Section 11: Electron Configuration and Periodic Trends · Constructing an Electron Configuration The aufbau principle states that the lowest energy orbitals in an atom are occupied

Section11:ElectronConfigurationandPeriodicTrends

ThefollowingmapsthevideosinthissectiontotheTexasEssentialKnowledgeandSkillsfor

ScienceTAC§112.35(c).

11.01TheBohrModeloftheAtom• Chemistry (6)(A)

11.02OrbitalEnergiesandElectronConfigurations• Chemistry (6)(E)

11.03CondensedElectronConfigurationsandExceptions• Chemistry (6)(E)

11.04CategoriesofElectrons• Chemistry (6)(E)

11.05TrendsinAtomicRadii• Chemistry (5)(C)

11.06TrendsinIonizationEnergy• Chemistry (5)(C)

11.07ElectronConfigurationforIons• Chemistry (6)(E)

11.08TrendsinIonicRadii• Chemistry (5)(C)

Note:Unlessstatedotherwise,anysampledataisfictitiousandusedsolely

forthepurposeofinstruction.

Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 1

11.01TheBohrModeloftheAtom

In1913,DanishphysicistNielsBohrproposedanewmodelofthehydrogenatom.Hismodel

hasthefollowingcharacteristics:

• Thehydrogenatomhasenergylevels(n)atwhichtheelectroncantravelinacircularorbitaroundthenucleus.Thesmallerthevalueofn,thesmallertheradiusoftheorbit

andthelowertheenergylevel.

• Anelectroninanatomdoesnotradiateenergywhileorbitingthenucleus.

• Theelectronwillmovetoanotherenergylevelonlyifitabsorbsoremitsaphotonthathasthesameenergyasthedifferencebetweenthetwoenergylevels.Inotherwords,ifanelectronisatenergylevel1(n1)andabsorbsaphoton,itcanmovetoenergylevel2

(n2),assumingthephotonaddedthesameamountofenergyasthedifferencebetween

n1andn2(n2–n1).

WhenstudyingtheBohrmodel,itisessentialtoknowafewkeyterms:

• Groundstate–whentheelectronisatthelowestenergyorbitalitcanoccupy(n=1forhydrogen)

• Excitedstate-whentheelectronisatahigherenergyorbitalthanthegroundstate(n=2,3,4,etc.forhydrogen)

• Ionize-theprocessbywhichanelectronisejectedfromanatom(nfinal=∞)

• Emission-atransitioninanatomwhereanelectroninahigherenergylevelreturnsto

alowerenergylevel(forexample,n=5ton=2)

• Absorption-atransitioninanatomwhereanelectroninalowerenergylevelmovesto

ahigherenergylevel(forexample,n=2ton=5)


Whenlightfromanelectricallyexcitedgaseousatomispassedthroughaslitandrefractedbya

prism,itcreatesalinespectrum.Thisconsistsofaseriesoffinelinesatspecificfrequencies,

separatedbyblackspaces.

Inahydrogenatom,anelectronundergoingemissioncanproducethreeseriesofspectrallines

thatareparticularlynoteworthy:

• Theultravioletspectrumisproducedwheretheelectrontransitionterminatesatn=1.Thewavelengthsofthelinesinthisseriesarebetween90and100nm.

• Thevisiblelightspectrumisproducedwheretheelectrontransitionterminatesatn=2.Thewavelengthsofthelinesinthisseriesarebetween400and750nm.

• Theinfraredseriesisproducedwhentheelectrontransitionterminatesatn=3.Thewavelengthsofthelinesinthisseriesareintherangeof1000sofnanometers.

1. Anelectroninahydrogenatomcanundergoanyofthetransitionsbelow,thuschangingits

energy.Whichtransitionisassociatedwiththeultravioletseries?

A. n=4ton=3B. n=1ton=2C. n=4ton=2D. n=3ton=1

2. Ofthetransitionsinthepreviousproblem,whichwillemittheradiationwiththeshortest

wavelength?


11.02OrbitalEnergiesandElectronConfigurations

Inamulti-electronatom,theelectronconfigurationisausefultoolfordeterminingthelocation

ofelectronsrelativetoothersastheyorbitthenucleus.Wecanthinkoftheelectron

configurationastheaddressoftheelectronsinanatom.

FourComponentsoftheAddress• Theprincipalenergylevel(n)describestheshellinwhichtheelectronorbitsthe

nucleus,accordingtotheBohrmodeloftheatom.Thevalueofnmustbeapositive

integer(1,2,3,...),andasthevalueincreases,theradiusoftheshellgrowslarger.

• Thedifferentenergylevelswithinaprincipalenergylevelarecalledthesubshellsorsublevels.Thesublevels,whichhavevaryingenergiesandshapes,aredenotedbytheletterss,p,d,andf.Thenumberofsublevelsinanygivenprincipalenergylevelisequal

ton.

• Theatomicorbitalisthethree-dimensionalvolumewithinasublevelwhereelectrons

havethehighestprobabilityofbeingfound.Everyorbitalcancontainamaximumof2

electrons.

o Thereis1sorbital,whichhasasphericalshape.

o Thereare3porbitals,eachhavingadumbbellshape.

o Thereare5dorbitals.Fourofthefivehaveadoubledumbbell(clover)shape.Onedorbitalhasadumbbellwithatorus(donut)shape.

o Thereare7forbitals,whoseshapesaremuchmorecomplexthanthoseofthedorbitals.

• Thespinofanelectronisapropertythatcanbedescribedaseitherclockwiseorcounter-clockwise.ThePauliExclusionPrinciplestatesthattominimizerepulsion,

electronsinthesameorbitalmusthaveopposingspins.

EnergyLevel(n) SublevelsPresent MaximumElectrons

1

2

3

4


1. Whichofthefollowingstatementsregardingenergylevelsandorbitalsisfalse?

A. Thenumberoforbitalsinthe5dsublevelis5.B. Theshapeofthe3sorbitalisspherical.C. Thereare3sublevelsinthen=3energylevel.D. Themaximumnumberofelectronsinann=3energylevelis9.

ConstructinganElectronConfiguration

Theaufbauprinciplestatesthatthelowestenergyorbitalsinanatomareoccupiedfirst.

Accordingly,whenwewriteanelectronconfiguration,westartfillingorbitalsinthelowest

valueenergylevel(n=1)andbuildupuntilwehaveaccountedforeveryelectronintheatom.

• Aneutralhydrogenatomhasoneelectron.Thelowestenergylevelisn=1anditcontainsonlyanssublevel,sotheelectronconfigurationforhydrogenis

1s1

Notethattheleadingnumberreferstothenvalue,theletter(s)referstothesubleveldesignation,andthesuperscriptdescribesthenumberofelectronsinthat

sublevel.

• Recallthatinthen=2energylevel,therearetwopossiblesublevels,sandp.Oncethessublevelisfilled,electronsbegintooccupythepsublevel.Considertheelectronconfigurationforcarbon:

1s22s22p2

• Writetheelectronconfigurationforneon,followingtheexamplegivenabove.

• Oncethessublevelandthepsublevelarecompletelyfull,wemuststartonthenext

energylevel,n=3,wherewecanmakeuseofs,p,anddorbitals.

• Hund’sRulestatesthateachorbitalinasublevelisassignedoneelectronbeforeanyorbitalisfilledwithasecondelectron.


2. Givetheelectronconfigurationsforthefollowingelements:

i. Boron

ii. Phosphorus


11.03CondensedElectronConfigurationsandExceptions

Foranatomwithmanyelectrons,wecanuseashortcuttoexpressitselectronconfiguration

usingthemostimmediatepreviousnoblegas.

• Forexample,theelectronconfigurationofcalciumis1s22s22p63s23p64s2.Usingtheshortcut,wecanexpressthisas[Ar]4s2.

• Writetheelectronconfigurationforchlorineusingtheshortcutdescribedabove.

FillingOrderofElectrons

Thedorbitalsofanenergylevelarefilledafterthesorbitalofthenextenergylevel.Forexample,the4ssublevelisfilledbeforethe3dsublevel.

Thediagrambelowillustratesthegeneralorderforfillingorbitals:

1s ®

2s ® 2p

3s ® 3p

4s ® 3d ® 4p

Writetheelectronconfigurationforvanadium,usingthenoblegasshortcut.

1. Theelectronconfigurationofanatomis1s22s22p63s23p64s23d5,whichreferstothegroundstateof

A. Mn.

B. Fe.C. Ca.

D. Ti.


ExceptionstotheAufbauPrinciple

Certainmetalswillviolatetheaufbauprinciplebecausetheyhaveincreasedstabilityduetoa

half-filledorfilleddsublevel.Theelectronconfigurationsofchromiumandcopper,two

exceptionstotheaufbauprinciple,areshownbelow:

Cr:1s22s22p63s23p64s13d5or[Ar]4s13d5

Cu:1s22s22p63s23p64s13d10or[Ar]4s13d10

2. Usetheperiodictabletogivethesymboloftheelementforeachofthefollowingelectron

configurations:

i. 1s22s

22p

5

ii. [He]2s22p

63s

1

iii. 1s22s22p63s23p64s13d10

iv. [Ar]4s23d5


11.04CategoriesofElectrons

Therearethreecategoriesofelectronsinanatom:

• Inner(core)electrons-Theelectronsinthelowerenergylevelsofanatomuptothe

mostimmediatepreviousnoblegas,plusanycompletedtransitionseries

Fe:[Ar]4s23d6®18innerelectrons

• Outerelectrons-Theelectronsinthehighestenergylevel(nvalue)ofanatom

Zn:[Ar]4s23d10®2outerelectrons

• Valenceelectrons-Theelectronsinvolvedincompoundformation

Formaingroupelements(1A–8A),thevalenceelectronsaretheouterelectrons.Forthetransitionmetals,allthe(n–1)delectronsarecountedaspartofthevalenceelectronsunlessthedsublevelisfilled.

Ge:[Ar]4s23d104p2®4valenceelectrons

Fe:[Ar]4s23d6®8valenceelectrons

Valenceelectrons=Totalelectrons-Innerelectrons

1. Selectthefalsestatementbelow.

A. TheVatomhas2outerelectrons.

B. ThePatomhas5valenceelectrons.

C. TheCuatomhas18innerelectrons.

D. TheNaatomhas10innerelectronsand1outerelectron.


11.05TrendsinAtomicRadii

Atomicradius–Whentwoatomsofthesameelementarejoinedtogether,onehalfofthe

distancebetweenthenucleiofthetwoatomsistheatomicradius.

• Theatomicradiusisthebestwaytopredictthesizeofanatom.

• Therearetwotrendsthathelppredictthesizeofanatom:

o Astheatomicnumberincreasesdownagroup,thechargeonthenucleus

increasesandthenumberofoccupiedenergylevelsincreases.

§ Thereisanincreaseinnuclearchargeandanincreaseinshielding,but

theshieldingeffectisgreaterthantheeffectoftheincreaseinnuclear

charge.

§ Thisresultsinanincreaseinsizedownagroup.

o Acrossaperiod,eachelementhasonemoreprotonandelectronthanthe

precedingelement.

§ Theshieldingeffectisconstantforallelementsinaperiod,whilethe

nuclearchargepullstheelectronsinthehighestoccupiedenergylevel

closertothenucleus.

§ Thisresultsinanincreaseinsize,righttoleft,acrossaperiod.

• Ingeneral,atomicradiiincreasefromtoptobottomwithinagroupandincreasefrom

righttoleftacrossaperiod.

1. Ineachpair,whichelementislarger?

i. Na,Rb

ii. Cl,Na

iii. Se,Br


11.06TrendsinIonizationEnergy

Ionizationenergy–Theenergyrequiredtoremoveanelectronfromanatom

• Ionizationenergyismeasuredwhentheatomisinitsgaseousstate.

Example:K(g)+energy(ionization)®K+(g)+e

-

• Thefirstionizationenergyistheenergyrequiredtoremovethefirstelectronfroman

atom.

• Ingeneral,thefirstionizationenergytrendistheoppositeoftheatomicradiustrend.

o Firstionizationenergytendstoincreasefrombottomtotopwithinagroupand

increasefromlefttorightacrossaperiod.

o Astheatomicnumberdecreasesfrombottomtotopinagroup,thevalence

electronsareclosertothenucleus,sotheyarehardertoremove.

o Asyougolefttorightacrossaperiod,theincreaseinnuclearchargecausesan

increaseintheattractionbetweenthenucleusandanelectron.

1. Usingonlytheperiodictable,ranktheelementsineachofthefollowingsetsinorderof

decreasingfirstionizationenergy:

i. P,Mg,O

ii. K,Ca,Sr

iii. I,Xe,Rb


11.07ElectronConfigurationforIons

• Anatomiselectricallyneutralbecauseithasanequalnumberofprotonsandelectrons.

Anionformswhenanatomorgroupofatomslosesorgainselectrons.

o Acation,orapositivelychargedion,isproducedwhenanatomlosesoneor

morevalenceelectrons.

o Ananion,oranegativelychargedion,isproducedwhenanatomgainsoneor

morevalenceelectrons.

• Thefirstelectronremovedbyanatomwillcomefromthehighestoccupiedenergylevel

(n)becausethoseelectronsexperiencelessnuclearcharge.

Ifthereismorethanonesubshellinthehighestoccupiedenergylevel,theatom

removeselectronsinthefsubshellfirst,thenthedsubshell,thenthepsubshell,andfinallythessubshell.

• Thefirstelectrongainedbyanatomwillgotothelowestunoccupiedenergylevelthat

hasroom.

Ifthereismorethanonesubshellinthelowestunoccupiedenergylevel,the

atomgainselectronsinthessubshellfirst,thenthepsubshell,thenthedsubshell,andfinallythefsubshell.

• Inbecominganion,anatomgainsorloseselectronstoreachanoblegasconfiguration.

Noblegaseshaveverylowreactivitybecausetheirhighestoccupiedenergylevelisfull

ofelectrons,whichmakesthemverystable.

• Somecationsdonotformcationswithanoblegasconfiguration;instead,theybecome

cationswithtwodifferentstableconfigurations:

o Pseudo–noblegasconfigurationoccurswhenanelementemptiesoutits

highestsandpsubshellstoleaveafilleddsubshellbehind.

o Inertpairconfigurationoccurswhenanelementemptiesoutitshighest

occupiedpsubshelltoleavefilledsanddsubshells.

1. Writetheelectronconfigurationforthefollowingions.

i. Al3+

ii. Se2-

iii. Ga3+

iv. Cr3+


2. Howmanyunpairedelectronsarepresentinthefullground-stateelectronconfigurationof

themonatomicionmostlikelytobeformedbyMg?

A. 1B. 2C. 3

D. Nounpairedelectrons


11.08TrendsinIonicRadii

• Theionicradiusistheapproximatedistancefromthecenterofaniontoitshighest

occupiedelectronorbital.Weobservethefollowingphenomena:

o Cationsaresmallerthantheparentatom–Whenelectronsareremoved,the

nuclearchargeincreases,sothesizeoftheatomdecreases.

Example:Na>Na+

o Anionsarelargerthantheparentatom–Whenelectronsaregained,thenuclear

chargedecreases,sothesizeoftheatomincreases.

Example:Cl->Cl

• Ionicradiifollowthetrendofatomicradii,increasingtoptobottomwithinagroupand

increasingrighttoleftacrossaperiod.

1. Ineachpair,whichionissmaller?

i. K+,Na

+

ii. Br-,Se

2-

iii. Fe2+,Fe3+


�


Documents

Section 11: Electron Configuration and Periodic Trends · Constructing an Electron Configuration The aufbau principle states that the lowest energy orbitals in an atom are occupied