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