ATOMIC AND MOLECULAR STRUCTURE MODELS AS A VISUAL AID IN THE TEACHING OR CHEMISTRY* Chemistry students can be assisted i n orienting themselves into the chemical of chemical structure point of view by suggestive models which aid en'suali~o~tion and the electron theory. Types of models suggested after tested classroom use i n secondary schools and colleges are: 1. Frame models to s h m atomic structure with attachable electrons. 2. Models representing outer shell of any atom with transferable electrons to s h m compound formation with electron transfer. 3. Elemenbl models without electrons to show valence exchange, molecular and compound formation, and weight proportion i n inorganic chemistry. 4. Flexible models of van't IIoff tetrahedronal carbon i n benzene forms of Boeyer, Armstrong, Vaubel, and Sachse. . . . . . . Most recent writers of textbooks for chemistry make use of the electron theory in explaining atomic and molecular structure and the process of chemical union. Secondary schools as well as colleges are faced with the problem of assisting the student to grasp this concept effectively. The beginning student finds difficulty in orienting himself into the modern points of view. Visual aids in teaching can facilitate the overcoming of this situation and also furnish the student valuable tools for increased familiarity with the applications of the theory,to chemical phenomena. Chemical education should not consist merely of a trained memory but should become an actual part of the student's world, in which and with which he can discover and learn for himself. The use of models as an aid in visualizing chemical change and structure is not new but the types of models in use are decidedly limited in availability and utility. Many instructors have constructed models with more or less success but due to differences in interpretation of theory these models show wide variation. I n my classes I have developed a system of models which possess flexibility and are adaptable t o the various phenomena of chemical union. The response of the students and the increased facility with which they grasp the general principles gives evidence of the value of this type of teaching technic. The structure of the atom is still problematical and the physicist and chemist have yet to meet in agreement upon a common cokept. The dynamic Bohr atom offers a logical and reasonable explanation of many physical phenomena, particularly the production of light of definite wavelength and the transfer of energy from one substance t o another. When * Presented before the Division of Chemical Education of the A. C . S. at the Buffalo meeting, August 3lSeptember 4, 1931. 301
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the chemist tries to apply this theory he is faced with the difficulty of making the free moving electrons compatible with valence, stable combination, and isomerism. The chemist, usually less interested in questions of energy, prefers to assume a static atom in which the electrons are held in fixed positions or a t least that their relative positions do not change. G. N. Lewis and Irving Langmuir have developed this static atom for our use and the phenomena of valence, stable combination, and isomerism are readily explained. The student, faced with both these views, must use both as far as they are helpful, keeping in mind the possibility that either may be abandoned as new information is derived from research of this character. The newer theories of Heisenberg, Schrodinger, and Dirac, with the concept of an electron cloud, have brought a mathematical atom which is yet to be evaluated fully. It seems logical that the beginning student should be given the electron theory of atomic structure early in the course and the use of models to aid the visualization of atomic structure will assist the teaching. From the study of atomic numbers and the periodic table we can determine the excess electrons which are outside the nucleus and after the hydrogen-helium period there is a tendency to group in octets which can be suggested by the corners of a cube. We use a 6-inch cubical wire frame painted white with sleeves a t each corner into which electrons, in the form of small wooden balls
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on rods, can be inserted. (Plate 1.) The helium nucleus with its two excess electrons is mounted in the center of,the frame and forms the first shell of every atom. Additional electrons are inserted into the corners of the frame until the total number of electron: agrees with the atomic number of the element being considered. In teaching with these models emphasis is placed upon the electrons and the frame merely represents the field of outside force which causes the electrons to pattern themselves about the nucleus. This spacial concept is a difficult one for beginning students and the cubical form is useful in developing the space idea visually. The helium nucleus is a black wooden ball made purposely large so that the number of protons and nuclear electrons present may be written upon it, or in the simpler elements i t is a cork ball into which pluses and minuses in thumbtack form may be placed to represent the nuclear charge. The true size concept of the nucleus must be left to the imagination. (See He in Plate 1.) Following this configuration, lithium has one electron in a comer of the frame or shell. Beryllium has two, boron three, and so on until we reach neon with a closed shell of eight electrons (Plates 1 and 2). The completed shell suggests the inert character and stability of the element as well as the zero valence because of no opportunity for further electron addition. With sodium (Plate 2) a second group of electrons is attached by extension rods to the first group of eight. Thus sodium adds one
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electron, magnesium two, aluminum three and so on, until argon with eight, completes the second shell of eight electrons (Plate 3), and again the inert and zero valence characteristics of argon are evident. Similarities are also noted between other elements such as carbon and silicon, and fluorine and chlorine in structure and in properties. In this manner we have built an atom of three shells, the hydrogen-helium nucleus, the second shell to neon, the third shell to argon. When we attempt the fourth shell, we can add an electron for potassium (Plates 3 and 4) and another for calcium, bnt then our configuration must change for the increasing weight of the atom is going to cause a building up of the third shell to eighteen electrons before we can proceed with the fourth. The student should now be able to complete this shell visually with the more complex atoms. We are, however, working on models to carry this idea through for future use. With this demonstration of atomic structure the student is then shown atomic combinations to f o m molecules (Plate 5) and then to form compounds. The tendency of compounds to form completed octets and the principles of valence electrons in combination is made clear by the buildmg of compounds as shown in the illustrations (Plates 6 and 8). After the student has acquired familiarity with the atomic structure from the frame model we can pass to a more simplified model which will show the transfer of electrons in the formation of compounds. The wire frame now gives way to a wooden ball IS/*" in diameter (Plate 1). There are eight sleeves in this ball to represent the eight positions of electrons in any shell. The student has learned from the previoua models to visualize the st of the structure. This ball then represents the outer shell of any atom. For recognition, the electrons, which are small wooden beads on rods, are significantly colored to suggest the various atoms; i. e., red symbolizes oxygen (fire); white, hydrogen (lightest); black, carbon (coal); blue, nitrogen (4/s sky); yellow, sulfur; green, chlorine; aluminum color, aluminum; copper color, copper; etc. The balls are all white, so that attention will be directed to the electrons, not the shells which the balls represent. By placing the proper number of electrons in any ball we can represent any atom which we choose, as the outer shell never has more than eight electrons. Molecules and compounds can now be constructed with coiled springs liking the electrons of atoms together where electrons are shared. (See under frames in Plates 6 and 7.) In this manner the transfer of electrons is distinctly shown as the colors identify themselves. The similarity of CHI and the NH4 ion in structure clearly shows a difference in electron position by this method. The secondary-school teacher may not feel required to teach the electron theory a t the present time but the advance of chemistry gives every evidence of the future necessity to include it as apractical theory. Whether or not the theory is now taught, models will assist in teaching molecular
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formation, valence combination and exchange, and weight proportions, as associated in molecular structure. The inorganic student will be greatly aided by any visual method that illustrates valence and balance of equations. The atomic model can be used in a simplified form which consists of a wooden ball with inserted sleeves to represent the maximum valence of the atom. Rigid rods and coiled springs to show strained bonds are prepared to fitthese sleeves so that molecular structure can be represented and valence combination illustrated. The balls are significantly colored to suggest the various elements, as in the previous modelsred symbolizes oxygen; white, hydrogen, etc. The approximate atomic weights are affixed to the units by plainly visible numerals. The student is thus familiarized with atomic weights and the association of atomic weights with the individual element is a valuable relationship which is often difficult for the elementary student to retain. The value becomes more evident in the ability of the student to visualize a t once weight proportions in a compound. (See Hi301 and Hz0 in Plate 6.) Valence becomes a more natural phenomenon by the use of models. The student actually sees the opportunity for combination. Inactive valence bonds are bridged by soft lead wires and in this manner variable valence is represented. Thus the formation of several oxides of nitrogen, carbon, and phosphorus becomes visually clear. The balancing of equations now follows in natural succession and atomic combination is more clear than by mere description. By allowing4he student to build compounds and balance equations with models a strong,association of theory and practice is formed and the educational value of this project method is a distinct achievement for the elementary pupil. The structural formulas of organic chemistry are difficult to represent in three dimensions on a blackboard, and the space relationship of these structures are difficult to visualize without assistance. The stereoscopic photographs such as are made by Adam Hilger of Bragg's crystal models are useful but they can be seen by only one student a t a time. Such pictures can be made by photographic models with a duplicating camera or by making two exposures at different angles. When mounted and used in the familiar stereoscope a good three-dimensional effect can be produced. (Plates 9-13.) This procedure is useful but the actual models are to be preferred for the student can construct formulas himself and thus see a t first hand the space arrangement. in diameter The carbon atom may be represented by a black ball with four equispacial sleeves indicating the tetravalency. The angles between any two of these sleeves should be 109'28', which will produce a true tetrahedronal atom typical of carbon, according to van't Hoff and KekulC. When rods are inserted in these atoms and compounds formed the student can clearly see the tetrahedral form.
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The c combined with the white hydrogen atoms previously referred to will demonstrate any of the hydrocarbons, as typified by the methane series (Plate 7); or the unsaturated hydrocarbons, such as ethylene and acetylene (Plate 14). The red ox);gen atom added to the hydrocarbons represent the alcohols. Methyl, ethyl, propyl, isopropyl, tertiary butyl, 2-pentanol, and stereoisomeric pentanol with asymmetric carbon are typical examples (Plate 15). It may be convenient when taking up nomenclature of isomers to use a carbon atom with the sleeves all in one plane. Further structures of alcohols can be made according to the needs of the instructor. A typical aldehyde, ketone, ketene, ether, ester, cyanide, and iso-cyanide can be easily constructed. In the case of the cyanides, the possible double valency of carbon is evident. (Plate 16.) Any of the exemplary organic acids, saturated and unsaturated as well as dibasic (Plate 18), can be modeled and compared with the preceding compounds. Stereoisomerism can be readily demonstrated. Such forms of tartaric and lactic acids are shown (Plates 17 and 18) and also d. 1., meso and racemic, and maleic and fumaric acids, cis and trans forms of geometrical isomers. The polymethylenes or cyclo forms of propane, butane, and peutaue show up well in demonstration (Plate 19); also cis and trans cyclopropane dicarboxylic acids and cis and trans hexahydro-p-phthallic acids. These models can be used in the formation of non-planar rings according to the developments of stereochemistry associated with Huckel and Ruzicka.
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(Plate 14.) If the rods used in forming these rings are made such a length that the distance between centers of the carbon atoms is 15 cm. they serve as a suggestive multiple of the 1.5 Angstrom units. With one ring structure of carbon atoms in hexagonal planar form (Plate 20) the instructor can, by adding springs and lead wires, demoustrate the traditional benzene ring forms of Claus, Ladenburg, Armstrong, Thiele, Kekul6, and modern conceptions as well. Renzene space molecular models to show the principles of Baeyer,*Vaubel, Sachse, and Kekul6 can be made by using small tetrahedra cut from sheet tin to represent the various space conditions. (Plate 21.) The tetrahedra are arranged around the frames according to the particular theory under consideration. The resourceful instructor will find many other opportunities for the use of models in teaching. If their use stimulates the learning process and simplifies the task of teaching, they are valuable to the teacher. If their use clarifies the theory and promotes mental skill in the subject, they are valuable to the student. Experience indicates that this is one valuable visual aid in teaching.
