The fairy story of valence


THE FAIRY STORY OF VALENCE. All agree that it is desirable for students to learn science, as far as possible, from experiment rather than on the autho...
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OCTOBER, 1947

THE FAIRY STORY OF VALENCE ANTHONY STANDEN The Interscience Encyclopedia, Brooklyn, New York

.. A L L AGREE that it is desirable for students to learn science, as far as possible, from experiment rather than on the authority of the teacher or the textbook. For teach'mg valence an allegedly ."experimental" method is sometimes used: 23 mg. of sodi?m, 24 mg. of magnesium, and 27 mg. of aluminum are weighed out and introduced into three inverted tubes containing dilute acid; the fact that the volumes of hydrogen produced are in the ratios 1:2:3 shows the students that the valences are in these ratios. I n order t o do this experiment, the atomic weights of the three metals must be known. We can know the atomic weight in two ways, one of which, the way u n i v e r ~ l l yused by professional chemists in practice, is t o lookupin the book. But if we may lookup the atomic weight, why not also the valence? The other way is by experiment, and if i t is important to teach students the experimental basis of valence, i t is also important t o teach them the experimental basis of atomic weights. Every method of atomic weight d e termination depends upon a knowledge of the equivalent weight, and when both of these are known the valence follows a t once as a necessary implication. Thus this method teaches valence by means of a fairy story. It makes no difference if it is only claimed that it demonstrates valence in a striking and memorable

manner. It is true that the student may he assisted in memorizing valences by this method, but he obtains this information a t the expense of confusion about the experimental basis of atomic weights. Another popular method of tiaching valence is t o go through the business of deriving formulas from analyses and atomic weights--skating over the implications of the fact that the atomic weight is sometimes not the same as the equivalent weight-and then discover valence, with an air of surprise, from examination of such formulas as NaCl, NaBr, WaI, MgC12, MgBr2, Mg12, etc. As if the law of equivalent proportions, which was introduced in a chapter so far back that it has by now been forgotten, did not.make this sort of regularity a logical necessity! This method has the extremely faint pedagogical merit of being true to history, for there was a period, in the first half of the 19th century, when the word "valence" was not used. We are now able to see that the full concept of valence could have been, and should have been, developed earlier; let us not blame the chemists of a hundred years ago, for they were probably no more stupid than ourselves, but let us not follow their mistakes in our teaching. The valence of an element does not require a plurality of the element's compounds in order t o reveal itself: we could

JOURNAL OF CHEMICAL EDUCATION

482

speak of the valence of an element that formed only one compound, and it would not be impossible for an element t,o form, say, only four compounds, with different elements and exhibit a different valence in each: But t,his method of teaching valence is, a t all events, experimental, for a determination of an equivalent weight is often made by the student himself. Of the various experimental methods of deciding which multiple of the equivalent weight to take for the atomic weight, a t least some are usually made more or less clear. Least experimental is the method of beginning with ~lectrons. A picture may be drawn of two chlorine atoms unit.ing t o form a molecule, and of a chlorine atom and a sodium atom forming two ions. A definition may even be attempted, "The valence of an atom is the excess or defect of the electrons in its outer orbit over a stable grouping," or similar words. Later on the necessity will arise for the pupil to learn and for the teacher to teach t,he vhole matter all over again, more nearly correctly. With the other two methods, the definition of "valence" must be greatly expanded, hut no actual misinformation has been given, and if a student is not made t o come to grips with some Expanding Definitions, he leaves the chemistry course with a very poor idea of what chemistry is actually like. But perhaps an even more serious objection to the electron method of teaching valence is its entire abandonment of the principle of learning from experiment. I n the body of science as a whole, our belief in electrons rests, presumably, upon very many experiments distributed among chemistry and physics in such a manner that it is a well-informed scientist who can give a good account of them. If valence is first described as depending on electrons, it is only possible to say t o the student, "Some of the experimental evidence for this will be made clear later in the course, and other parts of it are taken,up in advanced courses in physics." This is made necessary by the inverted order, from theories t o facts, instead af from facts to t,he t,heoriest,hat are supposed t o acconnt, for them. C

Leaining from experiment is done, t o a very slight extent, by experiments actually performed by the student himself. It is seldom possible for the student to do more than a few "token" experiments himself. Somewhat more indirect is the demonstration experiment performed by the instructor, which has, of course its own sphere of usefulness. Still more indirect, but also useful, is a clear description with diagrams if necessary, of the apparatus and a Gatement of the results. Teaching by experiment usually involves all three levels of indirection. For example, determination of atomic weights depends upon analyses of compounds (combining ratios or elements), densities of gases or vapors, and in some cases specific heats. I t would probably not be appropriate to have the student perform all the necessary experiments himself, but the tracher can devise a combination of direct experiment, demonstration experiment, and description of experiment that will enable the student to understand how .t,he necessary data are obtained. How, then, can valence be taught experimentally? .4n excellent beginning definition of valence is "the number of hooks on an atom," provided, of course, that it is made perfectly clear (1) that the language is p n r e l ~ analogical-we do not necessarily suppose ahat any part of an atom is actually hook-shaped, (2) that the "hooks" are such that each one of them engages with only one other "hook"-"push-buttons" would be a better analogy. But by what experiments do we count the "hooks" on an atom? Clearly by those experiments which lead t o a knowledge of its atomic weight. The teaching of valence experimentally is in no way different from the teaching of atomic weight,s experimentally.. Once an atomic weight has been determined, a student is being misled if he is given the idea that any further experiment is required in order to arrive a t the valence; al! that is required further ie: some hard, clear thinking, t o appreciate the significance of the fact that the atomic [email protected] a multiple of t,he equivalent might.