28/05/13Structure of the atomStructure of the atomA hundred years ago people thoughtthat the atom looked like a “plumpudding” – a sphere of positivecharge with negatively chargedelectrons spread through it…I did an experiment that provedthis idea was wrong. I called itthe “Rutherford ScatteringExperiment”Ernest Rutherford, British scientist:
28/05/13The Rutherford Scattering ExperimentThe Rutherford Scattering ExperimentAlphaparticles(positivecharge)Thin goldfoilSome particles passedthrough, some weredeflected backwardsConclusion – atom is made up of a small centralnucleus surrounded by electrons orbiting in shells
28/05/13The structure of the atomThe structure of the atomELECTRON –negative, massnearly nothingPROTON –positive, samemass asneutron (“1”)NEUTRON –neutral, samemass as proton(“1”)
28/05/13The structure of the atomThe structure of the atomParticle Relative Mass Relative ChargeProton 1 1Neutron 1 0Electron 0 -1MASS NUMBER = number ofprotons + number of neutronsSYMBOLPROTON NUMBER = number ofprotons (obviously)
28/05/13RadioactivityRadioactivityIf a substance is capable of ALWAYS emitting radiation under anyconditions we say it is ____________. There are three types ofradiation: ALPHA, _____ and GAMMA. These types of radiation arealways given off by rocks, _____, building materials, air and cosmic raysaround us – this is called BACKGROUND RADIATION. Each type iscapable of penetrating different materials:αβγSheet ofpaperFew mm of_________Few cm ofleadWords – aluminium, beta, food, radioactive
28/05/13IsotopesIsotopesAn isotope is an atom with a different number of neutrons:Each isotope has 8 protons – if it didn’t then it justwouldn’t be oxygen any more.Notice that the mass number is different. How manyneutrons does each isotope have?A “radioisotope” is simply an isotope that is radioactive –e.g. carbon 14, which is used in carbon dating.
28/05/13Types of radiationTypes of radiation1) Alpha (α) – an atom decays into anew atom and emits an alpha particle(2 protons and 2 neutrons – thenucleus of a helium atom)2) Beta (β) – an atom decays into anew atom by changing a neutron intoa proton and electron. The fastmoving, high energy electron is calleda beta particle.3) Gamma – after α or β decaysurplus energy is sometimes emitted.This is called gamma radiation andhas a very high frequency with shortwavelength. The atom is notchanged.UnstablenucleusUnstablenucleusUnstablenucleusNewnucleusNewnucleusNewnucleusAlphaparticleBetaparticleGammaradiation
28/05/13IonisationIonisationWhen radiation collides with neutral atoms or molecules italters their structure by knocking off electrons. This willleave behind IONS – this is called IONISINGRADIATION.α particleElectron
28/05/13Uses of radioactivityUses of radioactivity1) Medical uses – gamma rays can be used to destroycancerous cells or to sterilise medical instruments2) Tracers – a tracer is a small amount of radioactive materialused to detect things, e.g. a leak in a pipe:GammasourceTracers can also be used to developbetter plant fertilisers and inmedicine to detect tumours:The radiation from the radioactive source is picked up abovethe ground, enabling the leak in the pipe to be detected.
28/05/13Uses of radioactivity 2Uses of radioactivity 2RollersBetaemitterBetadetectorPaper
28/05/13Dangers of radioactivityDangers of radioactivityOUTSIDE the body β and γ aremore dangerous as α radiationis blocked by the skin.INSIDE the body an α sourcecauses the most damagebecause it is the most ionising.AlphaBetaGammaRadiation will ionise atoms in livingcells – this can damage them andcause cancer or leukaemia.
28/05/13Half lifeHalf lifeThe decay of radioisotopes can be used to measure thematerial’s age. The HALF-LIFE of an atom is the timetaken for HALF of the radioisotopes in a sample to decay…At startthere are 16radioisotopesAfter 1 halflife half havedecayed(that’s 8)After 3 halflives another2 havedecayed (14altogether)After 2 halflives anotherhalf havedecayed (12altogether)= radioisotope = new atom formed
28/05/13A radioactive decay graphA radioactive decay graphTimeCount1 half life
28/05/13Dating materials using half-livesDating materials using half-livesQuestion: Uranium decays into lead. The half life of uranium is4,000,000,000 years. A sample of radioactive rock contains 7 times asmuch lead as it does uranium. Calculate the age of the sample.88Answer: The sample was originally completely uranium……of thesample wasuranium482818Now only 4/8 ofthe uraniumremains – theother 4/8 is leadNow only 2/8 ofuranium remains– the other 6/8is leadNow only 1/8 ofuranium remains– the other 7/8is leadSo it must have taken 3 half lives for the sample to decay until only 1/8remained (which means that there is 7 times as much lead). Each halflife is 4,000,000,000 years so the sample is 12,000,000,000 years old.1 half lifelater…1 half lifelater…1 half lifelater…
28/05/13An exam question…An exam question…(AQA 2001 Higher Paper)Potassium decays into argon. The half life of potassium is1.3 billion years. A sample of rock from Mars is found tocontain three argon atoms for every atom of potassium.How old is the rock?(3 marks)The rock must be 2 half lives old – 2.6 billion years
28/05/13Chain reactionsChain reactionsEach fission reaction releasesneutrons that are used infurther reactions.
28/05/13Fission reactions summaryFission reactions summaryEach fission reaction releases energy in the form of_______. In a nuclear power plant this heat is used to boil_______, which is used to drive turbines etc. The energyfrom each reaction is very ______, but there are________ of reactions every second. The waste productsfrom these reactions are __________, which is whynuclear power plants are ___________.Words – radioactive, water, billions,controversial, heat, small