03periodicproperties-2

03periodicproperties-2 - Chemical Periodicity & the...

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Unformatted text preview: Chemical Periodicity & the Periodic Table Chemical Principles of Marine Systems SIO141/CHEM174 Friday, April 3, 2009 1 2 Can use WebElements to get this http://www.webelements.com 3 Shielding & Ionization Energies Energy of an electron in an atom is f(z2/n2) Thus would expect IE(Li) > IE(H) ? It's clearly not! IE(Li) = 513.3 kJ mol1, whereas IE(H) = 1312 kJ mol1 Why? 4 Shielding & nuclear charge Shielding or screening of the nucleus occurs, caused by the inner electrons so that the valence electron only "sees" part of the nuclear charge: an effective nuclear charge (Z* = z S). 5 Slater's rules (empirical) 1. 2. Z* = z - S, where z is the atomic number of the element and S is a screening (or shielding) constant. To determine S, electrons are divided into the following groups, each having a different shielding constant: 1s; 2s, 2p; 3s, 3p; 3d; 4s, 4p; 4d; 4f; 5s, 5p; etc. Thus the s and p of a given shell are grouped together, but the d and f are separated. The shells are considered to be arranged from inside out in the order named. The shielding constant S is formed as the sum of the following contributions: a. nothing from any shell outside the one considered; b. an amount 0.35 from each electron in the group considered (except in the 1s group, where 0.30 is used instead); c. if the shell considered is an s, p shell, an amount 0.85 from each electron in the next inner shell, and 1.00 from all electrons still further in; but if the shell is a d or f, an amount 1.00 from every electron inside it. 3. 6 Takes account of orbital filling order 7 An example Estimate the effective nuclear charge exerted on the outermost electron on the oxygen atom 8 Oxygen 1. First write down the full electronic structure 1s2, 2s2, 2p4 2. Divide it into groups (1s2), (2s2, 2p4) 3. S = ? x 0 + ? x 0.35 + ? x 0.85 + ? x 1.0 =5 x 0.35 + 2 x 0.85 = 3.45 4. Z* = z S = 8 3.45 = 4.55 9 Q1 in-class exercise Calculate using Slater's rules the Z* exerted on the outermost electron for each of the atoms in the 3rd period (Na to Ar). 10 Effective nuclear charge (Z*) Na"2.20 Mg"2.85 Al""3.50 Si""4.15 P" "4.80 S" "5.45 Cl""6.10 Ar""6.75 11 Q2 in-class exercise Use this information to explain (a) the trend in ionization energies, and (b) the trend in covalent radii. 12 13 Covalent radii 14 Effective nuclear charge (Z*) Na"2.20 Mg"2.85 Al""3.50 Si""4.15 P" "4.80 S" "5.45 Cl""6.10 Ar""6.75 It takes more energy to remove an outer electron as one goes across the group. Hence there is an increase in ionization energy. The outer electron(s) are held more tightly as one goes across the group. Hence there is a decrease in atomic size. 15 Usually a more rigorous quantum mechanical approach is used Z* calculations based on the self-consistent field (SCF) calculations of Clementi and Raimond http://www.webelements.com 16 Slater's Rules Na"2.20 Mg"2.85 Al""3.50 Si""4.15 P" "4.80 S" "5.45 Cl""6.10 Ar""6.75 17 Q3 How do properties like ionization energy and covalent radius vary down a group? WHY? 18 Li""1.28 Na"2.51 K" "3.49 As one descends a group, n increases and the outermost electrons are further from the nucleus. 19 Q4 What is the structure within a period due to? 20 Covalent radii Atomic radii are a somewhat nebulous quantity since an atom, unlike a billiard ball, has no well defined boundary. Its size thus depends on the its molecular environment. C -- C (CH3CH3)" " "F -- F (F 2) 154 pm 142 pm whereas CH3 -- F 138 pm 21 22 1s Group 1 2 4 3 4 5 6 7 8 9 10 11 12 13 5 14 6 15 7 16 8 1 2 H 17 9 He 18 10 1.008 4.003 s 3 Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra p B Al C Si Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At Ne Ar Kr Xe Rn 6.941 9.012 12 11 22.99 24.30 20 19 39.10 40.08 38 37 85.47 87.62 56 55 132.9 137.3 88 87 (223) (226) 10.81 12.01 14.01 16.00 19.00 20.18 15 18 14 17 16 13 26.98 28.09 30.97 32.06 35.45 39.95 33 36 32 35 34 31 d 21 22 23 24 25 26 27 28 29 30 Sc Y Lu Lr Ti Zr Hf Rf V Nb Ta Db Cr Mo W Sg Mn Tc (98) 75 Fe Ru Os Hs Co Rh Ir Mt Ni Pd Pt Ds Cu Ag Au Rg Zn Cd Hg Ga In Tl 44.96 47.87 50.94 52.00 54.94 55.84 58.93 58.69 63.55 65.38 69.72 72.64 74.92 78.96 79.90 83.80 46 53 43 52 41 45 49 42 51 47 54 44 50 40 48 39 88.91 91.22 92.91 95.96 72 73 74 71 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 78 85 84 77 81 83 79 86 76 82 80 Re Bh 175.0 178.5 181.0 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222) 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 (262) (267) (268) (271) (272) (270) (276) (281) (280) (285) (284) (289) (288) (293) UUb UUt UUq UUp Uuh UUs UUo (?) (294) f 57 58 59 60 61 62 63 64 65 66 67 68 69 70 La Ac Ce Th Pr Pa Nd U Pm Np Sm Pu Eu Am Gd Cm Tb Bk Dy Cf Ho Es Er Fm Tm Md Yb No 2007 138.9 140.1 140.9 144.2 (145) 150.4 152.0 157.2 158.9 162.5 164.9 167.3 168.9 173.0 95 92 90 94 98 91 100 101 102 96 93 89 99 97 (227) 232.0 231.0 238.0 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) 23 Ionic radii 24 Ionic radii of lanthanide(III) ions 125 3+ La 3+Pm3+ Pr 3+ 3+ Eu Ce 3+ Nd 3+ Tb 3+ 3+ Sm Ho 3+ 3+ 3+ Tm Gd Lu 3+ Dy 3+ Er Yb3+ 55 60 65 Atomic Number 70 r / pm 100 75 25 26 27 Simple periodic trends 28 Trends in chemical properties Regular change in valency found: LiH, BeH2, "BH3", CH4, NH3, H2O, HF Na2O, MgO, Al2O3, SiO2, P2O5, SO3, Cl2O7 29 Oxidation states It is convenient to define oxidation state as the formal charge remaining on an element once the other atoms have been removed as their normal ions: NH4Cl (4H+ + Cl) = N3"" " N(III) KMnO4 (K+ + 4O2) = Mn7+" Mn(VII) Note: arabic numbers denote ionic charge, roman numerals oxidation state (CrVIO42) 30 Q5 What are the oxidation states of the following species: 2+, HgCl 2, Ge(OH) , Mg 4 4 2+, IO ? UO2 3 31 Oxidation states of the elements. More common states in red. 32 33 Homework Download the table of electron configurations for the elements (or use WebElements) 1. Find those elements whose ground state configuration does not adhere to the electron filling order in the diagram given earlier. 2. The typical oxidation state for lanthanide ions is (III), however two of them have alternate stable oxidation states. Which ones? 34 Homework (cont.) 3. Is there a related anomaly in their ionic radii (for 3+ ions), or in their ionization energies? 4. Find out the atomic radii of the group 2 atoms (Be Ra). Explain what you find. 5. What is meant by "electronegativity"? How and why does it vary along the periodic table? (In addition to a "broad brush" approach, describe and explain some of the structure in the plot.) 35 ...
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This note was uploaded on 09/11/2009 for the course SIO 141 taught by Professor Dickson,a during the Spring '08 term at UCSD.

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