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Nuclear Chemistry - HSC Chemistry
Transcript of Nuclear Chemistry - HSC Chemistry
BUT different number of neutrons = different mass number. Isotopes of Helium Most elements exist as a mixture of 2 or more isotopes. The atomic WEIGHT is an average of these Write the 4 isotopes of helium in the format above. Radioactivity 1896 - Becquerel discovered that certain minerals containing URANIUM were emitting invisible radiation = RADIOACTIVE. spontaneous emissions of radiation by UNSTABLE nuclei of certain elements. (ie radioisotopes) 3 different types of radiation: Alpha Radiation A stream of HELIUM nuclei (2 protons and 2 neutrons).
Low penetrating power (will not pass through a sheet of paper.
Slow - 5% of speed of light.
All elements greater than 83. Beta Radiation A stream of ELECTRONS..... from the nucleus!?!
A NEUTRON decomposes to into a proton and an electron:
n p + e
moderate penetrating power (will pass through a sheet of paper, 0.5mm of Al but not 0.5mm Pb
moves up to 99% speed of light 1 0 1 1 -1 0 Gamma Radiation high energy electromagnetic radiation.
Very high penetrating power (is only stopped by 5cm of Pb or 15cm of concrete.
Is ONLY emitted with alpha or beta radiation The PROTON: NEUTRON ratio The nucleus can only be stable if the proton-neutron ratio is within certain, very narrow limits.
The graph shows all the know isotopes of all the elements. Example: If these isotopes of carbon were plotted in the graph, where would they lie? STABLE Carbon-14 lies just above the 'Line of Stability' inside the dotted line zone.
The nucleus is UNSTABLE because it has too many neutrons in its neutron:proton ratio.
TO ACHIEVE STABILITY, IT UNDERGOES RADIOACTIVE DECAY.
Carbon-14 emits a BETA particle and GAMMA energy and transmutes into a stable nitrogen atom. In Summary: An isotope is unstable:
If its atomic number is greater than 83.
If its ratio of neutrons to protons places it outside the zone of stability. We describe a nuclear reaction by writing a NUCLEAR EQUATION.
Write the atomic number as a subscript on the left of the symbol and the mass number as a superscript. THESE 2 NUMBERS MUST BALANCE AS THEY REPRESENT THE NUMBER OF PROTONS AND NEUTRONS Example: The radioisotope Uranium-238 undergoes radioactive decay, emitting alpha radiation. After it has lost a helium atom, Thorium is left: 238 92 4 2 234 90 U Th + He Example: Colbalt-60 is a beta emitter. Remember one of its neutrons has transformed into a proton and an electron. Co Ni + e 60 27 60 28 0 -1 This emission is accompanied by the emission of gamma rays. In ray treatment of cancer patients it is these gamma rays which are used to attack the cancer These were both examples of radioactive disintegration as they both involved the splitting off of a small fragment (beta or alpha particles) from certain nuclei. It is the basis of both atomic bombs and nuclear power plants as the amount of energy released per gram of uranium is enormous! Atomic Bomb Nuclear Reaction
= controlled nuclear fission Scientist have been able to produce about 23 transuranic elements with atomic numbers up to 118 - they are so unstable their existence is fleeting. How are they made?? NEUTRON BOMBARDMENT!
in a nuclear reactor
Some elements such as Uranium-238 are not fissile (does not undergo fission), instead when it it hit by a neutron a new element is formed: Particle Accelerators
-atomic nuclei are accelerated up to very high speeds by powerful magnetic fields, and then collided so that 2 nuclei fuse together: Lead Nucleus + Calcium Nucleus Nobelium nucleus Pb + Ca No 206 82 48 20 254 102 Linear Accelerators Cyclotrons Recent Discoveries
2. In 2009, New Scientitst magazine reported confirmation of the production of element 118. Atoms of this element may have been produced in 1999, but it took 10 years to repeat and confirm. 1. In 2004, a few atoms of elements 115 (Ununpentium) were APPARENTLY made by the reaction: Am + Ca Uup + 4 n 243 95 48 20 287 115 1 0 Bombardment with charged particles such as helium nucleus Half-Life of a radioisotope is the time required for half the atoms in a given sample to undergo radioactive decay. Example:
The half-life of iodine-131 is 8 days. If we start with 1g of iodine-131, then after 8 days we will only have 0.5g left (0.5g will have decayed to xenon). After a further 8 days 0.25g will have decayed leaving 0.25g, and so on. Commercial Radioisotopes Stats!
50 naturally occurring radioisotope
2000 radioisotpes have been reacted
most synthetic radioisotopes have short 1/2 lives (uses)
20 radioisotopes have found widespread uses in medicine and industry Production Place the target atoms inside a nuclear reactor and allow neutron bombardment to occur. ANSTO Small nuclear reactor in Lucas Heights. Used for research and to prepare useful radioisotopes for medical and industrial use Cobalt - 60 Co + n Co 59 27 1 0 60 27 Cobalt-60 is UNSTABLE and undergoes Beta Decay: Production: Co Ni + e + gamma rays 60 27 60 28 0 -1 Its these GAMMA rays that makes cobalt-60 so useful. By enclosing the radioactive material inside a heavily shielded container with a narrow aperture, a narrow beam of gamma rays can be produced. Industrial Uses Medical Uses Industrial Production of STERILE medical supplies (bandages and dressings) - the gamma radiation destroys bacterial cells and fungi spores. Aircraft manufacture - it is essential that welded parts are strong and free of defects. Gamma rays are used to 'see' inside joints using a gamma-ray camera. Medical Cancer treatment - the gamma rays are beamed into the tumor, destroying it. Iridium-192 (pg 86)
Technetium-99m (pg 86) Why use
Cobalt-60? Has a 1/2 life of 5.3 years - ie 6-10 years before it needs to be replaced.
The energy of gamma rays emitted is ideal for imaging welded metal joints and killing living cells.
Relatively CHEAP and easy to prepare. Transuranic Elements Song http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/radioa7.swf http://www.splung.com/content/sid/5/page/radioactivity Some half lives are less then a second in lenghts were as others can be billions of years! Radioactive Decay Series Often the radioactive nucleus of heavier elements can not become stable through a single emission. The product it forms becomes unstable and undergoes further decay. Detecting Radioactivity Photographic Film Photographic film DARKENS when exposed to radiation. The degree of darkening increases with exposure and intensity.
Today people working with radiation where photographic badges to monitor the level of exposure. http://www.darvill.clara.net/nucrad/detect.htm#film http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/radiation/radioactiverev5.shtml The Geiger Counter Ionisation counter It consists of a metal tube filled with argon gas. It is based on the fact that radiation causes the matter to be ionised, forming positive ions and free electrons. These are attracted to electrodes that conduct a current to a recording device. Scintillation Counter Use the fact that some substances such as zinc sulfide produce a flash of light (scintillation) when struck by ionising radiation because the electrons are excited. The flashes of light are counted electronically to measure the amount of radiation. Benefits and Problems of Radioisotopes Read and highlight the information given Read pg 89-90 of your text book and add anything extra to your notes