Raymond W. P a m Old Sun Community College, Gleichen, AB TOJ 1NO. Canada The relative urnitions of enerm levels in a given atom are influenced by the presence of other electrons and the nuclear charge. As well, the actual energies of identical orhitals in different atoms vary, hut the pattern of filling the orhitals is qualitarively very similar and is governed by the attainment of minimum enerev formalized hv the annlication of Hund's rule and the ~ a u l ~ ~ x c l u sprinEip~e. ion ~ k a few h exceptions the order of electron occupancy is as follows: Many mnemonic schemes for applying the Aufbau principle to arrive at the ground-state configuration of a neutral, isolated many-electron atom have been devised. They are similar in structure, most are a little cumbersome to construct, and all are rather awkward to use. Furthermore, caution must be used in employing these schemes for they often portray faulty impressions about the relative energies of given electron^'.^, especially in the case of transition metals and in cases where the atom is neither neutral, isolated, nor in the ground state3. This paper proposes a new mnemonic scheme for arriving at the ground-state electron configuration of a neutral, isolated atom. It is much easier to construct and read. Conventional schemes read diagonally. The one proposed herein reads in the normal left-to-right sequence. The construction is also very straightforward. The n s orhitals are arranged vertically in numerical sequence in a right-justified column. In the next column immediately to the left are the (n - 1)p orhitals again in numerical sequence. This is repeated for the (n - 2) d and (n - 3) f orhitals. The resulting pattern is shown below.
' J. Chem. Educ. 1962,39,289. J. Chem. Educ. 1978,55,2. J. Chem. Educ. 1982,59,757.
Note that a numerical sequence is followed horizontally, vertically, and, every second step, diagonally. The sequence of filling the orhitals is simply that followed in standard reading sequence; i.e., once the end of a line is reached one simply moves to the start of the next line and proceeds to its end, etc. A possible disadvantage is that energy increases from top to bottom, hut this can he remedied by inverting the structure even though this destroys the standard reading sequence. It is of interest to note that many of the exceptions to this simple scheme take place at the transition points from one line to the next (i.e., 4s-3d, 5 s 4 d , 6s-4f). d and f orhitals occupy the extreme left of the structure, and it is well known that these orhitals exhibit variant behavior when they are close to being filled or half-filled. In other words, these orhitals will usually half-fill or fill at the expense of pairing in the immediately preceding orbital; i.e., examine the electronic structure of Cr, Cu, Nh, Mo, Pb, and Ag. This trend increases in likelihood as each d and f orbital becomes more populated and as higher energy orhitals are successively filled; i.e., there is extreme interplay between the 5f and 6d orhitals of the actenoids. The mnemonic scheme outlined above serves to highlight these exceptions by the location of these orhitals in the structure.
Volume 66
Number 4
April 1989
319
