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 (t1/2) 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. 25.3
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 (106 times a chem rxn)(E=mc2)
• 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
Nuclear Power Plant
• Control Rods: Cadmium (Cd): absorb n0
• Moderator: water or graphite: slow n0
• 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