edited by
MARCIA C. BONNEAU Box 203
the trading post Presenting the Periodic System with Pictures lngmari Bolmgren Strandvagen 21 5-641 50 Katrineholrn Sweden Cut circles of cardboard with diameters proportional to the masses of the elements i n the main groups. Write the atomic mass and small letters of the element on the cardboard circles. The colors should vary systematically within the groups (for instance, red-orange-yellow-greenish yellow-green-blue and gray for the elements in the groups 18). Paste a piece of magnetic ribbon behind each of the cardboard circles. Finding a System At first place all the circles i n a n arbitrary order except the one representing germanium on the blackboard (Fig. 1).Start by telling the students that i t was difficult to be a chemical scholar a t the time of Mendeleev. There was ample knowledge but no system i n which to arrange the known information, so one had to learn facts by heart. Mendeleev tried to find a system. At that time chemists knew the relative atomic masses of the known elements and the chemical behavior of these elements. Tell the students that the circles on the board represent elemental substances known today and that they have sizes proportional to the known atomicmasses, and that elements with
the same color show similar chemical behavior. Our problem is to try to find a system for all this information in the same way that Mendeleev did. Following Mendeleev's Path After a discussion, that nearly always comes to the same conclusion a s that of Mendeleev-properties might change with mass-the students are told to arrange every circle according to size (Fig. 2). When the arrangement of Figure 2 is completed, ask the question: Is there any regularity in the row?
The students soon find that the colors seem to appear in the same order! Tell the students that this is the same conclusion of Mendeleev. The students are instructed to arrange the circles so that the same colors will be beneath each other (Fig. 3). Upon questioning the students report the observation that i n three places the colors do not fit. They are told that when Mendeleev found similar divergences with the elements known to him, he thought: I have a system, but it must depend on something even more important than atomic mass. Let us change the imperfections and later try ta find out the reason for the system!
So Mendeleev changed the places for rhe elements where the atomic massrd didn't n m r r with the svsrem. The sru-
dents are now told to do the same thing but find that an imperfection still remains (Fig. 4). They are asked if they
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Volume 72 Number 4 April 1995
337
hgure 6. 1 he
competed system
can find a solution to this difficulty They easily find that the system is consistent if they leave agap between Ga and As. They are then told that I had hidden the element that should be in this place, but in the time of Mendeleev several substances were unknown and that Mendeleev concluded the gaps like this were dependent on undiscovered elements. Furthermore, the students are told, that Mendeleev tried to imagine the properties of the unknown elements by observing the known elements around the empty places. When these elements were discovered his predictions turned out to be nearly perfect. The element germanium is now put into place and the students are told that Mendeleev called i t ehasilicon because of its place below silicon. Successful Dialogue and Memorable Figure I ha\v had thii d~alo&yllr with the students for some years and ha\vcnded uv with a diicus;ion ofwhnt the imvortnnr
factor is that leads to the periodic system. I t has worked very well. Doubtless, a certain aesthetically pleasing quality about the patterns of circles fosters and strengthens the visual memory.
338
Journal of Chemical Education
