UNIT 9: Nuclear Chemistry

Nuclear Chemistry Natural Radioactivity -- the changing, or decay, of an atom’s nucleus -- accompanied by the release of particles and energy -- changes one element into another (transmutation) -- occurs in some isotope of every element -- occurs in all atoms of elements number 83 and above

Types of Emissions (Table O) 1. Alpha particle: ( α ) mass = 4 u charge = +2 (2 neutrons, 2 protons & no electrons) 4He nucleus 2 Example: 238U  4He + 234Th 92 2 90 -- alpha radiation can be stopped by a few sheets of paper 2. Beta particle: ( β) 0e mass = near 0 charge = -1 -1 -- A high speed electron Notice top numbers and bottom numbers balance! Example: 14C  0e + 14N 6 -1 7 -- beta radiation results from a neutron disintegration 1n  1p + 0e 1 -1 -- beta radiation can be stopped by wood or metal

See Table N 0e 11B 3. Gamma ray: ( γ ) mass = 0 charge = 0 -- high energy electromagnetic radiation -- no change to nucleus -- stopped by lead, thick steel, or high-density concrete 4. Positron: ( β + ) 0e mass = near 0 charge = +1 +1 Example: 11C  0e + 11B 6 +1 5 -- a positron is the antimatter particle to an electron The proton : neutron ratio in the nucleus determines the decay mode too many neutrons beta too many protons positron very large nuclei alpha See Table N

Emissions can be separated by: 1. Testing their penetrating power 2. Seeing how they respond to an electric field

Every radioisotope follows a decay series, until it becomes a stable (non-radioactive) atom Example: U-238 to Pb-206

Half - Life the amount of time it takes for half the atoms of a radioisotope to decay U-238  4.5 billion years Po-214  0.00015 seconds See Reference Table N Example: if you have 100 grams of Cs-137: after 30.2 yr, 50 grams remain unchanged after 60.4 yr, 25 grams remain after 90.6 yr, 12.5 grams remain

Example: A company has 96 grams of Sr-90, and wants to dump it into the river. Legally they can only dump 3 grams. How long must they wait until only 3 grams are left? Solution: Half-life Sr-90 = 29.1 yrs Time Amount Left 0 ....................96 g 29.1 yr .............48 g 58.2 yr .............24 g 87.3 yr..............12 g 116.4 yr...............6 g 145.5 yr ..............3 g

The amount of C-14 is constant (formation = decay) The C-14 enters living things When an organism dies, it no longer takes in C-14, and the existing C-14 decays ... half-life = 5715 years So, after 5715 years, bones have half as much C-14 as living things After about 10 half-lives, too little C-14 is left to measure The changing of U-238 to Pb-206 (half-life 4.5 billion years) can be used to find the age of rocks 5715 22,860

Biological Effects of Radiation In low doses, radiation primarily affects the DNA of living cells -- it can change them into cancer cells The effect is greatest on dividing cells: 1. Blood cells -- leukemia; anemia 2. Skin cells -- skin cancer 3. Sex cells -- } birth defects 4. Unborn babies -- 5. Cancer cells -- radiation therapy attempts to destroy cancer cells without harming healthy cells Co-60 is used in cancer treatment I-131 is used to treat thyroid disorders

Radioisotopes can also be used: -- as tracers to study chemical or biological reactions -- to kill bacteria and sterilize food and other items Look for this symbol: -- to measure the thickness of various materials

Radioisotopes can be made artificially in an accelerator Accelerators use magnetic or electric fields to speed up a charged particle, and steer it into a collision with another atom -- 4He + 27Al  30P + 1n 2 13 15 0 alpha ordinary radioisotope neutron particle aluminum of phosphorus atom -- causing an artificial transmutation (artificial means people did it) Example: A scientist uses a particle accelerator to collide an alpha particle with a nitrogen-14 atom. A proton is produced, along with one other atom. What’s the other atom?

Nuclear Fission -- is the breaking of a large nucleus into pieces -- accompanied by the release of lots of energy -- occurs when a fissionable nucleus, usually U-235 or Pu-239 is hit by a particle such as a neutron

Top numbers and bottom numbers always balance: Protons: 92 = 56 + 36 Mass: 1 + 235 = 141 + 92 + 3

The neutrons released may be absorbed by another material, or they may strike another fissionable nucleus If enough other fissionable nuclei are present (a critical mass), the process continues -- a chain reaction If the mass of fissionable nuclei is less (subcritical), the reaction stops If the mass is very large (supercritical), a nuclear explosion results

Natural uranium is 0.7% U-235; not enough for a chain reaction It must be enriched to reach a critical mass mass of fuel is supercritical 95% U-235 mass of fuel is critical 3% U-235

Electricity from Nuclear Fission -- Nuclear Power A fission reaction generates heat, --which turns water to steam --which generates electricity as it spins a turbine

Advantages Disadvantages The Reactor Core consists of: --Fuel rods (Uranium, enriched for U-235; or Plutonium) --Control rods (Boron or Cadmium) absorb neutrons to stop or slow the chain reaction Advantages Disadvantages No air pollution problems (acid rain, global warming, etc) Chance of radiation release accident Does not use up fossil fuels— less dependence on foreign oil sources Used fuel (nuclear waste) remains dangerous for years May allow spread of nuclear weapons 3. Safer (?) 4. Cost (?) 4. Cost (?)

Nuclear Fusion 2H + 3H  4He + 1n --the combination of small nuclei to form larger atoms Example: 2H + 3H  4He + 1n 1 1 2 0 Deuterium + Tritium Two isotopes of Hydrogen -- releases even more energy than fission ( E = mc2 ) -- provides the energy for the sun and other stars Fusion could potentially serve as a power source: -- uses hydrogen for fuel ( from water) -- produces almost no radioactive wastes

BUT -- Fusion requires very high temperatures and pressures to overcome the repulsion between nuclei ( + and + ) -- about 10,000,000 ºC Producing these temperatures and containing the hot gases are problems yet to be solved