VALENCE BLOCKS for TEACHING INORGANIC CHEMISTRY FUNDAMENTALS ARTHUR H. BRYAN Baltimore City College. Baltimore. Maryland
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ROBLEMS of valence, ionization, molecular weights, chemical reactions, the structure of acids, bases, and salts, and so forth, are difficult fundamentals which the average high-school chemistry student has to overcome in order to achieve success in chemistry. Any teaching device which helps to visualize these difficult units or topics is apt to be of service to teacher and pupil. Jigsaw puzzles are still popular, and chemistry students may now learn while playing, merely by fitting blocks together and reading the equations, formulas, valence, ionization,
and molecular weights, and so forth, so formed. The blocks may be made of stiff paper, compressed paper, cardboard, and thin, or three-ply wood. Small sets may be used by pupils and large cardboard sets pinned on a board for teacher-demonstration purposes. These blocks were recently granted a United States patent right as a teaching device in chemistry.' Demonstrated before the Division of Chemical Education at the ninety-seventh meeting of the A. C. S., Baltimore, Md.. April 4. 1839.
(1) GENERAL PLAN OF THE BLOCKS Circles represent positive valences, e. g., Na, H, K, Ag, Ca, Ba, Al, and so forth. S p r e s represent negative valences, e. g., OH, Br, C1, I, SOa, Sot, Po,. Tphngles represent positive ion valencies which have more than one valence, e. g., Fe, Au, Cur represent elements having both positive and nepative valences such as S, N, As, and so forth. hi; owls represent organic radicals such as CH3, C& CsHs, and so forth. The number of arms on the blocks represents the valences, one, two, or three, and so forth. In positive
of the darker shades representing the more stable compounds. The metals with positive arms, colored blue, tend to form bases turning red litmus to blue. Hydrogen, it should be noted, while not a metal, acts like one. The non-metal blocks have negative anns and tend to form acids turning litmus red' Dark shades combined with light indicate unstable comP O U " ~ % e. g.. NazPO4. The t h e e Na blocks are dark blue and the PO1 block is pale pink.
elements and radicals, the arms point right, having arrows a t the ends, while the negative ion elements have radical arms pointing left with slots a t each end. By analogy with the litmus reaction the deeper colors, red and blue, are assigned to the more active elements or radicals. The lighter colors, pale blue and pink,
calcium blue circle block with two positive-ion right anns, and fits them onto two separate negative red square hydroxyl radical blocks, each with one left slotted arm giving him the formula Ca(OH)2. Mercury, iron, copper, and gold have two valences, making either ous or ic salts, and are therefore repre-
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DNALENT FORMULAS
For example, a student is required to construct the formula for calcium hydroxide. He finds the positive
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represent the less active elements or radicals. The sented by triangles with koo shades of blue, and two blocks may be kept in separate receptades correspond- or three pointed arms. To make the formula for ferric chloride, just fit three of the negative-ion red chlorine ing to the valences. c blocks to the triangular iron block, which is colored (2) USES FOR MAKING FORMULAS two shades of blue, and three right anns, making the To make a chemical formula the student merely fits formula FeCls. To make ferrous chloride, attach two together the blocks necessary to its completion like a chlorine blocks to two dark blue arms of the iron block, jigsaw puzzle. First the student might arrange his forming FeCla. separate sets of blocks, correlating them with the lessons ( 5 ) POLYVALENT FORMULAS in his chemistry text, in order to learn the valences, ionization, and so forth, of the common elements and The complicated polyvalent formulas representing several negative or positive valences are demonstrated compounds. by the rectangle blocks with both positive- and nega(3) MONOVALENT FORMULAS tive-ion arms, and both red and blue colors. To form To familiarize the student gradually with the blocks sulfur dioxide, the student picks out sulfur, a rectangleas teaching aids, he might pick out a blue circle and shaped block which has several valences as indicated any red square with one arm each and fit them to- by the six positive arms, and two negative ones cdoped gether, for example, sodium and chlorine. The sodium, both red and blue. Next, he fits two two-armed oxygen Na, as will be noted, carries a positive charge and the circles to four positive arms on the sulfur block, giving chlorine, C1, a negative one. The combined atomic the formula SOn. Two H (hydrogen) blocks may be weights 23 35 of sodium chloride are observed on fitted to two sulfur arms to form H a , hydrogen sulfide. each block and added up to make 59. The blocks are (The various valences and ions are marked on these colored various shades of red and blue, combinations blocks.)
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(6) CONSTRUCTING EQUATIONS To work out a single replacement equation, for example, the student sets up his formula for sodium chloride by fitting an Na and a C1 block together.
the information and ability to complete equations and solve problems. The blocks coordinate admirably with standard high-school chemistry texts, and may serve to amplify, make visual, and explain the problems incident to understanding chemical reactions and equations. SUMMARY
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In the classroom and laboratory, the teacher may illustrate many difficult chemical concepts with the large-sized demonstration models, and the student may follow the demonstration with a set of his own. The student should manipulate the blocks systematically, thus
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What would h a ~ n e nif the sodium chloride were treated with nitric acid (HNO~)? He fits together a hydrogen circle and a nitrate (Not) block forming HN03 (nitric acid), and places a plus sign between NaCl and HNOa followed by an arrow sign. He next switches the Na to the other side of the arrow, fitting it to NOa. The C1 unites with the H ; thus making the two formulas, HC1 and NaNOa for the new compounds formed (HN03 NaCl --t NaN03 HCl). The student might next take his hydrochloric acid formula, HCI, and combine it with K3P04,by joining three single-armed potassium blocks to it. He lines up three HC1 blocks with the &PO4 with a plus sign in between, but notes there are three negative arms on the PO4 to be satisfied with three potassium blocks. Three sets of blocks making HCI formnlas are obviously required. If he carries the blocks over to the other side of the + sign, the three H's unite with the three arms of the POa. The three K's unite with the three Cl's completing the equation 3HC1 + 3KC1 HJPO& A double set of blocks may be used to keep both sides of the equation before the student. Five types of typical reactions are easily demonstrated with the blocks by fitting theni together to form formulas and equations: (a) Direct combinqfion, 2Fe Oz-+2Fe0; (b) Simpledecomposition,2Hg0 --t 2Hg Oz(HgOa); (c) Singte replacement, Zn 2HC1- ZnClz Hz; ( d ) Reactions which go to an end, AgN03 NaCl + AgCl NaNOs; (e) Rarersible reaction, 2Fe 3H20 f Fez03 3Hz. When the student progresses in chemistry and prepares for his examination or quizzes, he should possess
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learning the valences and ionizations of the common elements or radicals covered in the course of study. By keeping each set of blocks together according to valence, the student may learn to associate the elements with their valences, metallic or non-metallic properties, ionization, acid or basic properties, learn the commonly used atomic weights, relative stabilities of compounds formed, and so forth. With the use of the blocks, equation writing becomes easy, and the student may master the fundamental concepts of inorganic chemistry by visualization and practical construction methods.
