化学专业英语《Chemistry English》课程教学资源（PPT课件）Lecture 02

Atomic structureNucleusThe mass number gives theThis contains protons andnumber of nucleons, that is theneutrons, called collectivelynumber of protons+neutrons.nucleonsNIsotopes are atoms with the samePPatomic number but different massNNPNnumbers. All the atoms of anelement have the same atomicNPNPnumber and it is this that makesNPthem all atoms of a particularThe atomic number gives theelement.number of protons in theThe masses of atoms of particularnucleus. It also gives theisotopes, called the relative isotopicnumber of electrons in themass, are expressed on a relativeneutral atom and the positionscale on which the mass of an atomof the element in the periodicof the isotope carbon-12 has 121tableunits exactly

1 Atomic structure Nucleus This contains protons and neutrons, called collectively nucleons. The mass number gives the number of nucleons, that is the number of protons+neutrons. The atomic number gives the number of protons in the nucleus. It also gives the number of electrons in the neutral atom and the position of the element in the periodic table Isotopes are atoms with the same atomic number but different mass numbers. All the atoms of an element have the same atomic number and it is this that makes them all atoms of a particular element. The masses of atoms of particular isotopes, called the relative isotopic mass, are expressed on a relative scale on which the mass of an atom of the isotope carbon–12 has 12 units exactly

Atomic structureRelative atomic massAll elements exist in several isotopic forms and it is useful to have anaverage value for the masses of the atoms of each element. This iscalled the relative atomic mass and is defined as the weighted mean ofthe masses of the naturally occurring isotopes of the element expressedon the carbon-12 scale. These are found using a mass spectrometer.IsotopeRelativeisotopemassRelativeabundancein naturalchlorine35CI3575%37CI3725%Relative atomic mass = 35X 0.75 + 37X 0.25 = 35.52

2 Atomic structure All elements exist in several isotopic forms and it is useful to have an average value for the masses of the atoms of each element. This is called the relative atomic mass and is defined as the weighted mean of the masses of the naturally occurring isotopes of the element expressed on the carbon-12 scale. These are found using a mass spectrometer. Isotope Relative isotope mass Relative abundance in natural chlorine 35Cl 35 75% 37Cl 37 25% Relative atomic mass = 35×0.75 + 37×0.25 = 35.5 Relative atomic mass

Atomic structureRADIOACTIVITYSome isotopes are stable, but others, often with unevennumbers of protons and/or neutrons are unstable. Thisinstability increases with atomic number, resulting fromthe growing repulsion between increasing numbers ofprotons. When an unstable isotope decays it gives offradiation known as radioactivity. This can be in one ofthree forms as this table shows.3

3 Atomic structure Some isotopes are stable, but others, often with uneven numbers of protons and/or neutrons are unstable. This instability increases with atomic number, resulting from the growing repulsion between increasing numbers of protons. When an unstable isotope decays it gives off radiation known as radioactivity. This can be in one of three forms as this table shows. RADIOACTIVITY

Atomic structureName ofradiationMadeofBehaviorinelectricfieldPenetratingpowerAlphahelium nucleideflected slightlystopped bypaperBetaelectronsdeflected a lotin other direction stopped by mm ofleadGammaelectromagneticradiation similartoX-rayspenetrates cm ofleadelectricfieldα,alpha particlescardleadβ,betaparticlesy,gammaraysleadAll radioactive decay is a first order rate process4

4 Atomic structure All radioactive decay is a first order rate process Name of radiation Made of Behavior in electric field Penetrating power Alpha helium nuclei deflected slightly stopped by paper Beta electrons deflected a lot in other direction stopped by mm of lead Gamma electromagnetic radiation similar to X-rays penetrates cm of lead

Atomic structureUses of isotopesIsotopes are often used as tracers. Very small tracers of anisotope can be detected and hence followed through a process.biology e.g.32P to study nutrient uptake in plantsmedicine e.g. 131I to study thyroid(甲状腺) functionindustry e.g. 57Fe to study wear and lubrication in enginesgenerating power e.g. 235U in fission reactors; 3H in fusiongeography e.g. 57Fe to study river flowarchaeology e.g. 14C in carbon dating5

5 Atomic structure Isotopes are often used as tracers. Very small tracers of an isotope can be detected and hence followed through a process. Uses of isotopes biology e.g. 32P to study nutrient uptake in plants medicine e.g. 131I to study thyroid(甲状腺) function industry e.g. 57Fe to study wear and lubrication in engines generating power e.g. 235U in fission reactors; 3H in fusion geography e.g. 57Fe to study river flow archaeology e.g. 14C in carbon dating

Atomic structureThe last example is well known. Nitrogen high in the atmosphereis converted into carbon-14 by cosmic rays coming from space.The amount of this carbon-14 relative to carbon-12 in theatmosphere was constant until the industrial revolution whenthe burning of fossil fuel began to dilute it. This means that allcarbon-containing objects such as wood and paper startedwith the same relative amount ofcarbon-14 as there was in theatmosphere.However, carbon-14 decays back into nitrogenwith a half-life of 5568 years, so by measuring the amount ofcarbon-14 in an object it can be dated to within 200 years.Objects like the Dead Sea scrolls and the Turin Shroud havebeen dated in this wayN = No(1/2) T/Tin26

6 Atomic structure The amount of this carbon-14 relative to carbon-12 in the atmosphere was constant until the industrial revolution when the burning of fossil fuel began to dilute it. This means that all carbon-containing objects such as wood and paper started with the same relative amount of carbon-14 as there was in the atmosphere. However, carbon-14 decays back into nitrogen with a half-life of 5568 years, so by measuring the amount of carbon-14 in an object it can be dated to within 200 years. Objects like the Dead Sea scrolls and the Turin Shroud have been dated in this way. N = N0 (1/2)T/T1/2 The last example is well known. Nitrogen high in the atmosphere is converted into carbon-14 by cosmic rays coming from space

CosmicRadiationDating of the Shroud of TurinCosmicraysentertheearth'satmosphereandcollidewithanatom,creatinganenergeticneutron.Sincethe 1354 AD,a yellowingWhentheneutronNeutronpiece of linen 14-ft long has beencolideswithanifrogenatom;astored in Turin, Italy. It bears thenitrogen-14(sevenprotons,sevenimage of a person who seems to beNeutron captureneutrons)atomfumsintoawearing a crown of thorns.carbon-14Carbon14Nitrogen14atom.CouldtheShroud裹尸布）ofTurinhave been the burial cloth of a personProtonPlantsabsorbcarbondioxideandincorporatecarbon-14who died two thousand years ago?throughphotosynthesis.In 1988, three laboratories weregiven four pieces of fabric; threecontrolpiecessimilarinwereAnimaisandpeopleeatpiantsandtakeinappearance, and one was a piececarbon-14.from the shroud. The labs all agreedthat theshroud was 608-728 yearsold, which means that it came intoFoowingdeathandexistence sometimebetween 1260burial,woodandboneloseC-14asitchanaesand1380AD,atimespanwhichtoN-14bybetadecayincludes the year the shroud was firstBetadecayNitrogen14shown to the public.Carbon14SAU4Howstufworks7

7 Since the 1354 AD, a yellowing piece of linen 14-ft long has been stored in Turin, Italy. It bears the image of a person who seems to be wearing a crown of thorns. Could the Shroud(裹尸布) of Turin have been the burial cloth of a person who died two thousand years ago? In 1988, three laboratories were given four pieces of fabric; three were control pieces similar in appearance, and one was a piece from the shroud. The labs all agreed that the shroud was 608-728 years old, which means that it came into existence sometime between 1260 and 1380 AD, a time span which includes the year the shroud was first shown to the public. Dating of the Shroud of Turin

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WordsAtomic structure Wordsnuclear:nucleus:nuclei:nucleonrelativeisotope mass;relativeatomic massweighted meanradioactive: radioactivityunstable:instabilitydecayfirst order rate: first order reaction; second order reactiondeflecttracer: isotopic tracing; isotopic tracer; radioisotopictracernutrient uptakethyroid: n, adj.= thyroid glandfission reaction; fusiongeographyarchaeologycarbon datingcosmic rays

9 Atomic structure Words Words nuclear; nucleus; nuclei; nucleon relative isotope mass; relative atomic mass weighted mean radioactive: radioactivity unstable: instability decay first order rate: first order reaction; second order reaction deflect tracer: isotopic tracing; isotopic tracer; radioisotopictracer nutrient uptake thyroid: n, adj.= thyroid gland fission reaction; fusion geography archaeology carbon dating cosmic rays

Electronic energy levels, orbitals and shells:how electrons are arranged in atomsENERGYLEVELSAtomicemission andabsorption88spectra tell us that the electrons in anatom have quantised ordefiniteamounts of energycalled energylevels22Increasingand thatthese energylevelshaveaenergyconvergentpattern.Rememberthat the energylevelforan1selectron is thesum ofitskinetic andpotential energy

10 Electronic energy levels, orbitals and shells: how electrons are arranged in atoms ENERGY LEVELS Atomic emission and absorption spectra tell us that the electrons in an atom have quantised or definite amounts of energy called energy levels and that these energy levels have a convergent pattern. Remember that the energy level for an electron is the sum of its kinetic and potential energy