edited - - hv -,
JAMES 0.SCHRECK UNIVERSITY OF NORTHERNCOLORADO
GREELEYCO 80639
Bright-Line Spectrum Analogy Richard A. Samsa Grove City High School, 51 1 Highland Avenue, Grove City, PA 16127
The bright-line emission spectrum of an element is caused by electrons dropping from a higher energy level to a lower energy level. When an electron drops to the lower energy level, it must lose a quantity of energy that is equal to the difference between the two energy levels. This energy is lost in the form of a nhoton of lieht with a snecific freouencv. and a specific color. Bv lookina a t the color of the lieht. we can infer what e n e k level change produced it. hisi is presented fairly well in most textbooks: however, i t is a pretty abstract idea for some high school students. ~onseq;entG, I have used the following analogy to illustrate this in a more concrete manner. In this analogy, a bag of sand will represent an electron. The floor, a chair, and a lab desk will simulate different energy levels. A meter stick set on some type of a fulcrum (perhaps a brick), will act as a lever. An eraser is placed on the side of the meter stick resting on the floor-it will represent the photon released when the electron returns to the lower level. The initial set u p is shown in the figure. In the first part of the analom. the bae of sand is a t the base of the labdesk and I inform the clasithat it represents an electron in its "ground state". Next. I bring a flame close to the bag and cause it to become "excited k d jump to a higher energy level (the lab desk). Iexplain that i t represents the electron in its "excited state", I move it to a location directly above the upper end of themeter stick below. When i t returns to the "ground state" (by being dropped), it lands on the meter stick and sends the eraser flying. Before I drop the sand bag, I tell the students to be sure to observe how high the eraser flies. I then repeat the procedure using a chair-instead of the lab desk-as the "excited state". The students observe the smaller distance the eraser flies as a result of this drop as compared to the distance produced by being dropped off the desk. Consequently, by seeing how
-
412
Journal of Chemical Education
high the eraser flies, the class can infer which drop produced the flight. In the same way that the class can infer which drop was made by looking at the energy given to the eraser, a scientist can infer which jumps are made by electrons by looking at given off when an atom is the specific frequency of liaht .. . excited by means of heat or electricity. Using the same reasoning, since there are only specific colors present for specific types of atoms, we can infer that there are only specific energy levels in atoms from which the electrons jump. I have found that this analow ... helm the students understand this concepr betterthan just explainingit, and it really doesstand out in their minds. For a more dramaticeffect vou could simulate theelectron and do the actual jumpingyourself. This could be hazardous because of the nussibilitv of breaking (and splintering) meter sticks or in&ring on&elf from the jumps. Either way, however, this analogy helps the students to get a "jump" on understanding the bright-line spectrum.
.
I
LAB DESK
/ -
Meter stick
lnltial set-up for the spectrum analogy.
I
\\
/Eraser
