Chemistry - an illustrated guide to science (794128), страница 30
Текст из файла (страница 30)
The aminoacids join together forming peptidebonds by the loss of water:-H2OH2N-CHR-COOH + H2N-CHR-COOH ➞H2N-CHR-CONH-CHR-COOH●3 ZwitterionsRR+H2N—CH—COO–H2N—CH—COOHAll of the amino acids in proteins arethe L-isomers.© Diagram Visual Information Ltd.4 ProteinsROR—H2N—CH—CHN—CH—Cpeptide link173MonosaccharidesCHEMISTRY OF CARBONKey words1 Chain structure112HO— C—HCHOCHO2H— C—OH33HO— C—HH— C—OHHO— C—HMonosaccharidesMon osaccharides are simple sugarsthat have between three and sixcarbon atoms. Those with six carbonatoms are known as the hexoses andhave the general formula C6 H12O6 .● Monosaccharides with an aldehydegroup ( -CHO) are called aldoses.● Glu cose has both an aldehyde groupand six carbon atoms and is thereforean aldohexose.45HO— C—H5H— C—OH6CH2OHD-glucoseCH2OHL-glucose1 Chain structure2 Ring structure6CH2OHCH2OHOCH5HCHOH4OH C23HFor simplicity, monosaccharides aresometimes displayed as vertical openchain structures to which the -H and–OH groups are attached.● Aldohexoses contain fourasymmetrical carbon atoms: C-2 , C-3,C-4 , and C-5 .
There are 8 differentpossible ways of arranging the –H and–OH groups on these carbon atoms,and each of these has two opticalisomers, making a total of 16.● The most important of these are thetwo optical isomers of glucose.● For glucose the D- and L- indicate theconfiguration of the –H and –OHgroups on C-5.●6Hhexosemonosaccharide●4H— C—OH6aldehydealdohexosealdoseanomerglucose1CC OHHOCOH5HCHOH4OH C23OHHα-D-glucose1CC HOHβ-D-glucose2 Ring structureIn reality, solid monsaccharides do notexist as open chain structures but asring structures.● In Howarth projections ofmonosaccharides, groups are shownon vertical bonds above and below aflat hexagonal ring.● D-glucose can exist in two separatecrystalline forms known and !-Dglucose and "-D-glucose.
These formsare known as an om ers.HOO6HCH2OH4H52HHO3OHOH13 Hexagonal ringHHβ-D-glucose●The hexagonal ring in amonosaccharide is not flat but in theform of a chair.© Diagram Visual Information Ltd.●3 Hexagonal ring174Disaccharides andpolysaccharidesCHEMISTRY OF CARBONKey wordscellulosedisaccharideglycogenmonosaccharidepolysaccharidestarchsucrose1 SucroseDi-and polysaccharidesA disaccharide is formed when twom on osaccharides join together. Amolecule of water is lost and aglycosidic link is formed.● A polysaccharide is a polymer formedby the joining of manymonosaccharide units.●HHOHSu crose, the sugar widely used onfoods, is a disaccharide.CH2OHHHOHHHOCH2OHOHHHO2 Cellulose●OHHO1 Sucrose●OCH2OHCellu lose, a polysaccharide, providesplant cells with a rigid structure.glycosidic bond3 StarchGlycogen is the storage polysaccharideof animals.● Starch is the storage polysaccharide ofplants.●2 CelluloseCH2OHHOHHOHOHOHHOHHHCH2OHOHOHOHHOHHOHOHHOHOHHHHOCH2OHHCH2OHCH2OHHHOCH2OHOH3 Starch© Diagram Visual Information Ltd.CH2OHOHHOHHOHHHOOHOHOHHHOHHCH2OHOHOHOHHHOHHOHHOHOHHOH175Ionizing radiationRADIOACTIVITYKey words1 Alpha particlesprotonα-radiation consistsof a stream of αparticlesProduces intenseionization in agasalpha particlebeta particlegamma radiationionizing radiationIonizing radiationneutron● Ion izin gradiation is any radiationcapable of displacing electrons fromatoms or molecules and so producingions.
Examples include alphaparticles, beta particles, and gam m aradiation .2 Beta particlesβ-radiation consistsof a stream of βparticles–electronProduces lessintenseionization in agas than aparticles3 Gamma radiationγ-radiation is a formof electromagneticradiationOnly weaklyionizes a gas1 Alpha particles● Analpha (!) particle has the samestructure as a helium nucleus (twoprotons and two neutrons).● Alpha particles are relatively heavy,high-energy particles with a positivecharge.● Alpha particles produce intenseionization in a gas.● Emission speeds are typically of theorder of 5–7 percent of the speed oflight.2 Beta particles● A betaWavelengthFrequency(") particle is a fast-movingelectron with a negative charge.● Beta particles produce less ionizationin a gas than alpha particles and onaverage produce only 1/1000th asmany ions per unit length.● Emission speeds can be as high as 99percent of the speed of light.<10 –12 m>10 21 Hz4 Radiation in laboratories3 Gamma radiation● Gammalead container(#) rays ionize gas only weaklyand on average produce only 1/1000thas many ions per unit length as betaparticles.4 Radiation in laboratoriesactive metal foilwire gauze coverof radiation used forlaboratory experiments are usuallysupplied mounted in a holder.
Theactive material is sealed in metal foil,which is protected by a wire gauzecover. When not in use, the material isstored in a small lead container.© Diagram Visual Information Ltd.● Sources4 mmplug176RADIOACTIVITYRadiation detectorsKey words1 Spark counterionizing radiationradiationradioactivity2 Cloud chambercDetectors● Radioactivityis invisible, but becauseit affects the atoms that it passes,scientists can easily detect it using avariety of methods.adbehmipjkf1 Spark counter● Highvoltage is applied between thestiff wire (anode) and the gauze(cathode) and reduced until it juststops sparking.● When a radium source is brought nearthe gauze, the air between the wireand the gauze is ionized, and sparksare seen and heard at irregularintervals.+–g2 Cloud chambero3 Geiger-Muller tube(GM tube)● Whenair containing ethanol vapor iscooled, it becomes saturated.
Ifion izin g radiation passes through thisair, further cooling causes the vapor tocondense on the ions created in theair. The result is a white line of tinyliquid droplets that shows up as atrack when illuminated.uw–3 GM tubel4 Testing absorbtion ofalpha, beta, and gammaradiationx–nyz– –– –––● Whenradiation enters the metal tube,either through the mica window orthrough the tube wall, it creates argonions and electrons. These areaccelerated toward the electrodes andcollide with other argon atoms. Onreaching the electrodes, the ionsproduce a current pulse, which isamplified before being fed to a pulsecounter.–++qrstv© Diagram Visual Information Ltd.4 Testing absorption● Theability of materials to absorbalpha, beta, and gamma radiation canbe tested by placing the materialbetween a radioactive source and aGM tube and comparing the count perminute with the count over the sameperiod when the material is removed.abcdefghstiff wire (anode)sparkswire gauze (cathode)radium sourceforcepsinsulating baseE.h.t.
supplycircular transparent plasticchamberi super-cooled vaporj transparent lidk felt strip soaked with alcoholand waterl basem radioactive sourcen foam spongeo crushed dry icep black metal base plateq mica windowr argon gas at low pressures anode wiret insulatoru cathode metal tubev pulse counterw electrons are pulled towardthe anode wire in anavalanchex sourcey absorbing materialz GM tube177Properties of radiations:penetration and range1 Penetration of radiationd2 Range of radiation in aireRADIOACTIVITYKey wordsalpha particlebeta particlegamma radiationradiationha1 Penetration● Alpha,beta, and gam m a radiationpenetrate by different amounts.● Alpha radiation is the least penetratingand is stopped by a sheet of paper orvery thin metal foil.● Beta radiation is stopped by aluminuma few millimeters thick.● Gamma radiation is most penetrating,and is only stopped by a thick block oflead.fibgckj2 Range● Theabcdα – sourceβ – sourceγ – sourcemetal foil1cme paperf aluminumg lead1mhijk2m3mα – a few centimetersβ – a few metersγ – many metersarea covered by γ radiation at 1m distances3 The inverse square law for gamma radiation penetrationy++● Gammarays are highly penetratingbecause they have relatively littleinteraction with matter.
There is verylittle absorption or scattering as theypass through air.● The intensity falls off with distanceaccording to the inverse square law:I= kd2where I is intensity, d is the distancefrom the source, and k is a constant.At a distance x, the intensity of thegamma radiation:+Count rate3 Gamma penetrationIx = kx2+At a distance 2x, the intensity of thegamma radiation:I 2x = k = k(2x) 24x2+As the distance increases by a factor of2, the intensity of the gamma radiationdecreases by a factor of 4.+0 105010 01d215020 0250 x© Diagram Visual Information Ltd.1mm 3mm 1cmpenetrating power of alpha, beta,and gamma radiation is reflected inthe distance that they can travelthrough air.
Alpha particles can onlytravel a few centimeters beforecolliding with air particles. Betaparticles travels a few meters, whilegamma radiation can travel manymeters.178RADIOACTIVITYKey wordsalpha particlebeta particleelectric fieldgamma radiationProperties of radiations:in fieldsRadiation1 Electric field2 Magnetic fieldmagnetElectric and magneticfields● Anelectric field is a field extendingoutward in all directions from acharged particle.● A magnetic field is an area of force thatexists around a magnetic body or acurrent-carrying conductor.
Alpha,beta, and gamma radiation behavedifferently in both.+N–SAlpha1 Electric field● Alpharadiation is composed ofpositively charged particles. A streamof alpha particles is deflected whenpassing through the electric fieldbetween two oppositely chargedplates. The particles are repelled fromthe positively charged plate andattracted toward the negativelycharged plate.● Beta radiation is composed ofnegatively charged particles. A streamof beta particles is deflected by anelectric field in the opposite directionto alpha particles. The deflection isgreater because the beta particles havea much smaller mass.● Gam m a radiation is not deflected byan electric field. This is evidence thatgamma radiation carries no charge.magnet+N–SBeta2 Magnetic field© Diagram Visual Information Ltd.radiation is deflected by a strongmagnetic field.
Weak magnetic fieldshave no noticeable effect due to thegreater mass of alpha particlescompared to beta particles.● Beta radiation is deflected by arelatively weak magnetic field. Betaradiation is deflected in the oppositedirection to alpha radiation, indicatingits particles carry an opposite charge.● Gamma radiation is not deflected by amagnetic field, indicating that gammaradiation carries no charge.magnet● Alpha+N–SGamma179Stable and unstableisotopesRADIOACTIVITYKey wordsisotopenucleonnuclide130120Stability● Thestability of isotopes is based on theratio of neutrons and protons in theirnucleus.
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