3The structure of the atom 11/04/201711/04/2017ELECTRON – negative, mass nearly nothingThe nucleus is around 10,000 times smaller then the atom!PROTON – positive, same mass as neutron (“1”)NEUTRON – neutral, same mass as proton (“1”)Atoms always have the same number of protons and electrons so they are neutral overall. They can gain or lose electrons to form ions.
4Structure of the atom11/04/201711/04/2017A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…Ernest Rutherford, British scientist:I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment”
5The Rutherford Scattering Experiment 11/04/201711/04/2017Alpha particles (positive charge, part of helium atom)Thin gold foilMost particles passed through, 1/8000 were deflected by more than 900Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.
6The structure of the atom 11/04/201711/04/2017ParticleRelative MassRelative ChargeProton1+1NeutronElectron1/2000 (i.e. 0)-1MASS NUMBER = number of protons + number of neutronsHe24SYMBOLPROTON NUMBER = number of protons (obviously)
7Mass and atomic number revision 11/04/201711/04/2017How many protons, neutrons and electrons?11116HBO1582335238NaClU111792
8Isotopes11/04/201711/04/2017An isotope is an atom with a different number of neutrons:Notice that the mass number is different. How many neutrons does each isotope have?O816O817O818Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.
10Introduction to Radioactivity 11/04/2017Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation at random intervals:RadiationThe nucleus is more stable after emitting some radiation – this is called “radioactive decay”. This process is NOT affected by temperature or other physical conditions.
11Ionisation11/04/2017Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons:Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer.
12Background Radiation 13% are man-made Radon gas Food Cosmic rays 11/04/201711/04/201713% are man-madeRadon gasFoodCosmic raysGamma raysMedicalNuclear power
13Background Radiation by Location 11/04/2017In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event:Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc
14Types of radiation11/04/201711/04/20171) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)Unstable nucleusNew nucleusAlpha particle2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.Beta particleNew nucleusUnstable nucleus3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.Words – frequency, proton, energy, neutrons, helium, betaUnstable nucleusNew nucleusGamma radiation
15Changes in Mass and Proton Number 11/04/2017Alpha decay:Am24195Npα+2374932Beta decay:Sr9038Yβ+9039-1
16Sheet of paper (or 6cm of air will do) Blocking Radiation11/04/2017Each type of radiation can be blocked by different materials:Sheet of paper (or 6cm of air will do)Few mm of aluminiumFew cm of lead
17Summary Property Alpha Beta Gamma Charge Mass Penetration ability 11/04/2017PropertyAlphaBetaGammaChargeMassPenetration abilityRange in airWhat is it?Ionising ability
18Deflection by Electric Fields 11/04/2017+2 protons, 2 neutrons, therefore charge = +2Alpha and beta particles have a charge:1 electron, therefore charge = -1-Because of this charge, they will be deflected by electric fields:+-+-1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?
19Deflection by Magnetic Fields 11/04/2017Recall:+2 protons, 2 neutrons, therefore charge = +21 electron, therefore charge = -1-Because of this charge, they will also be deflected by magnetic fields:+-1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?Region of magnetic field
20Uses of radioactivity 1 Sterilising medical instruments 11/04/2017Sterilising medical instrumentsGamma rays can be used to kill and sterilise germs without the need for heating. The same technique can be used to kill microbes in food so that it lasts longer.
21Uses of radioactivity 2 - Tracers 11/04/2017A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:Gamma sourceThe radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.Tracers can also be used in medicine to detect tumours:For medicinal tracers, you would probably use a beta source with a short half life – why?
22Uses of radioactivity 3 – Smoke Detectors 11/04/2017Smoke detectorsAlpha emitter+ve electrode-ve electrodeAlarmIonised air particlesIf smoke enters here a current no longer flows
23Uses of Radioactivity 4 - Treating Cancer 11/04/2017High energy gamma radiation can be used to kill cancerous cells. However, care must be taken in order to enure that the gamma radiation does not affect normal tissue as well.Radioactive iodine can be used to treat thyroid cancer. Iodine is needed by the thyroid so it naturally collects there. Radioactive iodine will then give out beta radiation and kill cancerous cells.
24Dangers of radioactivity 11/04/2017Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.AlphaBetaGammaOUTSIDE the body and are more dangerous as radiation is blocked by the skin.INSIDE the body an source causes the most damage because it is the most ionising.
25Half life11/04/2017The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…= radioisotope= new atom formedAfter 2 half lives another half have decayed (12 altogether)After 3 half lives another 2 have decayed (14 altogether)After 1 half life half have decayed (that’s 8)At start there are 16 radioisotopes
27Dating materials using half-lives 11/04/2017Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.Answer: The sample was originally completely uranium…1 half life later…1 half life later…1 half life later…8482818…of the sample was uraniumNow only 4/8 of the uranium remains – the other 4/8 is leadNow only 2/8 of uranium remains – the other 6/8 is leadNow only 1/8 of uranium remains – the other 7/8 is leadSo it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
28An exam question…11/04/2017Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain 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
40(This nebula is smaller and will only form a planet) 11/04/2017Planetary nebula(This nebula is smaller and will only form a planet)
41Stage 2: Protostar Gravity will slowly pull these particles together… 11/04/2017Gravity will slowly pull these particles together…As they move inwards their gravitational potential energy is converted into heat and a PROTOSTAR is formed
42Words – heavier, balanced, hydrogen, nuclear, temperatures Stage 3: Main Sequence11/04/2017In a main sequence star the forces of attraction pulling the particles inwards are _________ by forces acting outwards due to the huge __________ inside the star.Stars are basically ________ reactors that use _______ as a fuel. During its main sequence a star will release energy by converting hydrogen and helium (light elements) into _________ elements and this is why the universe now contains a number of heavier elements.Our sun is an example of a main sequence star – it’s in the middle of a 10 billion year life spanWords – heavier, balanced, hydrogen, nuclear, temperatures
43Stage 4: Red Giant11/04/2017Eventually the hydrogen and helium will run out. When this happens the star will become colder and redder and start to swell…If the star is relatively small (like our sun) the star will become a RED GIANTIf the star is big (at least 4 times the size of our sun) it will become a RED SUPERGIANT
44Stage 5: The Death11/04/2017What happens at this point depends on the size of the star…1) For SMALL stars the red giant will collapse under its own gravity and form a very dense white dwarf:Red giantWhite dwarfBlack dwarf
45This explosion is called a SUPERNOVA 2) If the star was a RED SUPERGIANT it will shrink and then EXPLODE, releasing massive amounts of energy, dust and gas.11/04/2017BeforeAfterThis explosion is called a SUPERNOVA
46If the star is big enough it could become a BLACK HOLE instead. 11/04/2017The dust and gas on the outside of the supernova are thrown away by the explosion and the remaining core turns into a NEUTRON STAR.If the star is big enough it could become a BLACK HOLE instead.
47Stage 6: Second generation stars 11/04/2017The dust and gas thrown out by a supernova can be used to form a new star…Our sun is believed to be a “______ ______ star” – this is because it contains some __________ elements along with hydrogen and ________. These heavier elements would have been the products of a previous star that have been thrown out by a ________. These heavier elements are also found on planets, indicating that they might have been made from remains of previous _______ as well.Words – helium, heavier, second generation, stars, supernova
48The Life Cycle of a Star summary 11/04/2017SMALLstarsProtostarBIGstarsMain sequenceRed giantRed super giantWhite dwarfSupernovaBlack dwarfNeutron starBlack holeBasically, it all depends on the size of the star!
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