Chemistry - an illustrated guide to science (794128), страница 31
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Although most nuclei arestable, some are not andspontaneously decay, emittingradiation.● The lightest stable n u clides (particularisotopes of an element) have almostequal numbers of protons andneutrons. The heavier stable nuclidesrequire more neutrons than protons.The heaviest stable nuclides haveapproximately 50 percent moreneutrons than protons.110Unstable region10 09080● Isotopestend to be more stable whenthey have even numbers of protonsand neutrons than when they haveodd. This is the result of the spins ofthe n u cleon s (the constituents of theatomic nucleus).
When two protons orneutrons have paired spins (spins inopposite directions), their combinedenergy is less than when they areunpaired.70band of stabilityN=Z6050Decay● Whenunstable nuclides disintegrate,they tend to produce new nuclidesthat are nearer to the stability line.This will continue until a stablenuclide is formed.● An unstable nuclide above the band ofstability decays by beta emission. Thisincreases the proton number anddecreases the neutron number. Thus,the neutron to proton ratio isdecreased.● An unstable nuclide below the band ofstability disintegrates so as to decreasethe proton number and increase theneutron to proton ratio. In heavynuclides this can occur by alphaemission.4030Unstable region2010001020304050Number of protons (Z)60708090© Diagram Visual Information Ltd.Number of neutrons NOdd-even rule180RADIOACTIVITYKey wordsalpha particlehalf-lifeisotopenuclideHalf-life1 Half-life1 Half-life● Half-lifeis the time required for halfthe nuclei in a sample of an isotope toundergo radioactive decay.● Radioactive decay is a completelyrandom process in which nucleidisintegrate independently of eachother or external factors such astemperature and pressure.N02 Rate of decayare always very large numbers ofactive n u clides even in small amountsof radioactive material, so statisticalmethods can be employed to predictthe fraction that will have decayed, onaverage, over a given period of time.● The rate of decay of a nuclide at anytime is directly proportional to thenumber of nuclei, N, of the nuclide:-dN ! N or dN = - "NN02● TheredtN042 Rate of decaydtN0© Diagram Visual Information Ltd.Nt = N0 e-"twhere N0 is the number of undecayedatoms at time t = 0 and Nt the numberof undecayed atoms after time t .● After one half life ( t 1/ 2 ) has passed, thenumber of undecayed atomsremaining in the sample will be N0 / 2 .Substituting this into the exponentiallaw equation for Nt and taking naturallogs of both sides provides amathematical relationship between thedecay constant and the half life of aradioactive atom:Number of undecayed nuclei, Nwhere N is the number of undecayednuclei and " is the decay constant.
Theminus sign indicates that the numberof undecayed nuclei falls with time.Integrating this gives the exponentiallaw equation:N02N04t (1/ 2) = 0 .693"N08Half-lifeHalf-lifeTypical radioactive decay curveHalf-lifeTime, t181Measuring half-lifeRADIOACTIVITYKey words1 Half-life of radonaealpha particlehalf-lifeisotopeib1 Half-life of radon● Thoriumfc+g–dAhionization chamberairradond.c. amplifierclipsf valvesg squeezable polyethylenebottleh thorium hydroxide powderi clock2 Exponential decay2 Exponential decay: decay curve for radon gas● A graphof current against time isplotted.● In this experiment, the half-life isindicated by the amount of time takenfor the current to fall to half of itsoriginal value.● The half-life of radon-220 isapproximately 55 seconds.y1.0Current (amps)3 Radon decay● Radon-220decays with the loss of analpha particle to form polonium-216,which decays to form lead-212.
Thehalf life of polonium-216 is 0.145seconds, and the half life of lead-212 is10.64 hours.0.50.250.1250050100Time (s)1503 Radon decayRa-222Po-216Pb-212200 x© Diagram Visual Information Ltd.abcdedecays to produce theradioactive isotope radon-220. Thisisotope is sometimes referred to asthoron.● The bottle containing thoriumhydroxide powder is squeezed a fewtimes to transfer some radon-220 tothe flask. The clips are then closed.● As the radon decays, the ionizationcurrent decreases. It is always ameasure of the number of alphaparticles present and, therefore, theproportion of radon-220 remaining.● The current is noted every 15 secondsfor 2 minutes and then every 60seconds for several minutes.182RADIOACTIVITYKey wordsalpha particlegamma radiationirradiationisotopeRadioactive isotopes1 Tracers2 Thyroid monitor3 Food preservation4 Sterilization1 Tracers● Radioactiveisotopes are used as tracersto monitor the movement ofsubstances in plants and animals.
Asolution containing radioactivephosphorus-32 is introduced into thestem of a plant. A Geiger counter isused to detect the movement of theisotope through the plant.2 Thyroid monitor● A solutioncontaining iodine-131 isintroduced to the bloodstream of apatient with a defective thyroid. AGeiger counter is used to detect theisotope and monitor thyroid activity.gammarays3 Food preservation● Foodis irradiated by exposing it togam m a radiation . Irradiationdestroys disease-causing bacteria aswell as those that spoil food, so theshelf life of food is extended.gammarays4 Sterilization● Gammaradiation is used to sterilizemedical equipment.bacteriadying5 Smoke detectors● Americium-241,a source of alpharadiation, is widely used in smokedetectors.
The alpha particles ionizethe air in the sensing circuit. Anysmoke particles interfere with this andcause a change in the current, whichtriggers an alarm.medical equipment5 Smoke detectors6 Duration of deathcurrent-sensingcircuit● All© Diagram Visual Information Ltd.6 Duration of deathorganisms contain a specific ratio ofradioactive carbon-14 to carbon-12.When an organism dies, no carbon-14is added. After death, carbon-14 decaysat a predictable rate: the half-life is5,700 years. By comparing the ratio ofcarbon-14 to carbon-12, it is possibleto say when an organism died.batteryelectriccurrentCarbon-12constantCarbon-14decreasesDeadorganismGrasshopper183Nuclear fusionRADIOACTIVITYKey words1 Nuclear fusionprotonfusionisotopeprotonprotonproton1 Nuclear fusion● Innuclear fu sion , two or more lightatomic nuclei join to make a moremassive one.
During the process,some of the mass of the nuclei isconverted into energy. Nuclear fusion,which first occurred during the BigBang, powers stars. It also occurs inhydrogen bombs. Currently scientistsare working to control fusion so it canbe used in nuclear reactors.heliumnucleuspositronpositronelectronelectron2 Deuterium● Deuteriumis an isotope of hydrogenknown as heavy hydrogen.
Thenucleus of a deuterium atom consistsof one neutron and one proton.● The fusion of two deuterium nucleiresults in the formation of a helium-3nucleus. A small amount of mass isconverted into energy:Mass of two deuterium nuclei =starlight2 Fusion of deuterium2 x 2.0 14 = 4.0 28 uMass of helium-3 nucleus plus aneutron =3.0 16 + 1.0 0 9 = 4.0 25 uMass converted to energy by fusion =4.0 28 – 4.0 25 = 0 .0 0 3 u21H21H32 He10ndeuteriumdeuteriumhelium-3neutronEnergy released by the fusion reaction= 4.5 x 10 -13 JEnergy released per kilogram ofdeuterium is approximately 9 x 10 13 J.3 Tritium21H31H42 He10ndeuteriumtritiumhelium-4neutronis another isotope ofhydrogen.
The nucleus of a tritiumatom consists of two neutrons and oneproton.● The fusion of a deuterium nucleus anda tritium nucleus results in theformation of a helium-4 nucleus andthe release of energy. The energyreleased per kilogram of deuteriumand tritium is approximately30 x 1013 J.● This reaction produces more energy,and the fusion takes place at a lowertemperature.© Diagram Visual Information Ltd.● Tritium3 Fusion of deuterium and tritium184RADIOACTIVITYKey wordschain reactionfissionNuclear fission1 Reaction with uraniumNNNNuclear fission● Innuclear fission , a heavy atomicnucleus divides to make two smallerones.
Some of the mass of the nuclei isconverted into energy during theprocess.1 Reaction with uraniuma nuclear reaction with uranium andslow-moving neutrons, the nucleus ofthe uranium-235 atom undergoesfission and forms two smaller nuclei(lanthanum-148 and bromine-85) plusthree neutrons. A small amount ofmass is converted to energy.N10none neutron+23592 U14857 Lauranium-235lanthanum-148+8535 Brbromine-85+3 01 nthree neutrons● In2 Chain reactionNN2 Chain reactionchain reaction is a series ofself-sustaining reactions in which theparticles released by one nucleustrigger the fission of at least as manyother nuclei.● Under normal circumstances, only avery small proportion of fissionneutrons act in this way.
However, ifthere is a sufficient amount of aradioactive isotope, a chain reactioncan start.● In an atomic bomb, an increasinguncontrolled chain reaction occurs ina very short time when two pieces ofuranium-235 (or plutonium-239) arerapidly brought together.● In a nuclear power station, the chainreaction is steady and controlled, soonly a limited number of fissionneutrons bring about further fissionreactions.© Diagram Visual Information Ltd.● A nuclearNNNNNNNNNNN1 neutron3 neutrons9 neutrons185Nuclear reactorRADIOACTIVITYconcrete sheildcooling waterconverted tosteam, whichdrives turbineKey wordsfissionisotopeuraniumNuclear reactoratoms splitting insidecore of the reactorgive off heatcarbon dioxide gas carriesheat away from uranium inthe core of the reactorgeneratorcooling waterturbineelectricitycooling towereither the metal or themetal oxide, is used as fuel in nuclearreactors.
The fuel is in the form of fuelrods, which are suspended in thereactor.● Naturally occurring uranium contains99.3 percent uranium-238 and only 0.7percent of the radioactive isotopeuranium-235. The uranium-235content must be increased toapproximately 3 percent before theuranium can be use as a fuel.● Uranium-235 undergoes spontaneousfission . However, in a nuclear powerstation, the fission is brought about bybombarding the uranium nuclei withneutrons.● The fission of one atom of uranium235 absorbs one neutron and releasesthree others. In order to increase thechances that these neutrons will strikeother uranium-235 atoms, they areslowed down by a moderator.● Control rods are suspended betweenthe fuel rods.
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