Parameters a, b and c (in eV) for Fitting the First Four Ionization Potentials to Eq 2"
Table 2.
b
0
a
2
13.60
16.97
c
4.12
$5
EA (calc)
EA (exp)
atom
1
4-0.75
4-0.75
H-
8.2 14.8 32.0 46.9 64.1 96.9 122.2 153.0
12 3 8 9 12 7 9 4
-0.6 +0.4 -0.9 -0.2 +0.8 -0.7 +0.9 +2.9
1.12
C-
1.47 3.45
0-
15 15 9 0.3 18 8 4 4
-1.4 -0.2 -1.0 4-0.1 +1.4 +0.2 +1.7 +3.2
2.07 3.61
Cl-
15
-I-2.9
3.36
Br-
3 4 5 6 7 8 9 10
3.439 3.442 3.473 3.470 3.462 3.532 3.521 3.535
11.2 15.1 22.1 26.8 31.3 38.7 43.3 48.5
11 12 13 14 15 16 17 18
1.705 1.679 1.694 1.764 1.828 1.687 1.685 1.680
29.3 30.7 35.4 39.6 43.9 42.2 44.3 46.3
121 135 179 217 258 253 279 305
36
1.234
76.6
1171
F-
S-
a 6 is the root mean square deviation in l o 4 eV. EA is electronaffinity.Calculated EA's were extrapolated by the fit to the IP's of the appropriate isoelectronic atoms. Experimental EA's and IP's taken from Tables E-64 and E-65 of ref. 10.
larger. I t can be understood by arguing that the "effective core" containing the nucleus with the first Q - 1 electrons becomes larger with increasing Q. The energy levels therefore become more closely spaced, which is manifested in a smaller effective quantum number. Along these lines we can understand the observation that n*/n decreases with increasing n (e.g., for Li, n = 2, n* = 1.98, while for Na, n =3, n* is 2.86), and that in the first three rows of the periodic table n* decreases monotonically with Q. For potassium, no increase in n* is observed, even though n has increased from 3 to 4..Instea&the^values of n* seem to continue the trend of the third row elements. A jump in n* to above 3 is observed only at the copper series (Q = 29), where the electronic configuration is 3d104s1. This is in line with the fact that up to copper, the lower lying 3d orbitals are being filled. Only
when Q = 29 is a "truly" 4s atom being ionized. A similar effect is observed for the fifth-row elements, where there is a sudden increase in n* for the silver series (Q = 47), whose electronic configuration is 4dlo5s1. Finally, let us turn attention to the question of determining electron affinities (EA). Already Glockler (6) has suggested to use the fit to the isoelectronic series in order to determine EA's by extrapolating to Z = Q - 1.For example, the EA of fluorine would be determined from extrapolating the Neon series (Q = 10) to Z = 9. I t is quite difficult to obtain accurate EA values in this way since one is attempting to determine small numbers from the extrapolation of very large numbers. One must therefore use a very accurate fit to the first few IP's in a series. We find that the best agreement with experimental EA's (10) is obtained by fitting eq 2 to the first 4 IP's in each series. the results are collected in Table 2. The calculated values are always somewhat smaller (0.4-0.5 eV) than the experimental, so that by adding a constant to the calculated values reasonable estimates of EA's may be obtained. This extrapolation is generally not reliable outside the first three rows in the periodic table. We conclude that a treatment of IP's of isoelectronic atoms somewhat more quantitatively than in most chemistry textbooks is indeed informative. The single-electron picture, eq 1, furnishes interesting observations regarding the trends in the screening and effective quantum-number parameters. The role of any real theory is to explain more fully these observed trends. Acknowledgment
I thank M. Cohen, M. Klapisch, D. R. Herschbach, and S. D. Peyerimhoff for discussions. Literature Cited 1. Mahan, B. H.; University Chemistry, 3rd ed.; Addison-Wesley: Reading, MA, 1975. 2. Pimentel, G. C.; Spratley R. D.,.Understanding Chemistry; Holden-Day: San Francisco, 1971. 3. Pauling, L. General Chemistry, 3rd ed.; Freeman: San Francisco, 1970. 4. Zener, C. Phys. Rev. 1930,36,51. 5. Slater, J. C. Phys. Rev. 1930,36,57. 6. Glockler, G. Phys. Rev. 1934,46,111. 7. Layzer, D. Ann. Phys. 1959,8,271. 8. Crossley, R. J. S. Proc. Phys. SOC.1964,83,375. 9. Cohen, M.; McEachran, R. P. Chem. Phys. Lett. 1981,84,622, and references therein. 10. Weast, R. C.; Astle, M. J., Eds. Handbook of Chemistry and Physics, 62nd ed.; CRC: Boca Raton, FL, 1981-1982. There is a misprint in line 8 of Table E-65.
Mnemonic for Z and E Nomenclature In systematic nomenclature the letters Z and E are used for specifying the configurations of geometrical isomers. Z is the first letter of the German word Zusammen, which means "together" and this is interpreted as meaning "on the same side". Similarly,E is the first letter of Entgegen, which means "opposite"understood to mean "on opposite sides". Z and E are used with reference to the ligands of highest atomic number priority. Thus the maleic and fumaric acids are Z and E isomers, respectively.
Maleic Acid
z
Fumaric Acid E
If the German words can be memorized, it is easily deduced that maleic acid is the Z isomer because zuSAMmEn contains parts of the English word Same. ,As an additional memory aid, we can observe that the shapes of the letters Z and E themselves, rewritten as
imply the reverse of the correct configurations. Using the latter method it is not necessary to memorize the German words. C. W. Thomas Bristol Polytechnic Frenchay, Bristol BS16 lQY,United Kingdom
14
Journal of Chemical Education
