mg of this isotope, what mass (mg) remains after 49.2 years? How long will it take for a sample of tritium to decay to one eighth of its activity? Estimate the length of time necessary for the sample to decay to 1% of its original activity.

Practice Problem A sample of Ca3(PO4)2 containing phosphorus-32 has an activity of 3.35 x 103 dpm. Exactly 2 days later, the activity if 3.18 x 103 dpm. Calculate the half-life of phosphorus-32.

Homework After reading sections 25.1 – 25.2 & 25.4 you should be able to do the following… P. 1007a-b (1-15 odd, 29, 31)