Chapter 22 Review:  Nuclear Chemistry

History
Just because it is interesting

Marie Curie

 Henri Becquerel (Physics professor, 1852 - 1908) discovered radioactivity in 1896 by placing a photographic plate near a box of uranium ore.  When the plate "developed" on its own, he knew the ore contained something giving off energy.  Marie Curie (1867 - 1934) and her husband Pierre worked with him, she processed the ore to discover the basics of radioactivity, and worked on this for the rest of her life. The discovery of radioactivity  inspired the Curies in their brilliant researches and analyses which led to the isolation of polonium, named after the country of Marie's birth, and radium. Madame Curie developed methods for the separation of radium from radioactive residues in sufficient quantities to allow for its characterization and the careful study of its properties, therapeutic properties in particular.  She also developed a portable X ray machine, roamed the battle fields of WWI, and X-rayed shot soldiers so that surgeons could locate the bullets. Becquerel was awarded half of the Nobel Prize for Physics in 1903, the other half being given to Pierre and Marie Curie for their study of the Becquerel radiation.  In 1911 Marie received a second Nobel Prize in recognition of her work in radioactivity.

 
  1.    Nuclear Reactions, section 22.1 
    1. We will look at two main types of reactions: 
      1. Radioactive Decay or Emission:  when an unstable atom emits a particle or energy.  This process is completely natural, humans can't control it, stop it, or slow it down.
      2. Transmutation:  when atoms are hit or bombarded by particles a reaction may occur.  These reactions occur naturally in space but not on Earth.  However, man made reactions can occur with the right equipment.
    2. Comparison of nuclear versus chemical reactions
      1. chemical:  breaking and forming bonds via electrons, nuclear:  changing one element into another via changes in the nucleus number of protons and neutrons
      2. chemical:  must start and end with the same number and type of atoms (ie start with 6 C atoms, must end with 6 C atoms), nuclear:  change elements (yes, we can create gold from lead)
      3. chemical:  DH is -1-1000 kJ, nuclear:  DH can be -1010 kJ
      4. chemical:  can affect the rate of reaction by changing temperature, pressure, concentration or adding a catalyst, nuclear:  can only affect the rate of reaction by changing concentration
    3. Nuclear particles involved in nuclear reactions  
      1. alpha particle = a = 42He = helium nucleus (2 protons and 2 neutrons, zero electrons) +2 charge, mass of 4 amu
      2. beta particle = b = 0-1e = an electron ejected from the nucleus, not to be confused with a normal electron orbiting the nucleus, -1 charge, mass of ~ 0 amu
      3. gamma ray = g = a high energy wave, actually not a particle, has no mass, just high energy rays, no charge, no mass
      4. position particle = b+ = 0+1e = a positively charged electron ejected from the nucleus, not to be confused with a normal electron orbiting the nucleus, +1 charge, mass of ~ 0 amu
      5. neutron = 10n = regular neutron, neutral, mass of 1 amu
      6. proton = 11p = 11H = normal proton, +1 charge, mass of 1 amu
    4. Radiation background discussion  
      1. Radiation is not all bad, in fact most radiation is harmless and necessary for life.  Radiation is simply energy in the form of waves or particles.  We are familiar with most types of radiation (sunlight, heat, radio and TV waves...) ALL waves travel at the same speed, the speed of light (c) which is 3.0 x 108 m/s.  So long radio waves(100 meters) and small X rays (picometers) all travel at the SAME speed.   c =  ln  is true for all radiation
      2. Types of radiation from high E (low l and n) to low E ( high l  and n ):  cosmic rays, gamma rays, X rays, UV rays, visible rays (visible light), IR rays (heat), micro rays (aka microwaves), FM radio, AM radio (TV, cell phone are in the radio region as well)  This order of all the radiation waves is termed the Electromagnetic Spectrum.  
      3. Alpha and Beta are also called radiation but they are particles with mass unlike the types listed in 2 above. 
      4. Many materials naturally emit some kind of radiation:  The sun emits all sorts of radiation (IR, UV and visible for starters), living things emit heat, rocks emit alpha and beta particles.  Human made devices found everywhere emit or accept radiation:  transmitters emit radio waves which our stereos and TV's receive, microwaves emit microwaves (imagine that), and on and on.
      5. If the radiation is high energy (alpha, beta, gamma) we term it radioactivity.  The materials that emit alpha and beta particles or gamma rays are termed radioactive.  Thus our radios are not radioactive but smoke detectors are (They consist of alpha emitters and a sensor. When smoke gets in the way, the alpha particle doesn't reach the sensor and the alarm goes off)  Note that alpha, beta and gamma radiation comes from the nucleus of a radioactive atom.
  2. Nuclear Reactions, section 22.2
    1. Radioactive Decay or Emissions Reactions:  Unstable atoms seek to change their number of protons or neutrons.  They can do this by high energy nuclear reactions.  Nuclear reactions actually change the identity of the atom by changing the number of protons (chemical reactions can never do this).        Table 22.1 and example 22.1, problems 22.1-2
    2. Beta emission (occurs when an atom has too many neutrons, or not enough protons).  In the nucleus a neutron splits into an electron and a proton (10n g  11p + 0-1e)  Since this electron originated from the nucleus, it is a beta particle.  Electrons/beta particles cannot live in the nucleus so they are ejected!  Example:  3215P g  0-1e + 3216S      The net effect is mass same, protons up one thus neutrons down one.  Example:  4220Ca   g  0-1e +  4221Sc
    3. Alpha emission (occurs when the nucleus is too large, typically elements larger than Bi)  The nucleus emits an alpha particle, or 2 protons and 2 neutrons.  Example:  23892U  g  42He  +  23490Th    The net effect is mass down 4 and protons down 2.  Example:   21084Po g  42He  +  20682Pb
    4. Positron emission (too many protons or not enough neutrons)  In the nucleus a proton splits into a positron and a neutron (11p  g  01e  +  10n)  The positron, like a beta particle, is ejected.  Example:  116C g  01e  +  115B     The net effect is mass same, protons down one thus neutrons up one.  Example:  3317Cl g  01e  +  3316S
    5. Gamma emission (occurs when a nucleus becomes highly energetic and unstable = metastable symbolized by m)  Example:  11m5B g  115B  +  00g   The effect is to release energy in the form of a gamma ray, mass same, protons same.  
    6. Electron capture (occurs when too many protons or not enough neutrons)  In the nucleus a proton captures a 1s electron to become a neutron (11p  +  0-1e  g 10n)  Example:  20884Po  +  0-1e  g  20883Bi     The net effect is mass same, protons down one thus neutrons up one.  Electron capture and positron emission have the same result.  Example:  116C + 0-1e  g   115B
    7. Balancing nuclear reactions 
      1. The mass numbers must be equal on both sides of the arrow
      2. The atomic number (charge or # of protons) must be equal on both sides of the arrow
      3. Example:  116C  +  0-1e   g  115B  (11 for mass on both sides total, 5 for charge on both sides total)
  3. Radioactive Decay Rates, section 22.3
    1. Atoms outside the "Band of Stability" will spontaneously emit/decay, which is called radioactivity.  (The 5 reactions above)  Man cannot stop or slow these natural decay reactions.  They have always existed, and always will. They are often multistep processes as the product atom may also be radioactive.  They it may take several reactions and billions of years for one atom to finally reach a stable isotopic form. 
    2. Terminology:  Parent is the reactant atom, daughter is the product atom.  The rate constant is called the decay constant. 
    3. All radioactive decay reactions are first order!  So recall these equations:  t1/2 - 0.693 / k  and  ln ( [A]t / [A]o) = -kt
    4. Example:  Protactinium-234, Pa, has a half life of 1.17 minutes.  How much of a sample would remain after 5.75 minutes?  Answer:  First find the decay or rate constant.  k = 0.693 / t1/2,   k = 0.693 / 1.17 min = 5.923 x 10-1 min-1.  Now use the first order integrated rate law.  ln (At / 100%) = -5.923 x 10-1 min-1 (5.75 min).  Solve for At = 3.32%.
    5. Now remember that the half life is the time it takes for 1/2 of a radioactive sample to decay. Table 22.2  If I have 100 grams, after one 1/2 life there is 50 grams, after a second 1/2 life there is 25 grams, after a third 1/2 life there is 12.5 grams and so forth.
    6. Example:  The half life for 90Sr is 29 years.  How much of a 500.0 gram sample remains after 87 years?  Answer:  87 years is 3 half lives so after one 1/2 life there is 250 g, after another 1/2 life there is 125 grams, after the third 1/2 life there is 62.5 grams. 
    7. t1/2 can not be changed.  Nothing we can do (temperature changes, pressure changes) has any affect on the half life.  We can not slow down or speed up the radioactive decay. 
    8. Here are some more half life values:  14C is 5730 years, 40K is 1280,000,000 years, 205Rn is 1.8 minutes, 222Rn is 3.82 days, 238U is 4500,000,000 years.
    9. Radioactive Dating 
      1. 14C is used to date anything that was living (bodies, plants, wood) accurately up to 60,000 years old.  Once something dies, no new carbon is incorporated into the body.  With time 12C remains but the 14C decays.  So the ratio of carbon-12 to carbon-14 (which is constant while alive) is no longer constant and changes with time.  So we can use this ratio to tell us the age since death.
      2. 238U is used to date rocks up to billions of years old.  Meteorites have been dated at 4.6 billion years of age.
      3. 40K is used to date minerals.
    10. Example 22.2-5, problems 22.3-8
  4. Stability, section 22.4
    1. Recall isotopes - atoms that have the same number of protons but different numbers of neutrons.  Most elements have 2 or more isotopes such as carbon which occurs as carbon-12 (12C), carbon-13 (13C), carbon-11 (11C) and carbon-14 (14C). 
    2. Some isotopes are unstable = radioactive.  Every element up to Bismuth, except Tc, has one or more stable isotopes, but several of them also have an unstable isotope.  Elements above Bismuth are all unstable (even thought it may take billions of years for some of them to decay).  There is a graph called the "Band of Stability" which plots the stable atoms by neutrons versus protons.  Figure 22.3
      1. Atoms up to calcium are stable in general when the protons = the neutrons.  Example oxygen-16, carbon-12, nitrogen-14. Some notable exceptions hydrogen -1, chlorine-35. 
      2. Atoms above calcium to bismuth are stable when there are more neutrons than protons.  So for these, if the neutrons are less than or equal to the protons, they are unstable.
      3. Atoms above Bismuth are all unstable no matter what.  They all need to lose protons and neutrons to make them smaller, they are just too darn fat. 
      4. Also in general, having an even number of protons and even number of neutrons is the most stable combination while having an odd number of protons and odd number of neutrons is the least stable combination. 
    3. Nuclear Binding Energy - E required to "break" up a nucleus.  Very high indeed.  Don't worry about calculations.
  5. Skip section 22.5
  6. Fission, section 22.6
    1. splitting of a large atom into smaller ones:  23592U + 10n  g  9038Sr  +  14354Xe  +  3 10n,  239Pu (plutonium) has a similar reaction
      1. Uranium is bombarded with neutrons and that initiates the reaction.  Since three more neutrons are products, they in turn hit other U atoms and continue the reaction.  This is a chain reaction and can result in an explosion if we have enough U atoms = critical mass. 
      2. The energy released by 200 grams of U is about 2 x 1013J.  Compare to 1 ton of coal which is about 5 x 107J.  It would take one million tons of coal to create the same energy of a handful of U. 
    2. Uncontrolled fission can be used in bombs = atomic bombs.  In WWII we dropped Little Boy and Fat Man on Hiroshima and Nagasaki - these were U and Pu bombs made with critical mass so they would result in a nuclear explosion. 
    3. Controlled fission can be used to generate electricity.  We control the rate by using Boron rods which can absorb neutrons to stop or slow the chain reaction. 
      1. Nuclear power plants use fission reactors to boil water which as steam turns turbines.  Check out diagrams in your text.
      2. 20% of US power is nuclear. We use old light water reactors in the US.  They are old and have problems:  the hot water put into lakes raises the lake temperature (thermal pollution) and can ruin the ecosystem of the lake, U waste pellets are still radioactive, the U ore must be enriched which requires expensive and dangerous processing, if the rods stick or the coolant is shut off meltdown can occur releasing radioactive material into the air = fallout.  Chernobyl had a meltdown - as of Sept 2005, 56 people have died from the Chernobyl accident.  This is terrible, but is the only nuclear power plant disaster in 50 years.  Don't forget how many people die each year from coal mines and plants.  Basically all our current energy plants have major downsides.  
      3. The worst case scenario with a nuclear power plant is core meltdown which results in a normal explosion like dynamite.  A nuclear explosion like a nuclear bomb is impossible at a power plant - they are nowhere near "critical mass" so no, a nuclear power plant can't explode like a nuclear bomb.  The problem is not the explosion, but the radioactive material being spread upon the wind to the environment. 
      4. Heavy water reactors - use D2O instead of H2O.  D is deuterium and is hydrogen with 1 proton and 1 neutron thus a mass of 2 so it is "heavy" compared to H.  Good - U ore does not need to be enriched.  This is a huge advantage.  Canada uses these types of nuclear fission power plants.
      5. Breeder reactors - names so because they actually produce fuel for nuclear power plants by adding other atoms to the U ore.  So the waste is actually useable fuel - this is a huge improvement.  Russia and France use these new designs.
    4. Nuclear Fusion  
      1. Fuse small atoms together into larger atoms:  2 21D   g  31H + 11H     Called thermonuclear reactions because require extremely high temperatures.  
      2. 98% of universe H and He 
      3. Stars are just giant fusion reactors:  Yellow mainstream stars like our sun fuse H into He, Red giants fuse He into Li, N, O, F, up to Fe
      4. H bomb same reaction as stars - H fusion
        1. We've already made fusion bombs since the goal IS to not have control. Never used in war yet but have run test explosions. 
        2. Uses LiD and ignition of the fusion reaction is done with a tiny fission reaction starter.
      5. If we could control H fusion we would solve all energy problems. 
        1. Good facts - H is cheap and plentiful, it is not toxic, the waste is Helium which is not toxic, no meltdown potential.
        2. Bad facts - We are not there technically yet, we can't control fusion yet, needs high temperatures, laser ignition is one possibility being researched.
  7. Transmutation, section 22.7
    1. Occur naturally in space, not on Earth however man has discovered how to do these reactions by hitting atoms with particles in a particle accelerator.
    2. Example:   105B + 10n  g 42a   + 73Li .  We hit Boron with neutrons and get alpha particles and Li.
    3. Example:   ?  + 11p   g 42a   +   126C   where ? = 157N.  
    4. Example 22.9, problem 22.15-16
  8. Uses and Biological Effects, sections 22.8 - 10  much of this info comes from a medical text.
    1. Uses besides bombs and power:  dating as mentioned earlier, medical scans for diagnosis, chemotherapy, and "tagging" isotopes to study reaction mechanisms (example:  14CO2 used to learn about photosynthesis reaction - follow the 14C "tag" during the reaction)
    2. Alpha radiation is stopped by the skin, thus usually not very harmful outside the body. Beta radiation is stopped by 1cm of lead, so can get inside the body and is worse.  Gamma radiation is stopped by 10cm of lead, so is considered the worst to be exposed to.  However, if alpha particles are swallowed or inhaled, they are in fact the most dangerous inside the body due to the +2 charge - they are powerful ionizers.  Alpha particles inside the body are more dangerous than gamma or beta particles.  
    3. Medical terms and measurements
      1. Radiation intensity is measured in curies (Ci) = 3.7 x 1010 counts per second or milli curies (mCi) = 3.7 x 107 counts per second or micro curies (mCi) = 3.7 x 104 counts per second.
      2. Intensity can also be measured in becquerels (Bq) = 1 count/sec when the intensity is low. 
      3. Roentgens (R) measure exposure to radiation, measures the energy delivered by the radiation
      4. Rads (rad = radiation absorbed dose) measure how much of the radiation is actually absorbed by the body.  
      5. Rems (roentgen equivalent man) measures radiation's effect of 1 R in humans for different types of radiation.  
    4. Average exposure to humans - yes this means YOU!
      1. natural sources:  cosmic rays, rocks, natural radiation inside body, Radon = 294 mrem per year
      2. artificial sources:  X rays in medicine, meds, consumer goods, nuclear power plants = 65 mrem per year (note the power plants are only 2 mrem)
      3. Radon is 200 mrem per year!  Radon is a noble gas yet all isotopes are radioactive.  Why is this a problem?  Radon being a gas is inhaled by us!  Its half life is 3.8 days.  Once inhaled, some is exhaled, but some decays into Po.  Po is a solid alpha emitter.  Since Po is a solid, it is not exhaled thus it just sits there in our lungs emitting alpha particles, which are most dangerous inside our body.  Causes lung cancer.  Accumulates in basement.  Problem in areas that have lots of basements (Canada, Colorado...)
      4. Levels of exposure
        1. 25 rem just noticeable in blood (rems, not mrems)
        2. 100 rem causes minor radiation sickness 
        3. 400 rem causes 50% chance of death
        4. 600 rem fatal  
        5. 50,000 rem needed to kill bacteria
        6. 1,000,000 rem needed to kill viruses (hardy aren't they?) 
        7. smaller but repeated doses over several years may cause cancer, especially leukemia
        8. sunlight exposure can cause skin cancer
    5. Medical Uses
      1. Diagnosis - radioactive isotopes are injected intravenously and detectors monitor where they go and accumulate.  Certain isotopes are more likely to go to some organ than others.  (Iodine collects in the thyroid)  Gamma emitters used most commonly because they can escape the body (they are not charged) and are used in small doses.
        1. 11C used in brain scans (PET = positron emission topography) to trace glucose - positron emitter
        2. 32P used to detect eye tumors
        3. 59Fe used to examine bone marrow
        4. 60Co used in cancer treatment
        5. 99mTc used to examine heart muscles, brain, liver, kidneys, bone marrow - gamma emitter
        6. 131I used to diagnosis thyroid problems - beta emitter
        7. 24Na used in saline solutions (NaCl(aq)) to follow in circulatory system - beta emitter
      2. Therapy
        1. destroys selected cells and tissues like cancer cells
        2. ionizing radiation (radiation that knocks off electrons producing cations) destroys cells
        3. used when cancer is spread out (not localized) thus surgery unlikely
        4. used after surgery to get any left over cancer cells
        5. used when cancer is moving (metastatic state) to other areas
        6. X rays, gamma rays, and proton beams are all focused on the suspected area to minimize the damage of normal cells
      3. CT scans (computer assisted tomography)
        1. X rays used, detected, and data compiled by computer to produce image
        2. used to detect brain tumors
        3. used to detect stroke damage
      4. MRI (magnetic resonance imaging)
        1. used on soft tissues that contain H atoms (water and fat)
        2. uses radio waves to reverse the spin of H atoms, each different H atom environment reverses at a different frequency so we can tell the different environments
      5. PET scan (positron emission topography)
        1. inject positron emitter (11C)
        2. positrons collide with electrons around other atoms and produce gamma rays
        3. detect the gamma rays and compile image
        4. show brain changes when we think (brain stimulated)