ch 4. atoms and elements - laney college · 2017. 8. 30. · 4. the atoms of a given element are...
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Ch 4. Atoms and Elements
Chem 30A
Early View of Atomic Structure
In the Beginning…
Greeks(~400BC)§ Fourelements–fire,earth,water,air§ Isma=ercon?nuousormadeofdiscretepar?cles(atomos)?
Alchemists(next2000years)§ Firstchemists§ Discoveredelements;acidprepara?on
RobertBoyle(1627-1691)§ Scienceshouldbegroundedinexperiments.§ Elementsaresubstancesthatcannotbebrokendown
intosimplersubstances.
Dalton’s Atomic Theory (1808)
1. Eachelementismadeof?nyindivisiblepar?clescalledatomsthatcannotbecreatednordestroyed(lawofconserva?onofma=er).
2. Atomsofanelementcannotbeconvertedtoatomsofanotherelement.Inchemicalreac?ons,atomsre-combinetoformdifferentsubstances(changethewaytheyareboundtoeachother).
Dalton’s Atomic Theory, continued
4. Theatomsofagivenelementareiden?cal,anddifferentfromthoseofanyotherelement.
5. Atomsofoneelementcancombinewithatomsofotherelementstoformcompounds.Agivencompoundalwayshasthesamerela?venumbersandtypesofatoms.
Hypothesized Atomic Structure, 1910
• LordKelvin,1910– Plumpuddingmodel
Rutherford’s Gold Foil Experiment
• 1911:ErnestRutherfordshotα-par?cles(posi?velychargedpar?cleswithmass7300xthatofelectron)throughgoldfoil.
Rutherford’s Gold Foil Experiment
Expected Observed
The Nuclear Atom
EarnestRutherfordconcludedthenuclearatom:(1911-1919)• Anatomismostlyemptyspacethatisoccupiedbyelectrons.
• Atomhasa?ny,densecenterofposi?vecharge:nucleus.
• Thenucleuscontainspar?clescalledprotons,whichhavechargeofsamemagnitudeaselectronsbutareposi?ve.
• Mostnucleialsocontainaneutralpar?cle:neutron.(withChadwick,1932)
The Nuclear Atom
• Protons:Foundinthenucleus;hasposi?vechargeequalinmagnitudetotheelectron’snega?vecharge.
• Neutrons:Foundinthenucleus;nocharge;virtuallysamemassasaproton.
• Electrons:Foundoutsidethenucleus,rela?velyfarfromthenucleus;nega?velycharged.
Modern Atomic Theory
Modern View of Atomic Structure
• Small,densenucleus(nucleusdiameter~10-15m).Nucleusaccountsforalmostallofatom’smass.
• Electronmovingfarfromnucleus(atomdiameter~10-10m).Electronaccountsforatom’ssize.
Modern View of Atomic Structure
Symbols for Atoms (Isotope Symbols)
ZAX ElementSymbol
uniqueone-ortwo-le=ersymbolAtomicNumber
(numberofprotons)
MassNumber(numberofprotons+neutrons
No.ofprotons=atomicnumberZ=IDENTIFIES ELEMENT!
No.ofneutrons=MassnumberA–AtomicnumberZNo.ofelectrons=No.ofprotons(sinceatomsareneutral)
ExProbs
Isotopes
Isotopes:Atomswithsamenumberofprotons,butdifferentnumbersofneutronsàSo,isotopeshavesameatomicnumber(sameelement),butdifferentmassnumbers
Sodium Isotopes
1123Na 11
24Na• 11=AtomicnumberZ(11protons)
• 23=MassnumberA• A-Z=12neutrons
• 11=AtomicnumberZ(11protons)• 24=MassnumberA• A-Z=13neutrons
“Sodium-23” “Sodium-24”
Hydrogen Isotopes
11H 1
2H 13HPro?um Deuterium Tri?um
Isotopes
• Showalmostiden?calchemicalproper?es(Thechemistryofatomisduetoitselectrons!).
• Innature,mostelementscontainmixtureofisotopes.
ExProbs
CarbonIsotope
NaturalAbundance
12C 98.89%13C 1.11%
Atomic Mass
• Atomshavevery?nymasses,soscien?stsmadeaunitforatomicmasstoavoidusingverysmallnumbers.
1atomicmassunit(amu)=1.660539×10–24g
Atomic Mass Unit
• Defini?onofatomicmassunitisbasedon12C:
Oneatomof12Chasamassofexactly12amu.So,1amu=1/12themassof12Catom =1.660539×10–24g
• Massesofallotheratomsarerela?vetothis12Cstandard.
Atomic Mass
• Ontheperiodictable,whydoesChaveamassof12.01amu,NOT12amu?
Theatomicmassontheperiodictableisanaverageatomicmass–weightedaverageofmassesofalltheisotopesofanelement.
Finding Atomic Mass of an Element
Atomicweightofelement=Σ[frac?onalabundanceixisotopicmassi]
ForC:(0.9889)(12amu)+(0.01110)(13.0034amu)= 12.01amu
(*Nocarbonatomactuallyhasamassof12.01amu–it’sanaveragemass.)
CarbonIsotope
NaturalAbundance
12C 98.89%13C 1.11%
Exprobs
Ions
Ions
• Atomsareneutral,havinganetchargeofzero.
• Whenelectronsareremovedfromoraddedtoaneutralatom,achargedpar?clecalledionisformed.
Cations
• Atomsloseoneormoreelectronstoformposi?veionscalledca?ons.
MgàMg2++2e–
Anions
• Atomsgainoneormoreelectronstoformnega?veionscalledanions.
Cl+e–àCl–
Isotope Symbols for Ions
• Ionscanalsoberepresentedbyisotopesymbols.
ExProbs
!"!!!!"
Elements and the Periodic Table
Periodic Table
• Awayoforganizingelements• FirstarrangedbyDmitriMendeleevin1869• Modernperiodictableisarrangedbyatomicnumber.
• Showsrecurringproper?es,sohelpspredictproper?esofelements
The Periodic Table of the Elements
Periodic Table Entries
Element Symbols
• Eachelementhasauniqueone-ortwo-le=ersymbol.
• Firstle=erisalwayscapitalizedandthesecondisnot.
• Thesymbolusuallyconsistsofthefirstoneortwole=ersoftheelement’sname. OxygenO KryptonKr
• Some?mesthesymbolistakenfromtheelementsoriginalLa?norGreekname.
GoldAu(aurum)LeadPb(plumbum)
Names and Symbols of Common Elements
Periodic Table
Periodic Table
Features of the Periodic Table
• Row=Period– Numberingofperiods(7periods)– Someperiodsarenamed:Lanthanides,Ac?nides
Features of the Periodic Table, cont’d
• Column=Group=Family– Numberingofgroups(18groups,AandBgroups)– Namesofgroups:
• Agroups=MainGroupelements• Bgroups=Transi?onMetalelements• 1A=alkalimetals• 2A=alkalineearthmetals• 7A=halogens• 8A=noblegases
Periodic Table Shows Periodic Patterns
Membersofsamegroupshavesimilarchemicalandphysicalproper?es.• Group1A(alkalimetals):Shinysosmetals,lowmeltpts,reactrapidlywithwatertoformalkaline(basic)products.
• Group2A(alkalineearthmetals):Shinymetals,lessreac?vethan1Ametals.
• Group7A(halogens):Colorful,corrosivenonmetals.
• Group8A(noblegases):Colorlessgases,dis?nctlackofchemicalreac?vity.
Metals, Nonmetals, and Metalloids
• Metals:Lesofstair-stepline• Nonmetals:Rightofstair-stepline• Metalloids/semimetals:Alongstair-stepline
Metal Elements
PhysicalProper?esofMetals– Efficientlyconductsheatandelectricity– Malleable(canbehammeredintothinsheets)– Duc?le(canbepulledintowires)– Lustrous(shiny)– Almostallmetalsaresolidsatnormaltemperatures(Excep?on:Mercuryisliquid.)
SodiumLithium
Nonmetal Elements
PhysicalProper?esofNonmetals– Donotconductelectricity.– Morevariablethanmetals.– Solidnonmetalsareusuallyhard,bri=le.– Mostnonmetalsaregasesorsolids[Excep?on:Bromineisliquid.]
Metalloid or Semimetal Elements
PhysicalProper?esofMetalloids– Havebothmetallicandnonmetallicproper?es.– Goodsemiconductors:Poorconductorsofelectricityatroomtemperature,butbecomemoderatelygoodconductorsathighertemperatureorwithaddi?onofimpuri?es
ExProbs
Periodic Table Shows Ion Formation Trend
Inachemicalreac?on:• Metalelementstendtoloseelectronsandformca?ons(posi?veions).
• Nonmetalelementstendtogainelectronsandformanions(nega?veions).
Common Ion Charges (Know!)
Metalsformca?ons.• Groups1A,2A,3A:
charge=group#• Transi?onsmetals:
Manyhavevariablecharges.
Nonmetalsformanions.• Group5A:charge=-3• Group6A:charge=-2• Group7A(halogens):
charge=-1• Group8A(noblegases):
charge=0
ExProbs
Periodic Table: Natural States of Elements Mostelementsaresolidsatroomtemperature.
Periodic Table: Natural States of Elements Someelementsexistasdiatomicmolecules(moleculesoftwoatoms):H2,N2,O2,allgroup7Aelements
Hydrogen
• Innaturalstate,hydrogenisanonmetal(ItisNOTanalkalimetal).
• Innaturalstate,hydrogenisadiatomicelement.
• Hydrogencanformca?on(H+=proton)oranion(H-=hydride).
• Ahydrogenatomdoesnothaveaneutron!
Expanded Periodic Table of the Elements