Why “periodic?”Why “periodic?”

Examination of properties reveals Examination of properties reveals whywhy

Learning objectivesLearning objectives

Define ionization energy and electron affinityDefine ionization energy and electron affinity Describe periodic trend in atomic and ionic Describe periodic trend in atomic and ionic

radius and ionization energyradius and ionization energy Predict order of atomic/ionic sizes using Predict order of atomic/ionic sizes using

concept of shielding and periodic tableconcept of shielding and periodic table

Properties show periodic variationProperties show periodic variation

Periodic trends in atomic sizePeriodic trends in atomic size

Atoms and ionsAtoms and ions

Ions are created by Ions are created by removing or adding removing or adding electronselectrons Positive ions are Positive ions are smallersmaller than the neutral than the neutral atomsatoms

Negative ions are Negative ions are largerlarger than the neutral atomsthan the neutral atoms

IsoelectronicIsoelectronic ions ions Isoelectronic ions have same number of electronsIsoelectronic ions have same number of electrons

Na [Ne]3sNa [Ne]3s11; Mg [Ne]3s; Mg [Ne]3s22; Al [Ne]3s; Al [Ne]3s223p3p11

NaNa++ [Ne] [Ne]++; Mg; Mg2+2+ [Ne] [Ne]2+2+; Al; Al3+3+ [Ne] [Ne]3+3+

P [Ne]3sP [Ne]3s223p3p33; S [Ne]3s; S [Ne]3s223p3p44; Cl [Ne]3s; Cl [Ne]3s223p3p55

PP3-3- [Ar] [Ar]3-3-; S; S2- 2- [Ar][Ar]2-2-; Cl; Cl-- [Ar] [Ar]-- Isoelectronic cations, higher charged ions are Isoelectronic cations, higher charged ions are

smaller (nuclear attraction is stronger)smaller (nuclear attraction is stronger) NaNa++ > Mg > Mg2+ 2+ > Al> Al3+3+

Isoelectronic anions, higher charged ions are Isoelectronic anions, higher charged ions are larger (nuclear attraction is weaker)larger (nuclear attraction is weaker) PP3- 3- >> SS2-2- > Cl > Cl--

Ionization energy: energy required to Ionization energy: energy required to remove electron from isolated gaseous remove electron from isolated gaseous

atom: A(g) = Aatom: A(g) = A++(g) + (g) + ee

Electron affinity: energy released when Electron affinity: energy released when electron is added to isolated gaseous electron is added to isolated gaseous

atom: A(g) + atom: A(g) + ee = A = A--(g)(g)

Explain these trendsExplain these trends

Atomic radius decreases across period, Atomic radius decreases across period, even though atomic number increaseseven though atomic number increases

Ionization energy increases – electrons Ionization energy increases – electrons more tightly heldmore tightly held

Shielding and effective nuclear chargeShielding and effective nuclear charge The “shell” picture helps to explain these observationsThe “shell” picture helps to explain these observations Electrons in same shell experience stronger attraction to Electrons in same shell experience stronger attraction to

nucleus as shell fillsnucleus as shell fills

Nearly full – high charge

Nearly empty –

low charge