NUCLEAR CHEMISTRY Studying Radioactivity
Classroom Notes History of Radiation (not testable but fun )
Introduction to Nuclear Chemistry • Wilhelm Conrad Roentgen studied cathode ray tubes and the light they produced in 1895. • While researching, he had a vial of barium platinocyanide crystals by his desk which fluoresced near the cathode ray tube. • When he placed the crystals in a box, away from the cathode ray tube, the still emitted light.
Discovery of x-Rays Roentgen asked his wife to place her hand on a photographic plate and exposed it to rays from the tube. When he developed the plate he saw the shadows of her bones and ring she was wearing. After some more experiments Roentgen named the rays “x-rays”.
X-rays • Within 3 months DIY x-ray machines were marketed and the rich and famous were constantly photographing their bones with jewelry on.
X-rays • Soon x-rays were a piece of art. In this photo a shoemaker placed his foot in the machine to see what it looked like and came up with an idea:
X-rays • The shoe fitting machine was invented in the late 1940s but banned in the 1970s because of the leakage of radiation to the environment.
X-Rays • The danger of x-rays were later understood when people became over exposed to x-rays as shown here.
Radioactive minerals • Antoine Henry Becquerel discovered the radioactivity of Uranium in the early 1900s.
Radioactive Minerals • After many women who worked in the watch factory became ill it was apparent that they had something in common. • They were painting with radium, which Marie Curie and her husband discovered in 1898.
Radiation • Since then there have been many developments made with radiation including nuclear power, nuclear medicine, warfare, etc.
History of radioactivity • https://www.youtube.com/watch?v=LDaZZnQCJw4
Video 13.1 Radioactive decay
Review: • Isotopes of the same element have the same number of _________ but differ in their number of ___________, which results in different __________. • Write the isotopic notation for C-12 and C-14. Explain their similarities and differences. • What does “radioactive” mean? • Which elements on the periodic table have no stable isotopes? protons neutrons mass 126C and 146C same element and protons but different mass and neutrons Decays/unstable Uranium and later.
Atomic Notation: Subtract atomic number from mass number to find the ________________ Number of neutrons
Radioactive Decay • The process by which nuclei emit particles and change into new elements. • This is the ONLY unit where elements can change into new elements ONLY be cause the number of protons are changing. • The rule still holds: You cannot change an elements number of protons with out changing its identity. • The reactions look different than normal.
Radioactivity • Sometimes an atom is unstable and will decay into a new atom. This type of atom is radioactive. • This is due to the proton-neutron ratio. The band of stability refers to atoms that are stable due to stable proton-neutron ratios.
I Xe 131 54 131 53 0 −1 0 −1 0 −1 e e + or Radioactivity • Atoms above the belt have too many neutrons and beta decay due to this. • The beta particle is an electron created when a neutron decays.
e 11 6 C B 0 1 11 5 + Radioactivity • Atoms below this belt have too many protons and positron decay. • The positron is the opposite of a beta particle.
Th He 234 90 4 2 238 92 U + Alpha decay • Atoms with 82 or more protons alpha decay. • Alpha particles are weak due to their mass. • Alpha particles are the helium nuclei. • What is the difference between alpha and helium particles?
0 0 Gamma decay • Strongest particle. • Accompanies most decay. • Usually not written due to the fact that it cannot change the mass or charge of any of the species.
Radiation is charged - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + alpha Radioactive source gamma beta
Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Detecting radioactivity • Particles such as ions are detected through interactions w/atoms or molecules using a film-badge • Photographic film in a small case, pinned to a person’s clothing which monitors exposure • Greater exposure of film greater exposure to radioactivity
Half Life • Time required for half of the sample to decay into a new, more stable element. • This can be used to date old artifacts such as rocks and things that were once living. • A long half life means that the substance stays relatively unchanged for a long period of time and is somewhat stable. • Short half lives are unstable elements, emitting particles at a regular basis.
Calculating Half Life • After one half life 50% or ½ the radioactive element is still present. • After two half lives 25% or 1/4 the radioactive element is still present. • After three half lives 12.5% or 1/8 the radioactive element is still present. • This continues forever, the number will never be zero. • The half life time is constant and nothing can change it. The half lives are listed on Table N.
1. The half life of Rn-222 is 3.8 days. If a basement holds 20 grams of Rn, how much will remain after 19 days? • Determine the number of half lives that have elapsed by dividing the total time by the half life time. • 19/3.8 = 5 half lives • Cut the original amount of the element in half the amount of half lives you calculated. • 20/2/2/2/2/2 = 0.625 grams
2. A cylinder contains 5L of Ne-19. It sits out for 103.2 seconds. What fraction of the sample is still remaining after is sat out? • Determine the number of half lives that have elapsed by dividing the total time by the half life time. (Look up the half life on Table N.) • 103.2/17.2 = 6 half lives • When starting you had a full sample. Start from 1 and cut in half the amount of half lives you calculated. • 1 ½ ¼ 1/8 1/16 1/32 1/64
3. The half like of Tc-99 (used in brain tumors) is 6 hours. If 10 micrograms are left for 24 hours, how much was administered to the patient originally? • Determine the number of half lives that have elapsed by dividing the total times by the half life time. • 24/6 = 4 half lives • Start with the ending mass and instead of halving, you double your amount the number of half lives you calculated, since we are going backwards! • 10*2*2*2*2 = 160 micrograms
4. A laboratory 400 grams sample of P-32 triggers 400 clicks per minute of the Geiger counter. How many days will it take the P to decay to only 50 grams and 50 clicks? • Find the number of half lives that have elapsed by dividing the original mass in half until it hits your final mass. • 400/2 = 200/2 = 100/2 = 50 3HL • Look up the half live on table N and multiple that time by the number of half lives you calculated. • 14.3 days * 3 = 42.9 days • Be sure to always include units in these answers!!!
5. An ancient scroll was discovered with 25% of the original C-14. How old is the scroll? • Find the number of half lives that have elapsed by dividing the total 100% in half until you get to the final percent. • 100/2 = 50/2 = 25 2 HL • Use table N to determine the half life of your element and multiple by the number of half lives you have calculated. • 5730 years * 2 = 11460 years
6. A radioactive sample is placed near a Geiger counter and monitored. In 20 hours the reading went from 500 counts to 125 counts per minute. How long is the half life? • Find the number of half lives that have elapsed by dividing the original counts in half until it hits your final count. • 500/2 = 250/2 = 125 3 HL • Divide the time elapsed by the number of half lives you calculated. • 20 hours/3 = 6.666Hours per half life.
7. A sample is placed near a Geiger counter and after 40 days it is re-massed and 25% remains. How long is the isotopes half life? • Find the number of half lives that elapsed by dividing the total 100% in half until you get to your final percent. • 100/2 = 50/2 = 25 2HL • Divide the time elapsed by the number of half lives you have calculated • 40 days/2 = 20 day half life
Video 13.3 Fission and Fusion
Transmutation • So far we have studied natural transmutation, which have reactions that have one reactant breaking down. All “decay” is natural. • Artificial transmutation involves a high speed particle bombarding the nucleus. This occurs in particle accelerators.
Ba Kr Th 91 36 He 234 90 4 2 142 56 235 92 U 238 92 U + + Transmutation • Natural and artificial transmutation both release energy by breaking down larger nuclei. • Natural transmutation begins with one reactant: • Artificial transmutation means a reactant is shot with a smaller particle such as a proton, neutron or electron. Artificial transmutation involves two reactants: n n 1 0 1 0 + 3 +
Fission A neutron is shot at a radioactive source which splits producing energy.
Fission This creates a chain reaction that must be controlled! The only way it will stop is if the radioactive source dies out. Therefore, many reactors only use a critical mass, which is the mass needed for a specific amount of energy made.
Fission Reactors The reaction’s energy is converted to steam which turns and turbine system, creating electrical energy from nuclear energy.
Fission Reactors Fuel rods contain the fissionable radioactive source. Control rods can regulate the neutrons absorbed. Cooling Fluid acts as a moderator, slowing neutrons down.
Nuclear power • In America, about 20% electricity generated by nuclear fission. Could it be More??????? • Yes please! So much cheaper than fossil fuels we currently use. But there may be risks to assess….
Fusion • Fusion is a better reaction to use since it harvests more energy, with less waste and less danger (no chain reactions). However, fusion is too expensive. • Tokamakapparati like the one shown at the right show promise for carrying out these reactions. They use magnetic fields to heat the material.