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SECTION1
STRUCTUREOFTHEATOM
7/1
Atomsarethebasicunitsofmatterandcanbedefinedasthesmallestparticlesofan
elementthatcanenterintochemicalcombinations.Teatomsofdifferentelements
havecharacteristicchemicalandphysicalpropertiesdeterminedbythenumberof
constituentparticlesofwhichtheyarecomposed.Atomsconsistofparticlescalled
protons,electrons,andneutrons.*Protonsarepositivelycharged,andelectronsare
negativelycharged;neutrons,astheirnameimplies,areunchargedparticles.
Tenucleusofanatomcontainsprotonsandneutronsandconstitutesthemajority
oftheatomicmass.Eachprotoncarriesanelectricchargeof+1,andthenumber
ofprotonsintheatomsofanelementcorrespondstotheatomicnumberofthat
element.Teatomicweightofanelementisthetotalmassofprotonsandneutronsin
theatomsofthatelement.Hence,theatomicnumberofcarbonis6anditsatomic
weightis12(Table2a,p.27).Tesmallestknownatom,hydrogen,weighs1.67×10–24
gramsandhasadiameterofabout50pm.**Althoughthenumberofprotonsinthe
atomsofagivenelementisconstant,thenumberofneutronsmayvary,givingriseto
isotopesofagivenelement.
Negativelychargedelectrons(whichhaveamassof1l1836thofthatoftheneutron)
arepresentinatomsinnumbersequaltoprotons.Electronsorbitthenucleusin
shells,zonesofelectrondensityatincreasingdistancesfromthenucleusoftheatom,
andwithintheshellsinorbitalsandsuborbitals.Techemicalpropertiesofelements
areforthemostpartdeterminedbythenumberofelectronsintheoutermostshellof
theiratoms(seeSection2).
Task2
Below(seep.24)aresomeofthemorecommonsymbolsandexpressionsusedin
mathematics.Readthemandthenverbalizethefollowing:
a)10×84;
b)72;
c)93;
d)125–25=100;
e)4×5=20;
f)40÷5=8;
g)6<12;
h)82>2;
i)B≠C;
j)c+b≤4;
k)a+b≥8;
l)20%;
m)3cm×9cm×6cm;
n)6:5
*Between1940and2008,manyelementaryparticlesinadditiontoprotonsandneutronswere
discovered.Forexampleitwasdiscoveredthatallmatterisformedfromthesamefundamentalparticles
dividedintotwofamiliesofsixleptonsandsixquarks.Teelectronisalepton,whereasprotonsand
neutronsaremadeofassemblagesofquarks.
**1pm(picometre)=10
–12
m
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