Jack W. Eichinger, Jr.
The Florida State University Tallahassee, 32306
Anticipating "Valences" from Electron Configurations
T h e current emphmis on the teaching of theoretical concepts at the expense of descriptive chemistry sometimes produces students who can talk glibly (if not knowledgeably) about atomic and molecular orbitals but who are unable to write correct formulas for the simplest substances. Is it possible to provide these people with a means for predicting "valences" from electron configurations? This paper suggests a procedure which works well for most metals including transition elements. The complex interactions which determine the total electronic energy of an atom or ion have been related theoretically to spectroscopic observables.' This approach, however, is often beyond the capabilities of beginners. Starting with the ground state electron configuration of a metal atom,= electrons are removed, one-by-one. The stability of each possible ion is predicted by the use of Table 1. For this purpose, ions are called "stable" if they are found in chemical compounds and take part in chemical reactions.
It is convenient to use the letters S, T, and U to designate the three types of ions under consideration. Type S ions are stable and no further ionization occurs; Type U ions are unstable, while Type T are intermediate between the other two types. Type T ions are stable hut they also undergo further ionization. In the imaginary Aufbau process, electron configurations are built up by adding a p r o t a to the nucleus (to increase the atomic number by one unit) and then adding an electron to the unfilled subshell which lies a t the lowest energy level. Ionization removes an electron but not a proton, so the electron ionized need not be the "last" or "diierentiating" electron added in the Aufbau process. For a transition metal such as vanadium, the "dierentiating" electron is in the 3d subshell but the first electron to be removed during ionization comes from the 4s subshell. The two processes are compared in the following "equations"; the electron occupancy of the two highest-numbered subshells is given in parentheses. Auj+
-
Bmed upon materid pnesented st Division of Chemical Education, 153rd National ACS Meeting, Miami Beach, Fla., April, 1967. under the title. "The Ionization of Metds." HOCHSTIIASSER, R. M., J. CHEM.ED&, 42, 154 (1965). 'EICHINGER, J., J. CAEM.EDUC.,34, 70 (1957); "Electron AND Chart," in Encyclopedia of Chemistw (2nd ed.), ( C L ~ K RAWLEY, Edilo~s),Reinhold, N. Y., 1966, pp. 374375.
+
++
process: zlTi(3d4sq proton electron + zrV(3da4@) rsV+(3dg4s1) electron
lonuation: ~V(3d?4s')
To use this method it is only necessary for the student to remove each successive electron from the subshell having the highest principle quantum number (4s electrons undergo ionization before 3d electrons, etc.). Within the same shell, d electrons ionize before p electrons and s electrons are the last to undergo ionization.
Volume 44, Number 1 1 , November 1967
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689
Table 1.
Types of Ions for Stability Predictions
T .v.~ S e T. m.e T Stable Stable No further ionization Further ionization
T. m.e U Unstable
Table 2.
Application of Classification Procedure to a Variety of Elements
OCCllPS .....-.
Most other8 except Li+
Any (P') M + (dlo)or (dlOsP). Ma+ (dlo) or ( d W ) Ma+ (da-O) Any MJ+ except (s')
NOTE: Particles of small size and high charge such as Bea+, Ala+, and CuS+usually form covalent bonds. I t is not possible to draw a sharp boundary between ionic and covalent substances.
Continuing with the ionization of vanadium, V+(d3s1) is Type U and unstable according to Tahle 1. It immediately loses a second electron to formV2+(d3)which belongs th the Type T classification. Vanadi"mi11) ion is stable but also undergoes further ionization to form VS+(d2). VanadiudIII) ion belongs to the stable Type S classification. Higher oxidation states of vanadium are covalent. A convenient summary of the ionization behavior of vanadium may he written as follows:
-a
IrV(d8s')
-c
'V+(das1)U
-a
VP+(d8)T
VaYd')S
The chemical symbol for V+ is starred because it is unstable and the classification (S, T, or U) is noted in each case. Having predicted that vanadium forms two stable ions, the student may now write correct formulas for compounds such as VBr,, VBs, etc. The following comparison of thallium and aluminum illustrates the so-called "inert pair," a filled s subshell found in some ions.
-e
sAKp6s2P 1) ,Tl(dWpl)
-6
-e
*M+(pw)U-+ *Ml+(pes')U -C
-0
-e
AIS+(p6)S
Tl+(dW)T + *TIa+(dlo~l)U Tla+(dlO)S
690 / Journal of Chemical Education
(see'note on Table 1)
T1+ is stable (Type T) whereas Al+ is unstable (Type by noting u), hi^ behavior might be that even by losing two more electrons,cannot achieve the noble-gas, p6, configuration; Alf, of course, has this added reason for quickly disposing of t,vo more electrons. Both elements exist in the +3 state, hut A13+ due to its smaller size has the greatest tendency towards covalency (see note in Tahle 1). Table 2 illustrates the application of this procedure to a wide variety of elements. Very few simple cations will be encountered which the rules cannot p r e d i ~ t . ~ More important, however, is the better understanding of electron configurations and the ionization process which students will gain by using thiq approach instead of memorizing ~ a l e n c i e s . ~ a Same exceptions: Hg2+ and Pos+ are known; Hf2+ and AuQ+are unknown. 4 Teachers wishing to try this approach me invited to write the author for sample worksheets.
