Ch. 25 Nuclear Chemistry Reactions involving the nucleus of the atom.
Radiation Radiation: penetrating rays and particles emitted by a radioactive source. Radioisotopes: radioactive isotopes Why is an atom radioactive? –The nucleus is unstable. Wrong ratio of p:n –Nuclear forces v. Electromagnetic Repulsion
Nuclear Stability Graph
Types of Radiation Alpha: Helium nucleus emitted from nucleus. Results in less p and n Beta: High speed electron ejected from the nucleus –n → p + e - Gamma: high energy photon
Summary of Radiation
How far will it go?
Alpha Decay
Beta Decay
Gamma Radiation
Gamma Ray on Electromagnetic Spectrum
25.2 Half life
Nuclear Stability Graph Red band=1:1 n:p ratio Blue band: stable n:p ratio for isotopes Falling outside this band =unstable
The n:p ratio determines the type of decay that will occur Too many neutrons: beta emission (n→p+e - ) Too few neutrons: positron emission (p→n) Too many p and n: alpha emission
Examples
Half-life A half-life (t 1/2 ) is the time required for one- half of the nuclei of a radioactive sample to decay to products After each half-life, half of the existing radioactive atoms have decayed into atoms of a new product
Half-lives can vary in length depending on the isotope. Some last nanoseconds, some last billions of years Decay of U-238:
What determines the type of decay a radioactive isotope will undergo? A radioactive sample has a half life of 4 day. How much of a 20 g sample will be left after 4 days? 8 days? 20 days? A radioactive sample has a half life of 5 hours. How much of a 150 g sample will be left after 25 hours?
Ch Fission v. Fusion
●Transmutation: the conversion of an atom of one element to an atom of another element ○Can occur by radioactive decay, or when particles bombard the nucleus of an atom ●Elements in the periodic table with atomic numbers above 92 are called transuranium elements. ○These all undergo transmutation ○All are man-made (not found naturally)
Fission vs. Fusion Splitting of large nuclei into smaller Releases lots of energy (10 6 times a chem rxn)(E=mc 2 ) Occurs in nuclear power plants Atomic bomb Combining smaller nuclei into larger ones Releases even more energy than fission (3-4 times greater) Occurs in sun/stars H-bomb, thermonuclear bomb
Fission reaction
Fusion
Nuclear power plant Nuclear power plants are sites of controlled nuclear chain reactions!
Nuclear Power Plant
Control Rods: Cadmium (Cd): absorb n 0 Moderator: water or graphite: slow n 0 Fuel: U-235 Nuclear Waste – Spent rods are stored in water tanks to cool and protect from radiation for many years.
Detection Film badge: photographic paper that gets exposed. Used only to monitor, not detect Geiger Counter: gas filled tube to detect radiation through audible clicks Scintillation Counter: phosphor-coated surface that produces flashes of light when exposed to radiation
Uses of Radiation Smoke detectors: Americium-241 Food irradiation: Cobalt-60 (gamma) Radioactive tracers to: –Study processes in plants (P-32) –Diagnose medical conditions Thyroid disorders: Iodine-131 Bone scans: Te-99 Medical treatment: cancer