Transcript Slide 1
What’s coming up??? • • • • • • • Oct 25 Oct 27 Oct 29 Nov 1 Nov 3,5 Nov 8,10 Nov 12 The atmosphere, part 1 Midterm … No lecture The atmosphere, part 2 Light, blackbodies, Bohr Postulates of QM, p-in-a-box Hydrogen and multi – e atoms Multi-electron atoms Ch. 8 • Nov 15 Periodic properties Ch. 10 • • • • • • • Periodic properties Valence-bond; Lewis structures Hybrid orbitals; VSEPR VSEPR MO theory MO theory Review for exam Ch. 10 Ch. 11 Ch. 11, 12 Ch. 12 Ch. 12 Ch. 12 Nov 17 Nov 19,22 Nov 24 Nov 26 Nov 29 Dec 1 Dec 2 Ch. 8 Ch. 9 Ch. 9 Ch. 9 Ch.9,10 SCREENING AND PENETRATION PENETRATION Is to get close to the nucleus SCREENING Is to block the view of other electrons of the nucleus LOOK AT RADIAL PROBABILITY DISTRIBUTIONS In a multi-electron atom Zeff(s) > Zeff(p) > Zeff(d) ENERGY LEVELS REVERSE!! E4s < E3d En A Z 2 eff 2 n 0.25 S-orbital penetrates closer to nucleus 0.2 0.15 0.1 0.05 0 0 5 10 15 20 25 30 Closer proximity to nucleus Higher effective nuclear charge, Zeff En A Z 2 eff 2 n The result is that 4s lies below 3d 5s 4d 4p 3d 4s 3p E 3s 2p 2s 1s MAGNETIC PROPERTIES Atoms, molecules or ions with all electrons paired are diamagnetic. Diamagnetic materials are weakly repelled by magnetic fields Example: NEON 1s Ne 2s 1s22s22p6 ALL ELECTRONS PAIRED 2p Spin quantum number Electrons are influenced by a magnetic field as though they were spinning charges. They are not really, but we think of them as having “spin up” or “spin down” levels. MAGNETIC PROPERTIES Atoms, molecules or ions with at least one unpaired electron are paramagnetic. Paramagnetic materials are strongly attracted to magnetic fields. Nitrogen atom is PARAMAGNETIC 1s N 1s22s22p3 2s 2p • Which of these is paramagnetic? – Na Ca – K+ O2 PERIODIC TRENDS We will look at some trends in experimentally determined properties such as: ATOMIC RADII IONIC RADII IONIZATION ENERGIES ELECTRON AFFINITIES TRENDS IN ATOMIC RADII Atomic radii increase down the group ATOMIC RADIUS ATOMIC RADIUS FOR GROUPS 1,2 and 13 to 18 Orbitals bigger as n increases Radial probability density extends further. TRENDS IN ATOMIC RADII Atomic radii decrease across a period ATOMIC RADIUS ATOMIC RADIUS ATOMIC RADIUS FOR GROUPS 1,2 and 13 to 18 SCREENING AND PENETRATION ATOMIC RADIUS DECREASES ACROSS PERIOD! WHY? AS WE ADD ELECTRONS The electrons go into same shell and do not screen the nucleus from each other As nuclear charge goes up, so does Zeff Therefore radial probability pulled in…. En A Z 2 eff 2 n Zeff increases .. no screening .. radius decreases 0.12 0.1 0.08 0.06 0.04 0.02 0 0 5 10 15 20 25 30 Zeff INCREASES 1 1 2 3 4 5 6 7 1s 2 18 RADIUS DECREASES 2s 3s 13 14 15 16 17 1s 2p 3p 4s 3d 4p 5s 4d 5p 6p 6s La 5d 7s Ac 6d DOWN GROUP... 4f 5f 2 3 4 5 6 7 1s 2 2s 3s 4s 5s 6s La 7s Ac 18 Zeff DECREASES 1 RADIUS INCREASES 1 13 14 15 16 17 1s 2p 3p 3d 4p 4d 5p 5d 6p 6d 4f 5f TRENDS IN IONIC RADII IONIC RADIUS Same reasons as for atomic radii……. Cations and anions…... IONIC RADIUS IONIC RADIUS FOR GROUPS 1,2 and 13 to 18 Cations are smaller than their corresponding neutral atoms. Na is 186 pm and Na+ is 95 pm One less electron electrons pulled in by nuclear charge Anions are larger than their corresponding neutral atoms. F is 64 pm and F- is 133 pm same nuclear charge and repulsion among electrons increases radius O < O– < O2– QUESTIONS…... EXAMPLES Which is bigger? Na or Rb Rb …. higher n, bigger orbitals K or Ca K …. poorer screening for Ca Ca or Ca2+ Br or Br- What about Ca …. bigger than cation Br …. smaller than anion ISOELECTRONIC SPECIES? QUESTION The species F-, Na+,Mg2+ have relative sizes in which order? 22s22p6 ALL 1s + 2+ 1 F < Na <Mg ALL are isoelectronic 2 F-> Na+>Mg2+ 3 Na+>Mg2+> F- 4 Na+=Mg2+= F- 5 Mg2+> Na+>F- QUESTION The species F-, Na+,Mg2+ have relative sizes in which order ? 1 F-< Na+<Mg2+ 2 F-> Na+>Mg2+ Check numbers 3 Na+>Mg2+> F- 4 Na+=Mg2+= F- 5 Mg2+> Na+>F- Na+ is 95 pm Mg2+ is 66 pm F- is 133 pm ELECTRON AFFINITY the energy change associated with the addition of an electron to a gaseous atom. X + e– X– F(g) + e– F–(g) DE= -328 kJmol-1 TWO DEFINITIONS! EA = DE the electron affinity is negative if DE<0. EA = -DE the electron affinity is positive if DE<0. In general no clear cut trends……. MORE NEGATIVE ELECTRON AFFINITY ELECTRON AFFINITY MORE NEGATIVE MORE NEGATIVE ELECTRON AFFINITY TRENDS IN EA ELECTRON AFFINITIES 400 Cl F -ELECTRON AFFINITY 300 S 200 C H 100 Si O Li Na B 0 -100 0 1 2 He 3 4 Be Al 5 6 7 8 9 10 11 N 12 13 15 16 17 18 Mg Ne -200 ATOMIC NUMBER What is special about … He, Ne and Ar? Be, N and Mg? 14 P Ar 19 • He, Ne, Ar: rare gases … adding electron to filled shell … must increase n • Be, Mg: alkali earths … adding electron to filled sub-shell … must increase l • N: Hund’s rule stability… adding electron to ½-filled degenerate p-shell • Q: what would you predict for… Cr, Fe? TRENDS IN IONIZATION ENERGIES M(g) M+(g) + eThe ionization energy of gaseous atoms of the elements have been measured…. And we find……. First Ionization energies decrease down the group IONIZATION ENERGY Zeff DECREASES IONIZATION ENERGY TRENDS IN FIRST IE Zeff INCREASES UP THE GROUP Electrons closer to nucleus more tightly held TRENDS IN FIRST IE Zeff INCREASES IONIZATION ENERGY Greater effective nuclear charge across period Poor shielding by electrons added n IONIZATION ENERGY I . E. A Z 2 eff 2 TRENDS IN FIRST IE IONIZATION ENERGY(kJ/mol) 2500 n=1 n=2 2000 n=3 1500 n=4 1000 500 0 0 11 2 2 155 166 133 144 GROUP NUMBER 177 8 18 9 Closed shells most stable The noble gases have the highest ionization energy!