A Simple Aid for Teaching the Theory of Atomic Structure Hung-cheh Chiang and Ching-Hwei Tseng Institute of Chemistry, National Taiwan Normal University No. 88 Sec. 5, Roosevelt Rd., Taipei, Taiwan, Republic of China
In the course of both high school and undergraduate college chemistry, students are often puzzled by the theory of atomic structure. In order to help them learn this more easily, we have designed a simple demonstration to show the Pauli exclusion principle, Hund's rule, quantum numbers, electronic energy levels, and electron configurations. The exercise involves hanging valence electrons (represented by pieces of rubber) onto a game hoard which depicts the energy sequence of a given atom. Materials 1 wooden plate (60 cm X 40 cm X 1cm) 33 iron nails (1.5cm in length) 2 rubber tubes of different colors (16 cm X 1em), cut these into 1-cm pieces, yielding 16 pieces of each color I blank index cards Set-Up
The game board, or orbital plate, is cut from a piece of wood. I t is labeled as indicated in Figure 1, where n is the principal quantum number, s, p, d , and f are the suborbits, and the number in parentheses after the suborbital designation indicates the angular momentum quantum number. The spin quantum number will be designated by using differently colored pieces of rubber tubing which should be cut as described above. Hammer the nails (which will be used as holders for electrons) into position as noted. Each suborhit should have the number of electron nails that corresponds to the maximum number of electrons that can be held in that suhorbit. Seven index cards should be labeled as shown in Figure 2. These n-value cards will provide the players with standard information about the principal quantum numbers.
The Rules Remember that the goal of this exercise is to fill out the game hoard with cards and pieces of tubing so that it provides a visual representation of the electron configuration within the atom. Total the number of electrons within the given atom and compare to the electron totals on the n-value cards. The n-value card with a total that is either less than or equal to the number of electrons in the atom of interest is then chosen and hung onto the upper left nail. If the number of electrons is equal to the n-value card, you are done. If i t is not, determine how many more electrons are present. Take that number of pieces of cut rubber tubing and hane them. as re~resentativesof valence electrons. on the nails accirding to the following energy sequence. US; differently colored nieces of tuhine to renresent electrons with opposite spin. he energy sequence ofthe sublevels with the &me n value is: ns < (n - 2)f < (n - l)d < np if n < 6, then (n - 2)f is absent if n < 4, both (n - l)d and (n - 2)f are absent
216
Journal of Chemical Education
Example: 24Cr The student would choose the fourth card, because 24 is smaller than the 36 required by the fifth card. There are 18 electrons on the fourth card, thus the remaining six electrons of Cr will distribute themselves in the fourth energy level according to the rules. Rule one ns < (n - 2)f < (n - l)d < np translates to 4s < 2f 3d < 4p
