Chapter 8 Lecture Notes

Al e al ea 44 kjmol still small but positive si e si

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Al + e → Al EA = +44 kJ/mol still small but positive Si + e → Si EA = +134 kJ/mol P + e → P EA = +72 kJ/mol S + e → S EA = +200 kJ/mol Cl + e → Cl EA = + 349 kJ/mol Ar + e → Ar EA <0, we cannot add an electron to the filled shells of Ar Note that for Mg to Al to Si the EA increases, but then we have EA(P)<EA(Si). This is because the half-filled p-shell of P is favourable, and an added electron would have to go in with antiparallel spin. Then again EA(P)<EA(S)<EA(Cl), but Ar with its filled s 2 p 6 configuration cannot accept another electron at all. The trend from top to bottom within a group also has no regularity, as you can see from the alkali metals and the halogens. Electron Affinities are quantities mainly of theoretical interest. While Ionization Energies can be determined from spectroscopic measurements, EA values are calculated indirectly, and in some cases can be found only from theoretical calculations. Still, EA values will 8
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be important to us, and to chemistry, when we combine EA and IE values in a new quantity called the electronegativity (EN), which we will encounter in Chapter 9. Don’t confuse Electron Affinity and Electronegativity! Example . 1. Of the following pairs of atoms, predict which atom wil have the highest EA value. a) Li and Be b) F and O c) F and Ne d) C and N e) Be and B Answer . a) Li, because Be has a s 2 configuration; b) F, because it is "to the right” of O; c) F, because Ne will not have a positive EA d) C (!) because adding an electron to N’s p 3 with three parallel spins, where the added electron would have to be antiparallel, is unfavourable e) B, because Be will have a very low or zero EA, just like Mg. Exercises 1. Write the electron configuration of a) Ca 2+ b) F c) Sc 3+ d) Mn 2+ e) Cu 2+ f) Sn 4+ , g) Pb 2+ h) S 2− i) I 2. For each of the following pairs, which atom or ion has the largest radius? a) Na and Mg b) K and Rb c) S and Cl d) F and Ne e) C and N f) Na and Na + g) F and F h) K + and Ca 2+ i) S 2− and Cl 3. For each of the following pairs, which atom has the highest 1 st IE value? a) Na and K b) Na and Mg c) C and N d) N and O e) O and F f) Na and Na + 4. Explain in words why the IE of N is higher than the IE of O 5. Explain in words why the Electron Affinity (EA) of Mg is lower than the EA of Na Additional problems in Chang: Chapter 8: problems 8.22, 8.24, 8.26, 8.32, 8.38, 8.44, 8.52, 8.62, 8.64 9
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