250 CHAPTER15. IDENTICALPARTICLES
energyincreasesbecause Z=4,andthebindingenergyofeachofthe 2 sstatesis
greater. Butalthoughthe 2 sshellisfilled,the 2 pshellisquitecloseinenergy,and
thepresenceofanearbyatommaybeenoughtocausearearrangementinwhichone
ofthe 2 selectronsgoesintothe 2 pstate. SoBerylliumisnotaschemicallyinertas
Helium. Ingeneral,themoststableelectronconfigurationsarethoseforwhich: (i)
thevalencelevelisfilled;and(ii)thereisalargejumpinenergytothenextlevel;
e.g. whenallllevelsforagivennarefilled.
Roughly, then, this isthe picture: The various electronlevels can be lumped
together into groups whichhave aboutthe same energy. These groups, andthe
maximumnumberofelectronsallowedineachgroup,are
1 s 2 electrons
2 s, 2 p 8 ”
3 s, 3 p 8 ”
4 s, 3 d, 4 p 18 ”
5 s, 4 d, 5 p 18 ”
6 s, 4 f, 5 d, 6 p 32 ”
7 s, 6 d, 5 f,...(15.71)
AsonegoesupinZineachgroup,thebindingenergytendstoincrease,duetothefact
thatchargeofthenucleusincreases.Thustheionizationenergybuildstoamaximum,
whichisthecasewhenalloftheenergylevelsinagrouparefilled,asseeninFig.
[15.4]. Thesemaximaoccur fortheinert gases: Helium,Neon,Argon,Krypton.
However,asZ increasesfurtherandthelevelsofthenexthighergroupbegintobe
filled,theionizationenergydropssharplyfromitsvalueattheinertgas.Thereason
isthatthesuddendecreaseinbindingenergyofthenexthighershellpredominates
overtheincrease inZ. Itshouldbenotedthat unlessashelliscompletelyfilled,
ahigh ionizationenergy does notmeanthat an elementis necessarilyveryinert.
Chlorine,forexample,hasanionizationenergy(13eV)whichisnearlyashighas
theinertgasArgon(15. 8 eV).ButthevalencelevelofChlorineisnotfilled;thereis
roomforonemoreelectron.Andbecausethebindingenergyofthevalencelevelisso
high,itispossibleforChlorinetocaptureanelectron,therebyfillingitsvalencelevel.
Thisiswhy,whenSodiumandChlorinearebroughtintoproximity,itisenergetically
favorableforSodiumtoloseits valenceelectron(this onlycosts 5. 1 eV),whichis
morethancompensatedbyaloweringoftheChlorineenergybythevalenceelectron
boundstateenergy(− 13 eV).ThepositivelychargedSodiumionisthenboundtothe
negativelychargedChlorine ionbyordinaryelectrostaticattraction. Theresulting
compound,asyouprobablyknowfromhighschoolchemistry,^5 isordinarytablesalt:
NaCl.
Thegroupsshownabovein(15.71)areassociatedwithrowsintheperiodtable.
Thefirstgroup,correspondingtothe1slevel,isoccupiedonlybyHydrogenandHe-
lium.Thenexttwogroupsarethe”short”rowsofthePeriodicTable,andconstitute
(^5) Dotheystillteachchemistryinhighschool?