E. C. M. Chen University of Houston Victoria Center Victorio, Texos 77901 and W. E. Wentworth University of Houston Houston, Texos 77004
The Experimental Values of Atomic Electron Affinities
I
Their selection a n d periodic behavior
The electron affinity and the ionization potential are important physical properties of the elements and are discussed in freshman, organic, physical, inorganic, and analytical chemistry textbooks and have been discussed in several recent articles in this Journal (1-3). The electron affinity of an atom (A) is defined as the energy released when an atom and an electron react to form a negative ion in the gas phase a t O"K, i.e., the energy for the reaction A(g) + e
=
A-(g)
(1)
where T = 0°K; while the ionization potential is the energy required to remove an electron from an atom a t O°K. The atoms and ions are all assumed to he in their ground electronic state. Since most atoms have more than one electron, it is customary to consider the energy for the formation of the monopositive ion as the first ionization potential, the energy for the formation of the dipositive ion from the monopositive ion as the second ionization potential, and so forth. In keeping with this practice, B m k s , Meyers, Sicilio, and Nearing (1) suggest that the energy change for the reverse of reaction (1) he designated as the zeroth ionization potential. A knowledge of atomic electron affinities and ionization potentials is necessary for the understanding of any phenomenon or reaction involving atomic ions in the gaseous, liquid, or solid phases. Several illustrative examples are: (1) in gases, ion-molecule reactions, electron impact and attachment phenomenon, and radiation chemistry; (2) in solution, radiation chemist~y,kinetics and mechanisms of electrochemical processes, solvation energies, solution acidities, electron donor acceptor complexes, and the electronic spectra of negative ions; (3) in solids, the estimation of lattice energies, superconductivities, and the effects of radiation upon solids. In the theoretical area, experimental values can he used to test quantum mechanical calculations and various theories of electronegativities. Accurate experimental values of ionization potentials (2) have been known for some time hut, until very recently, there have been very few accurate experimental measurements of electron affinities. Prior to 1970, only the electron affinities of hydrogen, fluorine, chlorine, bromine, iodine, carbon, oxygen, and sulfur had been definitively measured. However, in the past four years, reliable experimental values for almost all of the group A elements and the group IB elements have been obtained. This dramatic increase in experimental results has been due to three factors: (1) the development of new intense negative ion sources; (2) the development of new, very accurate methods of measuring photodetachment cross-sections; and (3) the application of older methods of measuring electron affinities such as surface ionization and conventional photodetachment techniques to many other elements. These recent advances have generally not been communicated to those in the area of chemical education and have not even been included in standard reference sources such as the Chemical Rubber Handbook and Lange's Handbook of Chemistry. Therefore it is the purpose of this paper to first review the various methods for determining atomic electron affinities, then to select the current "best values" 486 / Journal of ChemicalEducation
of atomic electron affinities, and finally to correlate these values with their oosition in the oeriodic table. Estimates of the electron afknities of other-elements in the periodic table will he made from these correlations. Through the yeah. there ha\,e been a number of review articles concerning the methods of determining electron affinities 14-11,. Thus we wdl onlv hrieflv diicuss the soecific experimental techniques which have led to the current "best values" of atomic electron affinities. The most precise values of atomic electron affinities have been obtained from photodetachment thresholds, i.e. the onset for the reaction
In this method it is necessary to have: (1) a source of negative ions; (2) a light source; and (3) a method of measuring the detachment cross-section as a function of energy. There are two "conventional" photodetachment experiments: the crossed beam experiments pioneered by the group a t the National Bureau of Standards in Washington D.C. (6, 7, II), in which the electrons detached from a negative ion beam are measured as a function of the energy of the crossed light beam; and the measurement of the ahsorption spectra of negative ions generated by alkali halide vapors in a high temperature shock wave with a conventional high resolution spectrograph, developed by Berry and coworkers (10). These latter workers (10) also determined atomic electron affinities by measuring the emission snectra associated with the reverse of the vrocess given in eqn. (2). There are two new variations of the photodetachment method which have been developed and have improved the precision of electron affinities. The first uses a Rhodamine 6G laser with a one angstrom band width as a light source and determines the detachment cross-section by measuring the production of neutrals. The other new technique, photoelectron spectroscopy of negative ions, utilizes a fixed wavelength laser beam (4880 A) to cross a beam of negative ions and energy, analyzes the electrons photodetached in a small angle. The energy scale is calibrated with a oeak from a known electron affinitv. These techniques ha;e been developed and applied by'workers at the Joint Institute for Laboratow Astroohvsics a t Boul. . der, Colorado (12). The surface ionization method has contributed a number of our current "best values." Two variations have been used: the "self surface ionization" (13) procedure in which the ratio of oositive to neeative ions emitted from a heated filament of the test material is measured as a function of temperature and is related to the electron affinity; and the "relative" method in which the equilibrium between neutral atoms and negative ions in the vicinity of an inert heated filament is used to estimate electron affinities. Originally Bakulina and Ionov (14) measured the ratio of negative ion currents formed when beams of two different atoms simultaneously impacted a heated metal filament as a function of temperature. The typical Ln (illid versus 1/T plot gives the relative electron affinities. The procedure was modified by Zandberg and coworkers (15) by
-
also measuring the positive ion currents hut a t a single temperature. A knowledge of the ionization potentials of the two elements leads to the relative electron affinities. Iodine was originally used as the reference material but in the later work, silver was used. An absolute surface ionization value of 1.90 i 0.1 eV was used for the electron affinity of silver in order to convert the relative measurements to absolute ones (15). At that particular time, there were no other results for comparison. Later, the electron affinity of Ag was determined by photoelectron spectroscopy to be 1.303 i 0.00'7 eV and the electron affinities of several of the elements measured bv "relative" surface ionization measurements were obtained from conventional photodetachment studies. The use of the recent value for silver puts the surface ionization determinations in agreement with the photodetachment values and supports the photodetachment results which are considered the best. Two other less accurate techniques have supplied the only experimental values for two elements. These are the use of charge exchange cross-sections for the reaction of highly energetic negative ions (10-50 KeV) with a neutral gas and the use of appearance potentials for negative ions in a conventional mass spectrometer. In the cross-section measurements, the ratio of the electron affinity of the test element to the electron affinity of hydrogen is ohtained (16). Another type of experimental data which is available is the simple observation of the existence of various neeative ions. This generally implies a positive electron affin~;~ for that species, except in the case of certain long-lived metastable ions of elements with half or completely filled shells, which have been observed experimentally and have been predicted theoretically. The electron affinities of the rare gases, group 0, the alkaline earth metals, group IIA, and the group IIB elements can he considered together since they all have a filled shell and should have a negative or zero electron affinity. It was once thought that negative ions of these elements do not exist but, in fact, metastable He-, Be-, and Mg- have all been observed mass spectrometrically (17-19). These ions correspond to the negative ion formed from an excited state of the neutral. Thus the ground state for the negative ion lies above the ground state for the neutral and the electron affinity as normally defined is negative. For example, the ground state for H e is (ls.2s.2~) 4P,while those for Be- and M g are (ns,npZ) 4P.The definition of the electron affinity has been frequently modified to include positive values of metastable ions relative to the excited state of the neutral. Thus Weissf20) has calculated the "electron affinity" of Be and Mg (n.s,np) 3P to he 0.24 and 0.32 eV, respectively. Schulz (21) has recently reviewed negative ion states of this sort. In this article we will adopt the conventional definition of the electron affinity and consider the electron affinities of the group 0 and group I1 elements as being negative or zero. The "best values" for the electron affinities of the group I elements come from dye-laser photodetachment studies (12, 23, 24). The alkali metal results are confirmed by the conventional photodetachment results of Feldman, Rackwitz, Heinicke, and Kaiser (22). The best value for Li was ohtained by Patterson, Hotop, Kasdan, Norcross, and Lineberger from photoelectron spectroscopy which also yielded confirmatory results for Na, Rb, and Cs (12). The electron affinities of Cu, Au, and Ag have been determined by dye laser photodetachment or photoelectron spectroscopy (23, 24). Confirmation for Cu and Au is obtained from "relative" surface ionization measurements if the photoelectron spectroscopy value for Ag is used to obtain absolute electron affinities(1.5). The conventional photodetachment method has placed an upper limit of 0.5 eV on the electron affinities of B, Ga, In, and TI f22), members of group IDA. A conflicting
value of 0.86 eV has been ohtained for B from charge exchange cross-sections (16). There is no experimental value for the electron affinity of Al but a theoretical estimate of 0.52 eV has been obtained by Clementi and McLean (25). The only value for In comes from "relative" surface ionization measurements referenced to Ag (26) while the only value for TI comes from appearance potential measurements (27). Conventional photodetachment measurements have supplied electron affinities for C, Ge, Sn, and P b among the group IVA elements (22). The result for carbon has been confirmed by two groups (28, 29). Surface ionization measurements relative to silver are in aereement withthe photodetachment results for Ge and S : and supply the only value for Si (15.26). The electron affinities of the VA elements, except of N, have been determined by conventional photodetachment (22) and the values for Sh and Bi have been confirmed bv ~* surface ionization measurements relative to Ag (15, 26). The nitrogen negative ion has been observed by Fogel, Kozlov, and Kalmykov (30) from the reaction
+
N+ 2e = N (3) The cross-section for this reaction is quite small and it was concluded that the electron affinity of N is small, negative, or zero. Among the VIA elements, the values for 0 (31), S (32), and Se (33) have been determined by dye-laser photodetachment. The 0 and S values confirmed and extended the precision of earlier results. The value for Te was obtained by conventional photodetachment (22) and there is no experimental value for Po. The best current values for the electron affinities of the halogens, group VIIA, come from absorption and emission photodetachment. I t is interesting that the lack of precise agreement between the results of Popp (34), and Berry and Reimann (35) for F led the latter to reinvestigate their results and to discover an experimental artifact which caused a bias error in their results. Their more recent result (36) is in agreement with that of Popp. The results for C1 and Br have been obtained by two groups (37, 38) and are confirmed hy surface ionization measurements relative to I (39). The value for I was also obtained by
PERIOD Figure 1. Electron affinity of the elernentsverrus period
Volume 52. Number 8. August 1975 / 487
Atomic Electron Affinitie. (Valuer in parentheses are not experimental. See note added in proof1
0.75415
IIA
beam photodetachment (40). The only experimental value for At is a Born Haber Cycle determination in which an energy term has been extrapolated (41). The remaining elements with experimental electron affinities are Cr, by beam photodetachment f22), Pt, by dyelaser photodetachment, and Mo, Ta, W, and Re by selfsurface ionization (13). All of the experimental results are summarized in the table. The results which Lave been confirmed by two or more groups using the same or different techniques will probably not be changed in magnitude but will surely he determined more accurately as the new photodetachment methods are applied to more elements. The values given in parentheses are not determined experimentally. The theoretical value for the electron affinity of hydrogen is included in this table because of its greater precision, although experimental values agree with the theoretical calculation made by Perkeris (42) using a variational procedure with a 444 parameter wave function. The experimental electron affinities of the atoms are plotted against the period in Figure 1. The smooth variation for the group A families is quite good, especially for the second through the fifth periods. This allows one to estimate the electron affinities of At, Po, and Fr by a vertical extrapolation. For the IA, IIIA, and IVA groups, there is a decrease in going from the first to the second period while the opposite is true for the VA, VIA, and W A groups. 488 / Journal of ChemicalEducation
L
I 0
1
2
3
4
5
6
7
COLUMN Figure 2. Electron affinity for the elements versus group number: lines connect elements of the same period.
A plot of electron affinities of atoms versus column number is shown in Figure 2. The lines connect the elements in the same period. The values of the second and zeroth column are not experimentally determined. The regularity of these curves and the general increase with
Note Added in Proof
We have recently become aware of laser photoelectron speetrascopy values for B, 0.24 f 0.01 eV and Al, 0.46 f 0.03 eV. (W. C. Lineberger, A. Kasdan and J. Carlsten, to be published). In addition, a dye laser photodetachment value for the electron affinity of Te, 1.9708 0.0003 eV (W. C. Lineberger and J. Slater, to be published), and a conventional photodetachment value of the electron affinity of Ni, 1.15 + 0.10 eV (Kaiser, H. J., Rackwitz, R., and Feldman, D., Verhandl. DPG (VI) 9,393 (1974) have been recently obtained. These values are the "best values" and should be listed in the table and plotted in Figures 1, 2, and 3. The most significant change is the value for boron which confirms the earlier upper limit obtained by conventional photodetachment (22). These results were listed in a ureurint of a review entitled "Rinding Energies in Atomic ~ e g a t k eions" by H. Hotop and WIG. Lineberger to appear in The Journal of Physical and Chemical 20
40 ATOMIC
Figure 3. Electron atfinity for the elements perimental: theoretical or extrapolated.
60
80
NUMBER
versus atomic number. 0 Ex-
group number are a manifestation of the Pauli Principle which states that two electrons will stay apart if they have the same spin quantum number as the added electron. Thus the greater the number of electrons in the outer shell of the parent neutral atom having the same spin quantum number as the added electron, the greater the binding energy of the extra electron. The IIA elements have one electron with the same spin quantum number as the added electron while the group IVA elements have two such electrons, the group VA, none, the group VIA, one, and the group VIIA, two. This gives the trend shown in Figure 2, especially the decrease for the group VA elements. On the basis of the horizontal regularity exhibited in the filling of the p electrons, Zollweg (43) developed a method of estimating the B group and the group VIII elements. The experimental values of the IB elements were used to anchor the scale so that these estimates were necessarily revised when Hotop, Bennett, and Lineberger (23) obtained new experimental results for the IB elements. Zollweg (43) alsoused a horizontal analysis technique to very roughly estimate the electron affinities of some of the lanthanides. It is not the purpose of this article to elaborate upon these extrapolation procedures and we mention them only because in some cases they represent the only available estimates for these elements. Clementi and coworkers have calculated the electron affinities of all of the group B and VIII elements in the third row using Hartree Fock values and semi-empirical estimations of relativistic and correlation corrections (44). The results of this paper can be summarized by examining Figure 3 in which the electron affinities of the elements, including those discussed above, are plotted against the atomic number. The periodic nature of the electron affinity is quite apparent. The authors gratefully acknowledge the support of the Robert A. Welch Foundation Grant Number E-095.
Reference Data. Literature Cited I11 Bmoks. D . W., Meyen, E. A , Sieilio, F., and Nearing. J . C.. J. CHEM. EDUC.. 50.487 (1973). 121 Liabman. J . F . . J.CHEM. EDUC., 50. 831 (1973). 13) Holm. J.L., J.CHEM.EDUC., 51,460(1974). I41 Massey, H. S. W.. ''Negative lonn," 2nd Ed.. Cambridge University Prrs8. cam^ h"A-
195"
.
...,...-, (341 Popp, H. Z.,Naturlors~h.,22a.25411967). I351 Beriy, R.S., and Reimann, C. W . , J Chem Phys., 38,154011963). (361 Berry, R . S . . and Milstein, R . M . J . Chm.Phys.. 55,414611971). (371 Muck.G..sndPopp,H..Z.Nnfur/orsch.. 2%. 1ZL3(19681. 1%) Frank, H.. Neiaer. M..and Popp. H . . Z Naturloneh., 2%. 1617 11970). 139) Bakulina. I. N., and Ionou. N. I.. Zh. Fiz. Khim.. 53. 2069 (1959). I401 Steiner, B . W., Branseomb, L. M., and Seman, M. L.. "Afomic Colllnion prn. ces88es.il
(Editor MeDowell. M. R. C.). Amsterdam. North Holland. 19M. D.
c*"
"0,.
(411 Ladd. M.F.C.. andLee, W.H., JInorg. Nucl. Cham.. 20.163(1961) (121 Perkeris, C.L.,Phya. R s u . 126. L470lL962). 143) Zoilweg, R.J..J. Chem.Phvr.. 50.4251 (1969). 144) Clementi, E.. Phys Re", 135%9801L964).
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