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Atomic Models and Radioactivity NCEA AS 2.5 Text Chapter: 14 History Greeks: 4 types of atoms, earth, air, fire, water Used these atoms to explain why things happened Eg stones fell to the earth because they were made of earth atoms Atomos = “indivisible” History Early 1800s John Dalton, an observer of weather and discoverer of color blindness among other things, came up with atomic theory All matter is made up of small indivisble particles known as “atoms” Atoms were solid spheres Drew the first molecular diagrams History J.J. Thompson (1856 -1940) Studied the “mysterious cathode rays In 1903 he proposed the “Plum pudding model” for the atom the atom is a sphere of positively charged matter with electrons embedded like the currents in a “plum pudding” Thompson’s Model History Ernest Rutherford Famous for his gold foil experiment Atom is mainly empty space Small dense positively charged nucleus Electrons orbiting the nucleus (This is the model you have to be able to explain for this achievement standard) Since Rutherford…. With the help of quantum theory that was being developed by Planck, Einstein and others, the model continued to evolve… Neils Bohr (1913): electrons occupy fixed energy levels (not fixed positions) Louis de Broglie (1924): electrons are waves Erwin Schrodinger (1925): electrons are matter waves whose position is based on a statistical probability (enter quantum mechanics) Chadwick (1935): Discovers the neutron. Rutherford’s Gold Foil Experiment He fired alpha particles at a very thin piece of gold foil and measured the angles they were scattered at. The Results Observation 1 Most passed right through the gold foil Explanation 1 Atoms are mostly empty space The Results Observation 2 Some were deflected Explanation 2 The atom contains a positive charge in its centre or nucleus that deflects alpha particles (which are positively charged) The Results Observation 3 A rare few bounced directly backwards Explanation 3 The positive charge must be small and densely packed so only a few alpha particles hit it directly head-on and bounce back Rutherford’s Model Positive nucleus surrounded by orbiting negatively charged electrons The Nucleus Writing nuclei X = element symbol A = mass number or nucleon number (the number of p+n) Z = atomic number (the number of protons) How many of each sub-atomic particle does potassium have? A Z X 39 K 19 Isotopes Atoms with the same atomic number but different mass numbers Eg: 12 1 6 1 C H 2 1 3 1 H (deuterium ) 13 6 H (tritium ) 14 6 C C Just Interesting In 1896 Henri Becquerel discovered by chance that a rock containing uranium when left on wrapped photographic paper was able to darken it. Pierre and Marie Curie, French chemists did most of the early work on radio active substances. They discovered radium which is far more active than uranium Radioactivity Radioactivity is a natural decay process Radioactivity is unaffected by external conditions like temperature Radioactivity involves the nucleus of the atom Some elements spontaneously emit particles or energy from their nucleus In a nuclear reaction atoms can change from one element to another. Nuclear Reactions 3 types: Radioactive Decay – the spontaneous emission of particles from the nucleus of an atom Nuclear Fission – splitting one large nuclei into two smaller ones Nuclear Fusion – combining two small nuclei into one large one. Radioactivity 3 types: Alpha a Beta b Gamma g Named in order of their discovery. Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them. The Nucleus In small atoms, the number of protons and neutrons are usually the same (roughly) In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation Alpha Particles Helium nucleus Charge of +2 Mass of 4 (a.m.u) Travel slowly ie. 10% of light speed Don’t travel very far ie. A few cms in air Low penetration power – can be stopped by a piece of paper Very good ionising power – because they’re big and slow. Beta Particles An electron from the nucleus Charge of -1 Same mass as an electron (effectively 0) Travel relatively fast – up to 95% of light speed Travel about 30 cms in air Average penetration power – can be stopped by a few mm of Aluminium Average ionising power Gamma Radiation A wave of electromagnetic radiation (energy) No charge No mass Travels at light speed Travels several metres in air High penetration power – Several cms of lead needed to stop it Low ionising power – because no mass Radiation One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field: b g a Balancing Nuclear Equations The atomic numbers are conserved. That means they have to add up to the same number on both sides of the equation. The same applies to the atomic masses. I Xe 131 53 131 54 0 ? 1 b Note: Both mass and charge must be conserved Try this 226 88 Ra Rn He g 222 86 4 2 (ie 226=222+4, 88=86+2) Another One Cobalt 60 decays to Nickel 60 emitting a a gamma ray and ? Co Ni e g 60 27 60 28 0 1 Again, mass and charge are conserved NB. the a or b symbols can be used instead of He or e Why have 2 different atoms got the same atomic mass? (type 1 question) Last one for now 1 0 n p e 1 1 0 1 That’s how the nucleus is able to emit electrons What is bigger a proton or neutron? (Type 1 question again) Think about it and don’t yell out Half-life The time it take for the decay rate to have halved, or…. The time taken for half of the original atoms to have decayed Usually shown on a graph Half-life Half Life 4500 4000 No. of Atoms 3500 3000 2500 2000 1500 1000 500 0 0 1 2 3 Time in days What ½ is being shown here? 4 5 Answer These The half-life of iodine-128 is 25 minutes. How long would it take the radioactivity of a sample to drop from……………. 3000 decays per second to 750 decays per second 2 half lives 50minutes 3000 to 375 decays per second 3 half lives 75 minutes Answer These The half life of thorium-234 is 24 days. Initially a sample contains 6.4 x 1020 atoms. How many thorium atoms will remain after 72 days? 8 x 1019 atoms How long will it take for 6.3 x 1020 thorium atoms to decay. 144 days Are studying for 2.7 yet????? The background count in S2 averages out at 6 min-1 . If a radio active source has a count of 246 min-1 and a week later 21 min-1 what is the ½ life? 24015 4 x ½ lives 42 hours A sample of atoms 16.5x109 has a 168 hour ½ life. How many decayed atoms are there 3 weeks later? 16.5x109 – (16.5x109 x ½ x ½ x ½) = 14.4 x 109 A radioactive sample has a ½ life of 3.6 days and 1.5 g of radioactive sample remain. How much was there 14.4 days ago? 14.4/3.6=4 So 4 half lives ago 1.5x2x2x2x2= ? 24g Detecting Radioactivity Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas Geiger-Muller tube filled with low pressure Ar End: thin mica window +Cathode: metal cylinder 400V DC Supply - Anode: central wire Counter or speaker Uses of Radioactivity Radiation therapy to treat cancer - g Power stations - 24a, -10b, g Sterilization - g Carbon dating - -10b Nuclear medicine eg tracers - -10b, g Smoke detectors - 24a Detecting leaks in metal pipes - g Blowing up your neighbors (not) Nuclear Fission 1 0 Breaking large unstable nuclei into smaller ones. Releases a lot of energy Lots of possible combinations of fragments from one initial nucleus Eg: n U Ba Kr 3 n 235 92 141 56 92 36 1 ? 0 n Nuclear Fission U Only one neutron is needed to start the reaction, but several are produced This starts a “chain reaction” Kr Ba n Kr Ba n n U U U n n n Kr Ba n n n Kr Ba n n n Nuclear Fission If the chain reaction is controlled it can be used in a nuclear reactor If it is uncontrolled it explodes as a nuclear or atomic bomb Nuclear Fusion The joining of two small nuclei to form one larger one Again, a lot of energy is produced This is the process that powers the sun Eg: 2 1 H H 3 1 4? 2 He n 1 0 Nuclear Fusion Fusion requires extreme temperature and pressure to occur, and has not practically and economically been used in power generation (yet….) Hydrogen bombs have been successfully made, but require a fission reaction to provide the necessary temp and pressure.