The Nucleus and Radioactivity Radioactivity: Spontaneous changes in the nucleus that emit energy as radiation (particles or rays) Nuclei contain protons and neutrons; some combinations of these particles are unstable Examples of Radioactive Nuclei Include: Uranium, Plutonium Hydrogen-3 Potassium-38 Radioactive Decay: Emission of radiation produced by unstable nuclei changing to a more stable state Alpha rays α: positive charge Beta rays β: negative charge Gamma rays γ: no charge Types of Radiation Include: α and β rays consist of streams of particles γ rays consist of electromagnetic radiation α particle: contains 2 protons and 2 neutrons identical to helium nucleus travel only short distances β particle: electrons produced in the nucleus, then emitted γ Ray: High-energy ray similar to an X ray Travel great distances Daughter Nuclei: New nuclei that result from unstable nuclei undergoing radioactive decay Example: Uranium-238 gives up an α particle, resulting in a daughter nucleus of a different element, Thorium (Th) travel greater distances than α particles Summary of Radiation Types Alpha Decay When a radioactive nucleus emits an alpha particle, a new nucleus results. The mass number of the new nucleus is 4 less than that of the initial nucleus. The atomic number is decreased by 2. 1
In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers for the nuclei of the reactant and the products must be equal. 251 Cf 247 Cm + 4 He 98 96 2 Write an equation for the alpha decay of Rn-222. 222 Rn new nucleus + 4 He 86 2 Mass number: 222 4 = 218 Atomic number: 86 2 = 84 Symbol of element 84 = Po 222 Rn 218 Po + 4 He 86 84 2 Beta Decay The unstable nucleus converts a neutron into a proton (emitting an electron from the nucleus) The mass number of the new nucleus remains the same The atomic number of the new nucleus increases by 1 1 n 0 e + 1 H 0-1 1 Example: Potassium - 42 is a beta emitter. 42 K new nucleus + 0 e 19-1 Mass number : (same) = 42 Atomic number: 19 + 1 = 20 Symbol of element 20 = Ca 42 K 42 Ca + 0 e 19 20-1 2
Learning Check Solution Write the nuclear equation for the beta decay of Co-60. 60 Co 27 Write the nuclear equation for the beta decay of Co-60. 60 Co 60 Ni + 0 e 27 28 1 Gamma γ Radiation Summary of Radiation Gamma radiation is energy emitted from an unstable nucleus indicated by m. In a nuclear equation for gamma emission, the mass number and the atomic number are the same. 99m Tc 43 43 99 Tc + γ Some radioactive isotopes are more stable than others, and therefore decay more slowly Radioactive Decay Over Time Half-Life: Time required for half of the unstable nuclei in a sample to decay Example: A Potassium-38 sample weighs 100 grams. 8 minutes later, the sample is weighed again and found to weigh 50 g. The half-life of potassium-38 is 8 minutes 3
Half-Lives Vary Dramatically Between Elements Half-Life Calculations After one half-life, 40 mg of a radioisotope will decay to 20 mg. After two half-lives, 10 mg of radioisotope remain. Initial 40 mg 40 mg x 1 x 1 = 10 mg 2 2 1 half-life 2 half-lives 20 mg 10 mg 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 Half life = 13 hrs Number of half lives = 2 Amount remaining = 64 mg x 1 x 1 = 16 mg 2 2 13 hrs 13 hrs 64 mg 32 mg 16 mg Radiation and Health Free Radicals: Very reactive compounds that can cause mutations, cancer; usually caused by long-term exposure to lowlevel radiation Uses of Radioisotopes Medical: diagnosing and disease (cancer, thyroid, brain scans) Radiation Sickness: Illness and symptoms caused by shortterm exposure to intense radiation 4
Imaging Techniques Using Radioisotopes PET Scans (Positron Emission Tomography): gamma rays create a 3D image of organs, used to analyze blood flow, metabolic activity and brain function Health/Agriculture: food irradiation Radioactive dating: determine age of fossils CT (Computed Tomography): X-rays are used to create series of images of the brain, identifying brain damage and hemorrhaging MRI (Magnetic Resonance Imaging): H protons in magnetic field are used to create color images of soft tissue Nuclear Power Plants: Alternative energy source Units of Radiation Curie (Ci): number of disintegrations per second per gram of radium; 3.7 x 10 10 disintegrations per second Rad (Radiation Absorbed Dose): amount of material able to deliver 2.4x10-3 cal of energy to 1 kg of tissue Rem (Radiation Equivalent in humans): amount of biological damage caused by different types of radiation In 1934 Radioactivity was Artificially Induced for the first time!! High-energy particles (such as neutrons) can create unstable nuclei that then undergo radioactive decay (Cyclotrons and Linear Accelerators) 5
Nuclear Fission: Process in which large nuclei split into smaller nuclei when bombarded with neutrons, releasing large amounts of energy Example: When a neutron bombards U-235, an unstable nucleus of U-236 forms smaller nuclei such as Kr-91 and Ba-142. Chain Reaction: Nuclear reaction in which the products of a reaction cause that reaction to occur repeatedly Nuclear Fusion: Process in which small nuclei combine (fuse) to form larger nuclei Example: Hydrogen nuclei combine to form Helium nuclei 6