EFFECTS OF MOLECULAR SHAPES' LAURENCE S. FOSTER Watertown Arsenal Laboratory, Watertown, Massachusetts
F O R a long time the general course in freshman chemis- cording to the elements, is founded upon the properties try has been becoming more theoretical and less factual. that are governed by constitutional peculiarities. No longer is it believed desirable to stuff students full "The perspective obtained thereby is quite different of facts as a turkey is filled with stuffing. The student in nature from that which results n-hen inorganic chemisneeds facts, but only enough to enable him to think try is treated element for element.. . . It should.. . . about theories. He needs theories so that he does not be clear that a correctly drawn up inorganic structural need to know so many facts. If a student is asked chemistry, taken in conjunction with the organic, "What are the probable valences of astatine?" he does provides a magnificent and internally consistent overnot have to go to the laboratory to find out. He can all picture of the structure and constitution of matter." To point out rather dramatically the difference that make a reasonably good guess from the position of element 85 among the halogens in the periodic table. structure may introduce into molecules, consider the If he is asked "What are the valences of tungsten?" two simple and comparable substances, COzand SiOz. Carbon and silicon are in the same periodic group: he may not be able to predict these so readily, but it is still conceivable that he will predict some of the valen- both have a valence of four in these compounds. ces of tnngsten from his knowledge of chromium with Carbonates, with the -C03 radical are similar to metareasonably good results. The periodic table thus silicates with the S i O s group; yet how different the permits a certain amount of deductive reasoning, and two simple substances COz and SiOz appear to be! the student -gradually learns what can be deduced The differences are directly related to differences in structure. The existence of the carbonate radical, from it. Many physical and chemical properties of substances C03--, shows that there is room for t,hree oxygen atoms are, however, not dependent alone upon the location around the carbon atom. The SiO? metasilicate radiof the constituent elements in the periodic table. The cal has a similar structure, but there is also the orthoshapes of the "molecules" are also very important. silicate radical, SiOa--, that shows the silicon atom to be This is a neglected subject in general chemistry and large enough to become surrounded by four oxygen atoms. No orthocarbonate is kn0n.u. The carbon atom should no longer be omitted. is too small to hold four oxygen atoms. h slight difA new hook has just appeared by Walter Hiickelnow available in English translation-with the title ference in size of the two atoms, silicon and carbon, is "Structural Chemistry of Inorganic compound^."^ the only apparent difference between them. This, The preface states, "This work attempts to furnish however, is enough to explain the difference between inorganic chemistry with that which organic chemistry the two compounds, COz and SiOl. In solid silicon has long possessed as a basis for its systematization, dioxide there is room enough to have each silicon atom namely, a structural and constitutional theory in one bonded to four oxygen atoms in a tetrahedral arrangeembracing representation. This simultaneously points ment and a giant molecule of SiOl with a melting point of the way to a systematization of inorganic compounds, 1700°C. forms. CO~cannotbe bonded together simiwhich, other than that based upon a classification ac- larly, so it forms only a weakly bonded solid that sublimes a t -78'C. The problem is actually a little 'Presented a t the 13th Summer Conference of the New Engmore complicated than this. The oxygen atoms in land Association of Chemistry Teachers, University of Rhode Island, Kingston, R. I., August 21, 1951, as part of a Symposium COXare linear:
on the Content of the General Chemistrv Course. "Elsevier Publishing Co., New YO&, 1950. Translated by L. H. LONG.
MARCH, 1952
157
The SiOemolecule on the other hand, when formed into a giant molecule, no longer has a linear configuration, hut consists of SiO. tetrahedra linked together so that every oxygen atom is common to two tetrahedra (thus retaining the SiO*composition). The difference between COz and SiOa is not directly a result of the electronic configuration but in spite of it. By rights, COz should not have a linear configuration, hut due to repulsion of the oxygen ions for each other a linear molecule results. Another example shows again the importance of molecular shape. This example probably makes the difference between life on the planet Earth or the lack of it,. The example is water, which has an extraordinarily high melting point and hoiling point. Water is also unusual in that its ice is less dense than the liquid; ice floats on top of ponds instead of sinking to the bottom. Of all the simple hydrides of the subgroup B elements, water has by far the highest boiling p0int.J Instead of being a gas like methane, it boils nearly 300" higher than methane. Methane has a symmetrical shape: H H:C:H .. H
Water might he thought also to have a symmetrical shape: H-O--H, but numerous laboratory experiments show that this cannot be so. The water molecule is bent with an angle of 105' between the two hydrogenoxygen bonds.
What is the explanation of these various structures? A good theory has been evolved based on the arrangement of the electrons around the nuclei of the various atoms. As you will see from a table of atomic structures. electrons are classified as s, p, d, andf. The p orbitals evidently have a preferred orientation in space, along lines a t right angles to each other, whereas s orbitals have no preferred orientation. W ~ t hthis basic information it is possible to explain the molecular shapes after both s and p electrons are used in making bonds, and a process called "hybridization" occurs. Let us illustrate this by an example. The structure of oxygen is S P
8 2) 2 . 4
There are two covalent bonds to an oxygen atom in water, and the oxygen atom makes use of two p orbitals to bind the hydrogen atoms. Since two p orbitals arrange themselves at right angles, the two bonds should be at right angles, but are 105O instead of 90°,because of the repulsion of the protons for each other. Hydrogen sulfide also has this structure, but the angle is 92', the proton repulsion being less. The COz molecule uses all the valence electrons of the carbon atom 8
S P
8 2) 2.2
The result is a hybrid bond involving use of both s and p electrons -since the s orbital has no directional property, it points away from the p orbital and v e get an angle of 180' between the bonds, that is a linear molecule. When there are four covalent bonds they may point towards the corner of a square or towards those of a tetrahedron. The tetrahedral arrangement, as in methane, arises from the use of three p orbitals and one s orbital: the square arrangement involves the use of one s, two p, and one d orbital. This is what causes the four molecules of water in the hydrated cupric ion to have this arrangement. The table shows a summary of these arrangements.
Recently the location of the hydrogen atoms in ice has been settled by neutron bombardment experiments. Because of too feeble scattering by protons, electron diffraction and X-ray diffraction experiments do not reveal the hydrogen, but neutrons give their location easily. Laboriously, over a long period of time, scientists have evolved ideas concerning the geometric shapes of Electronic inomanic structures. For example, the four mole- Number of orbitals Svan'al arringernenf Ezamples cules of ammonia around the copier ion in the complex cmalencei 2 S,P linear CO! ion, [Cu (NH,),] ", are a t the corners of a square instead d, P linear of being at the corners of a tetrahedron. The water BCI,, CO,-3 3, P, P trigonal plane molecules in [Cu (HzO)J++ are also at the corners of a P,P, P trigonal pyramid KH3, PHs square. The other water molecule in CuS04.5Hz0is 4 8, P unsymmetrical plane 4 s, p, p, p tetrahedral CHS,SnCL attached to the SO4 radical. With nickel the 4NH2 square plane [Cu(HnOhI++ d, s, p, p moleculei can he either in a square or in a tetrahedron; 5 d, s, p, p, p bipymmid PFr, MoCl. d, d, a, p, p squarepyramid IF, when in a square the complex is red, and when in a 6 d, d, s, p; p octahedron SFa, [PtCla]-tetrahedron, the color is blue-green. 8 d, 4 d , d, s, It is seen, thus, that while the primary valence deterp, p, p dodecahedron [Mo(CN)8]-' mines the stoichiometric formula, the properties depend also upon the structural relationships. In fact, strut- The electrons may be promoted from a lower shell to ture may even determine which valence is exhibited. accomplish the necessary electronic configuration. The methods used for det,ecting the structure are in PAULING,L., 'LGenelaI chemistry," W. H. 8 See the Freeman and Co., San Francisco, 1947, p. 271. both physical and chemical and are too complicated
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JOURNAL OF CHEMICAL EDUCATION
for examination a t this t i e . The classical methods of determining the physical properties-melting point, boiling point, solubilities, molecular weights, electrical conductivities, and magnetic properties-give clues to differences in structure, information as to presence of covalent or ionic t -v- ~ bonds, e and the kind of svmmetrv. Ferric chloride has a molecular weight corresponding to the formula Fe2Cla. What is the structure? The diagram gives a partial answer. C1 :
.
C1
Fe
.
C1
\
\ C1
'.
CI
Fe
.
.'
C1
The important point to be emphasised is that the cal and physical properties depend upon the structure, and a satisfactory explanation for many Seeming irregularities in behavior can only be explained by ref-
erence to the structure of the molecules or of the crystal. It is recommended that in high-school and freshman courses more attention be given to simple cases, such as the anomalous properties of water and ammonia, to show the strong influence that the stmctural factors have. LITERATURE NPHOLM,R. S., "The shape of inorganic molecules," Sn'mec Nezus, No. 20, Penguin Books, Ltd.,May, 1951, pp. 9-32. L., "The Nature of the Chemical Bond," 2nd ed., CorPAULING, nell University Press, Ithscrt, N. Y., 1945. WELLS,A. F., "Structural Inorganic Chemistry," 2nd ed., Oxford University Press, New York, 1950. Hitcm4 W., "Structural Chemistry of Inorganic Compounds," Elsevier Publishing Co., New York, 1950, ~ o l sI. and 11. MAXTED,E. B., "Modern Advances in Inorganic Chemistry," Clssendor,Press, Oxford, 194,, N. V., "The Chemical Elements and Their Corn pounds," Clarendon Press, Oxford, 1950.
~,GW,CK,
