Chemistry - an illustrated guide to science (794128), страница 6
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Most nonmetallic elements have simple covalentstructures and have very low boilin gpoin ts.● Elements with metallic and giantcovalent structures have very highboiling points (see diagram). Theboiling points of tran sition m etals aregenerally much higher than those ofthe grou p 1 and grou p 2 metals.3,0 0 0b2,50 0Boiling point °C2,0 0 01,50 01,0 0 050 0© Diagram Visual Information Ltd.00abcde10CarbonSiliconVanadiumMolybdenumRhenium20304050Atomic number607080903350.5atomic massatomic volumedensityelementgroupgroup 8lanthanide seriesmolenoble gasesperiodatom ic volu m e is the volume ofone m ole of the atoms of an elem en t.It can be found by dividing the atom icm ass of one mole of atoms by thedensity of the element:Atomic volume = Atomic massDensity● Sincethere are 6.023 x 1023 atoms permole of atoms, it would seem possibleto use the atomic volume to calculatethe volume of a single atom, and thusits radius.
However there are twoproblems with doing this. First, thestate of an element, and therefore itsdensity, changes with temperature andpressure. Second, using the atomicvolume to calculate the volume of asingle atom assumes that an elementconsists of atoms that are not bondedto each other. This is true only of thegrou p 8 elements (n oble gases). Forthese reasons, it is not possible toconsider the volume of an atom inisolation, but only as part of thestructure of an element.● In general, atomic volume increasesdown a grou p. Across a period, itdecreases and then increases.● The diagram highlights the elementwith the highest atomic volume in thefirst six periods (minus the lan than ideseries).Elements with peak atomicvolumesHeKRbCsRnHeliumPotassiumRubidiumCesiumRadon© Diagram Visual Information Ltd.22.2321.418.317.214.810.29.18.68.58.99.610.913.522.6Pt Au Hg Tl Pb Bi Po At RnKey words● The39.242.9ELEMENTS AND COMPOUNDSAtomic volume71.06 Cs Ba La Hf Ta W Re Os IrXeI25.620.418.216.415.813.010.38.88.38.18.59.410.914.2Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb TeY16.134.025.616.513.113.311.89.27.16.66.67.17.45 Rb Sr55.77.332.2Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrV8.910.614.726.044.94 K Ca Sc Ti24.218.7Cl ArS15.5P16.911.6Al Si10.014.023.73 Na Mg16.8F Ne17.1O14.0N17.3C5.4B4.34.913.02 Li Be31.814.11 HHeAtomic volumes of theelements34ELEMENTS AND COMPOUNDSKey wordsatomic massatomic numberatomic volumeperiodicityVariation of atomicvolumesd70Periodicity● As60ce50bAtomic volume cm3mol–1early as the Middle Ages, scientistsrecognized that elements could bedifferentiated by their properties andthat these physical and chemicalproperties were periodic.● The German chemist Lothar Meyerdemonstrated periodicity by plottingatom ic volu m es against atomicweights (the term atom ic m ass is nowused).● This periodicity is better shown byplotting atomic volumes againstatom ic n u m ber.● You can see periodicity most clearly bythe pattern between potassium (b)and rubidium (c), and betweenrubidium (c) and cesium (d) in thediagram.
These correspond to thechanging values across period 4 andperiod 5, respectively.40a3020© Diagram Visual Information Ltd.1000abcde10HeliumPotassiumRubidiumCesiumRadon20304050Atomic number6070809035Atomic massELEMENTS AND COMPOUNDSKey words1 Carbon-12atomic massisotope121 Carbon-12( 6 C)● To+++ +++2 LithiumIsotopeNatural abundanceLithium-6( 63Li)7.5%Lithium-7( 73Li)92.5%compare the masses of differentatoms accurately, scientists need astandard mass against which all othermasses can be calculated. Masses aregiven relative to this standard.● The isotope carbon-12 is used as thestandard.
On this scale, atoms ofcarbon-12 are given a mass of exactly12. The atom ic m asses of all otheratoms are given relative to thisstandard.● If an element contained only oneisotope, its atomic mass would be therelative mass of that isotope. However,most elements contain a mixture ofseveral isotopes in varyingproportions.● Natural abundance gives theproportion of each isotope in a sampleof the element.● If more than one isotope of anelement is present, the atomic mass iscalculated by taking an average thattakes into account the relativeproportion of each isotope. Diagrams2 and 3 illustrate how the atomic massof common isotopes of lithium andchlorine would be calculated.2 Lithium● Thereare two common isotopes oflithium: lithium-6 and lithium-7.● The atomic mass of lithium is 6.925,but for most calculates a value of 7 issufficiently accurate.The relative atomic mass of lithium is given by:(6 × 7.5) + (7 × 92.5)= 6.9251003 Chlorine3 ChlorineNatural abundance3575.77%3724.23%Chlorine-35( 17 Cl)Chlorine-37( 17 Cl)The relative atomic mass of lithiumis given by:(35 × 75.77) + (37 × 24.23)= 35.4846100● Thereare also two common isotopesof chlorine: chlorine-35 and chlorine37.● The atomic mass of chlorine is35.4846, but for most calculations avalue of 35.5 is sufficiently accurate.● Rounding the atomic mass of chlorineto the nearest whole number wouldlead to significant errors incalculations.© Diagram Visual Information Ltd.Isotope2122232425atom ic m ass of an elem en t is theaverage of the relative masses of itsisotopes.
It provides the relative massof an “average” atom of the element,which is useful for calculations.● The atomic mass is represented by thesymbol A(r)● The atomic mass of an isotope is itsmass relative to the isotope carbon-12.● The atomic mass of an isotope is thesum of the protons and neutrons in itsnucleus.● The atomic masses of the elements arepresented below the element on theperiodic table at right.Atomic number● Theatom ic n u m ber of an element isthe number of protons in its nucleus.● The atomic number is usuallyrepresented by Z.● The number of neutrons in thenucleus of an isotope is:● TheA(r) – Zatomic numbers of the elementsare presented above the element inthe periodic table at right.Ba88.9057–71Hf91.2272Zr47.8740TiTa92.9173Nb50.9441VW95.9474Mo52.0 042CrRe98.0075Tc54.9443Mn2627Rh58.9345Co28Pd58.6946Ni29Ag63.5547Cu30Cd65.4148ZnOsIrPtAuHg101.07 102.90 106.42 10 6.90 112.418078797677Ru55.8444FeGaTl114.8281In69.7249CPb118.7182Sn72.6450Ge28.0 832Si12.0114Bi121.7683Sb74.9251As30.9733P14.0 115N7Ra226Fr2232625926158Ce57LaNdPm6126460BhSg266Sm62277HsMtEu63268227AcPaU232.04 231.0 3 238.03Th237Np244Pu243Am138.90 140.12 140 .91 144.24 145.0 0 150.36 151.9689909192939495PrDbRfTb65272RgDy66285Ho67284Uub Uut210116Po68ErTm69288Yb70292Uuq Uup Uuh289I79.9053Br35.4535Cl19.0017F9247Cm247Bk251CfEs252257Fm258MdNeXe83.8054Kr39.9536Ar20 .1818Lu71210At259NoLr262220Rn127.60 126.90 131.29848586Te78.9652Se32.0 634S16.0016O8157.25 158.92 162.50 164.93 167.26 168.93 173.04 174.9796979899100101102103Gd64271Ds132.90 137.33178.49 180.95 183.84 186.21 190.23 192.22 195.08 196.97 200 .59 20 4.38 207.20 208.9889–103 1048887106111112113114115108110107109105CsYSr87.6256Rb85.475544.963940.083839.1037ScCaK27.0031AlMg24.3120NaB10 .8113Be9.0112Li6.941123.00195464.0 010● The1.0083Atomic mass2atomic massatomic numberelementisotopeHeKey words1ELEMENTS AND COMPOUNDSH© Diagram Visual Information Ltd.36Periodic table withmasses and numbers37Calculating the molecularmass of compounds1 Diatomic molecule (chlorine)ELEMENTS AND COMPOUNDSKey wordsatomic masscovalentcompounddiatomicmoleculeionic compoundlatticemolecular massCalculating molecular masschlorineCl = 35.452 Covalent compound (ethanol)You calculate the m olecu lar m ass of acompound the same way regardless ofstructure:1.
Multiply the number of atoms in anelement by its atom ic m ass.2. Repeat this process for each elementin the compound, then3. Add the numbers.1 Diatonic molecule(chlorine)● Theelement chlorine exists as adiatom ic m olecu le Cl 2.Atomic mass of chlorine = 35.5Molecular mass of chlorinehydrogen= 2 x 35.5 = 71carbonH= 1C = 12O = 162 Covalent compound(ethanol)● Ethanoloxygenis a simple covalen tcom pou n d that has the formulaC2H5OH.Atomic mass of carbon = 12 ;hydrogen = 1; oxygen = 16 .Molecular mass of ethanol= (2 x 12) + (6 x 1) + (1 x 16) = 463 Ionic compound (sodium chloride)3 Ionic compound (sodiumchloride)com pou n ds do not exist asmolecules but as a giant latticecomposed of ions in a fixed ratio.
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