12C1403Lecture 13, Wednesday, October 19, 2005Chapter 17 Many-Electron Atoms andChemical Bonding17.1 Many-Electron Atoms and thePeriodic Table (Done)17.2 Experimental Measures of OrbitalEnergies17.3 Sizes of Atoms and Ions17.4 Properties of the Chemical Bond17.5 Ionic and Covalent Bonds17.6 Oxidation States and ChemicalBonding3Summary: The Periodic Table built up by electron configurations: theground state electron configurations of the valence electrons of theelements417.2 Experimental Measures of OrbitalEnergiesPhotoelectron spectroscopy. Measuring the energy ofelectrons in orbitals by kicking them out with photonsPeriodic trends in ionization energies and electronaffinitiesEffective nuclear charge and screening by innerelectrons. How these influence ionization energies andelectron affinitiesIonization energies. How strongly atoms hold on to electronsElectron affinities. How strongly atoms add an electron5En = -(Zeff2/n2)Ry = energy of electron in orbitalrn = (n2/Zeff)a0 = “average” radius of a orbitalThe Bohr one electron atom as a starting point for theelectron configurations of multielectron atoms.Some important periodic properties of atoms:Energy required to remove and add an electron (En)Size of atoms (rn)Rules: (1) Larger Zeff more energy required to remove e-(2) Smaller r more energy require to remove e-Replace Z (actual charge) with Zeff (effective charge)6Rule: In many electron atoms, for a given value of n, the value of Zeff decreases withincreasing l, because screening decreases with increasing lFor a given n: s < p < d < fEffective nuclear charge: ZeffEffective nuclear charge, Zeff: the net positive charge attracting an electron in an atom.An approximation to this net charge isZeff(effective nuclear charge) = Z(actual nuclear charge) - Zcore(core electrons)The core electrons are in subshell between the electron in question and the nucleus.The core electrons are said to “shield” the outer electrons from the full force of thenucleusSince the energy of an orbital depends on Zeff, in a many electron atom, for a given valueof n, the energy of a orbital increases with increasing value of l.7Electron shielding (screening) of the nuclear charge byother electronsWhy is the energy of a 3s orbital lower than than of a3p orbital? Why is the energy of a 3p orbital lowerthan the energy of a 3d orbital?A qualitative explanation is found in the concept ofeffective nuclear charge “seen” by an electron8Effective charge, Zeff, seen by valence electrons*:Rule: Zeff increases going across a period ot the table*Note x-axis is incorrect. What should it be?9Let’s take a look at some experimental data:The ionization energy (IE) of an atom is the minimum energy requiredto remove an electron from a gaseous atom.X(g) X+(g) + e- The first ionization energy IE1 is the energy required to remove thefirst electron from the atom.The second ionization energy IE2, is the energy required toremove the second electron from the +1 positive ion of the atomand so on.Ionization energies (ionization potentials):10Periodic trends ionization energies of the representativeelements: What are the correlations across and down?11How do we obtain ionization energies?Photoelectron spectroscopy is an important technique that providesthe energies of electrons in different orbitals and providesoverwhelming evidence for the existence of shells and subshells.Photoelectron spectroscopy is the photoelectric effect explainedby Einstein applied to gases:A photon hν hits an electron and ejects it from the atom; part ofthe energy of the photon goes into overcoming the attraction ofthe electron for the nucleus (the ionization energy, IE) and theremainder appears as kinetic energy: hν = Φ (ΙΕ) + (1/2)mv2Or: Φ (ΙΕ) = hν - (1/2)mv2 (total photon energy - KE)12Photoelectron spectroscopy: the photoelectriceffect for ejecting electrons from gaseous atoms.1s 2s 2pΦ (ionization energy, IE) = hν - (1/2)mv2Energy diagram forNe: 1s22s22p61s2IonizedNe atom2s22p6-1.6 Ry-3.6 Ry-64 Ry0 Ry13Experimental data and theoretical ideasQuestion: Explain the “two slopes” for the ionizationenergies of carbon.Slope 2Slope 1146C 1s22s22p26C+5 1s12s02p06C+4 1s22s02p06C+3 1s22s12p06C+2 1s22s22p06C+1 1s22s22p1It takes more and more energy to remove an electron from anincreasingly positively charged atom.The first smaller slope is due to removal of n = 2 electrons, thesecond larger slope is due to removal of n = 1 electrons.2s1s2p15Looking for trends: Ionization energies in tabular formLots of data but hard to see trends16Periodic trends of the first ionization energies of the representativeelements mapped on the periodic table.IE IncresaseIE DecreaseAre correlations more apparent? What are they?17Looking for trends. Ionization energies as a graph: PeriodictrendsBig drop after noble gases:ns2np6 ns2np6(n +1)s118Ionization energies of the main group elements intopographical relief form: family relationships.Why is the IEof H so muchlarger than theIE of Li?Why are the IEsof the alkalimetals sosimilar?Why do noblegases have thehighest IE?19First and Second ionization energies in graphical formWhy is theIE of Ngreater thanthat of C orO?N OC20IE13Li [He]2s (↑) 4Be [He]2s2 (↑↓)5B [He]2s22p1 (↑ ) 6C [He]2s22p2 (↑↑)7N [He]2s22p3 (↑↑↑)8O [He]2s22p4 (↑↓↑↑)9F [He]2s22p5 (↑↓↑↓↑)10Ne [He]2s22p6 (↑↓↑↓↑↓)11Na [Ne]3s1Using electron configurations to explain Ionization EnergiesRemoval of an electron from a neutral atom: IE1En = -(Zeff2/n2)En = -(Zeff2) if n is fixed(across a period)En = -(1/n2) if Zeff is fixed(down a column)Zeff increases21IE2 (removal from E+)3Li+[He]+4Be+ [He] 2s1 (↑ )5B+ [He] 2s2 (↑↓)6C+ [He] 2s22p2 (↑ )7N+ [He] 2s22p3 (↑↑)8O+ [He] 2s22p4 (↑↑↑)9F+ [He] 2s22p5 (↑↓↑↑)10Ne+ [He] 2s22p6 (↑↓↑↓↑)11Na+ [Ne]+Using electron configurations to explain Ionization EnergiesRemoval of the second electron from the atomic cation: IE2Li+Na+HeNe22Periodic trends for the representative elements.Within a row (period) IE1 general increases withincreasing Z.Within each column (family or group) the IE1generally decreases with increasing Z.The energy needed to remove an electron from theoutermost occupied shell depends on both Eeff and reff.Eeff increases and reff decreases (both increase IE)as one goes across a period. Exceptions are due tospecial stability of subshells.23lA Family IE, Volts Valence