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Atomic Structure 1 The Atom and Isotopes http://commons.wikimedia.org/wiki/File:Stylised_Lithium_Atom.png 1 Can We “See” Atoms? http://www.aip.org/history/einstein/atoms.htm Scanning Tunnelling Microscope Image of Silicon Atoms (X 34,000,000) 2 Can We “See” Atoms? New Scientist 3 Changing Models of the Atom http://www.docstoc.com/docs/2197377/History-of-the-Atom-Timeline TOK: If we can’t see these particles, how do we know that atoms are really like this? 4 A Grade 9 Version of the Atom energy levels proton nucleus electron neutron What is the name of this atom? carbon-12 12 6 C What is incorrect about this representation? many things - sizes, colours, spacing + ? 5 Subatomic Particles approximate particle location charge proton nucleus +1 1 neutron nucleus 0 1 electron energy levels -1 1/2000 mass* * in atomic mass units (amu) 1 amu = 1.67 x 10-24 g 6 Terminology Related to Atomic Structure Define: atomic number (Z) = the number of protons in an atom • unique for each element mass number (A) = the number of protons PLUS neutrons in an atom • always a whole number • NOT the atomic mass on the Periodic Table Symbols: A Z X charge (ions only) 7 Terminology Related to Atomic Structure Types of “particles” ATOMS IONS = particles with the same number of protons (+) and electrons (-) = charged particles = formed when an atom loses or gains electrons + ions (cations) ISOTOPES – ions (anions) ... wait for a few slides ... 8 Terminology Related to Atomic Structure Deduce: number of protons = atomic number (Z) number of neutrons = mass number (A) - atomic number (Z) number of electrons for neutral atoms: # e = number of protons for cations (+ ions): # e = number of protons - charge for anions (- ions): # e = number of protons + charge 9 Terminology Related to Atomic Structure Give the number of protons, neutrons and electrons in: 32 P p = 15 n = 32 - 15 = 17 e = 15 I p = 53 n = 131- 53 = 78 e = 53 131 18 O 2- p=8 27 Al 3+ p = 13 carbon-14 p=6 n = 18 - 8 = 10 e = 8 + 2 = 10 n = 27 - 13 = 14 n = 14 - 6 = 8 e = 13 - 3 = 10 e=6 10 Isotopes Define: Isotopes are atoms of the same element with different numbers of neutrons. Isotopes are atoms with the same atomic number but different mass numbers. http://education.jlab.org/glossary/isotope.html 1 1 H 2 1 H 3 1 H 11 Isotopes - Properties chemical properties • properties related to how a chemical reacts with other chemicals • examples = flammability, reactivity, pH • isotopes of an element have the SAME chemical properties example - use of iodine supplements The thyroid gland regulates your metabolism using a hormone called thyroxin which contains iodine. Radioactive iodine isotopes are waste products from the decay of uranium. If you ingest radioactive iodine, it accumulates in your thyroid gland when cells there produce and store thyroxine. This is an example of how both the radioactive and non-radioactive isotopes of iodine have the same chemical properties - they react in the same way. Over time, the build up of thyroxine with radioactive iodine atoms can lead to thyroid cancer. In order to prevent the accumulation of radioactive iodine in the thyroid, people may take iodine supplements which contain non-radioactive iodine. If there is more non-radioactive iodine available, then more of it will be used to make thyroxin. 12 Isotopes - Properties physical properties • properties related to how a chemical looks, feels, tastes • examples = boiling and melting point, density, solubility • isotopes of an element have DIFFERENT physical properties example - uranium enrichment These are fuel rods in a nuclear reactor and they contain radioactive uranium-235. However, natural uranium contains more than 99% non-radioactive 238U and only 0.71% radioactive 235U. Uranium-235 is needed for the fission reactions that generate energy in nuclear power plants. In order to increase the amount of 235U, a process of isotope separation is used. Chemicals with different physical properties can be separated from each other. One physical property that can be used to separate the isotopes of an element is density. A process called centrifugation can be used to separate chemicals with different densities. Uranium enrichment processes take advantage of the different densities of 235U and 238U to increase the amount of radioactive 235U in a sample - up to 3-4% in reactor grade uranium and up to 90% in weapons-grade uranium. http://www.world-nuclear.org/info/inf28.html 13 Radioisotopes Radioisotopes are isotopes that have “unstable” nuclei Unstable nuclei emit “radiation” as they try to become stable. 14C (unstable): 6p 8n 14N (stable): 7p 7n beta radiation http://www.windows2universe.org/physical_science/physics/atom_particle/radioactive_decay.html In this example of a nuclear reaction, a neutron is converted to a proton, with the release of beta radiation. There are many other types of nuclear reactions. 14 Types of Radiation “Radiation” refers to certain particles or energy released when an unstable nucleus decays. 1. alpha radiation • helium nuclei • symbol = 4 2 He • low energy, but particles have mass 1I. beta radiation • high speed electrons • symbol = 0-1 e • high energy, but particles have little mass 1II. gamma radiation • a form of electromagnetic radiation • symbol = • very high energy, but no particles 15 Types of Radiation penetrating power of different forms of radiation Different types of radiation have different “penetrating powers”, depending on their mass and energy. Alpha particles (helium nuclei) are large and slow, and can blocked by paper or skin. Beta particles (high speed electrons) are small but fast, and are blocked by aluminum sheets. Gamma rays are not particles and have high energy, and are only blocked by lead sheets. 16 Uses of Radioisotopes general uses: energy production sterilization of surgical equipment food preservation detect cracks in structures smoke detectors medical diagnosis and treatment 17 Uses of Radioisotopes carbon-14: radioactive dating The amount of carbon-14 in a living organism is constant. But when it dies, the levels of carbon-14 decrease as the radioisotope decays. This decay occurs at a regular rate, known as the half-life of the radioisotope. The half life of carbon-14 is 5730 years. The amount of carbon-12, however, is the same in both living and dead matter. By measuring the ratio of carbon-12 to carbon-14 in a specimen, scientists can calculate the age of the specimen. http://commons.wikimedia.org/wiki/File:Guanajuato_mummy_02.jpg cobalt-60 : cancer radiation therapy Cobalt-60 releases gamma radiation, which is very effective at destroying the fast dividing cells in cancerous tumours. Normal cells may also be affected, but they are able to recover if treatment is carefully controlled. It has been used for over 50 years to treat a variety of types of cancers, including skin, larynx, brain, breast and uterine cancers, as well as leukemia (a cancer of the blood). http://www.medscape.com/viewarticle/580420_3 18 Uses of Radioisotopes iodine-131: medical diagnosis/tracer Radioisotopes have the same chemical properties as other atoms of the same element. This means they will be used by cells to make compounds in the same way as non-radioactive isotopes. However, the radioactive isotopes are easily detected and this makes them useful as a “medical tracer”. They can be used to track the movement or accumulation of a particular chemical in the body, as shown in the photo to the right. This is the result of a test for thyroid cancer, where radioactive sodium iodide is injected into the patient. The dark areas show the presence of thyroid tumours. This isotope has a short halflife of 8 days, so the small amount used in the test be cleared from the body quickly. Iodine-131 is also used to treat thyroid cancers. http://www.medscape.com/viewarticle/580420_3 iodine-125: cancer radiation therapy Iodine-125 has a longer half-life (80 days) than iodine-131. It is used in the treatment of prostate and brain cancer. 19