R. 1. Sanderson State University
of
An Aid to Teaching Electronic
lowa
Iowa City
Configurations of Atoms
A
simple, crude, but effective visual aid, for demonstration during lectures on atomic structure, is shown in the accompanying photo. It consists of a series of shelves arranged to show qualitatively the differences in energy levels among the various atomic orbitals. Each orbital is represented by a beaker (400 ml, tall form) capable of holding two Z1/* in. diameter Styrofoam halls,' representing electrons. Each shelf represents a particular combination of principal and orbital quantum numbers and is of appropriate length to hold the correct number of beakers (orbitals). Each s shelf can accommodate only one beaker, each p shelf, three, each d shelf, five, and if the framework shown were extended to include j shelves, each would accommodate seven beakers. The shelf edges corresponding to each principal quantum level are painted a distinctive color: in the assembly illustrated, these colors are black (for n = I), green, orange, blue, and red. This makes the stairstep arrangement of orbital quantum levels a t the same principal qnant,um level easily visible to the students (although it does not show well in the photo).
Starting with thc enipty shclvcs and a supply of beakers and balls, one can demonstrate the building up of the atoms of elements of successively higher atomic number, starting with hydrogen, quickly and easily. When each alkali metal structure is reached, all the beakers (orbitals) of that particular outermost 'Inexpensive Styrofoam ~pheresin white, red, green, and blue can be purchased from the Star Band Co., Portsmouth, Virginia. The colored spheres have firnler surfaces, with greater resistance to abrasion.
262 / Journal of Chemical Education
principal quantum level are placed on the shelves, and then filled in with balls as the atomic number increases across the period, observing the order of filling, s-pd-j, and the rule that within a given sublevel, electrons do not pair until each available orbital contains a single electron. The electronic configuration illustrated is that of iron. In this as in all such representations, the students should be told that the relative heights of the shelves (energy levels), only qualitatively shown here, represent the order of building up successive elements but not necessarily the order of electron energies within a given atom. For example, the significance of the slightly higher level of the 3d shelf in the photo is not that the 3d electrons of iron are less stably held than the 4s electrons, but rather that in the successive building up of atoms beyond argon, the more stable sublevel when the nuclear charge is 19 (K) and 20 (Ca) is 4s. However, with nuclear charge 21 (Sc) this order is reversed and from here on, 3d electrons are more stable than 4s. As each electron is added, in the successive fillmg of atomic orbitals, it is helpful to tabulate on the blackboard the four quantum numbers that specify its position. For example, for the hydrogen electron, one would write: Electron No.
n
1
1
1 0
mr 0
m, '/n
Orbital Type 8
Even though, a t the elementary level, students cannot he expected to comprehend fully, they can acquire in this manner a useful and logical picture of the significance and reasonableness of atomic structure. They need only accept the restrictions that 1 may have only values 0 through (n - I), m , may have values of 0 through + I , and m, only together with the Panli exclusion principle according to which no two electrons in an atom may have an identical set of four quantum numbers. The support of this work by the National Science Foundation, Course Content Improvement Section, is gratefully acknowledged. The shelves were kindly constructed by Mr. J. H. Thomas. Related Bibliography
EVERETT,D. H., J. CEEM.EDUC.,36,298 (1959). LON~-~ET-HIGGINS, H. C., J. CHEM.EDUC.,34, 30 (1957). WALKER.W. R.. AND CURTHOYS.G. C.. J. CEEM.EDUC., . 33,. 69 (1956).
WEIS, N. D.,
'
AND
MEEK,J. S., J. CBEM. EDUC;35, 570 (1958).
