VALENCE DEFINED E. A. FLOOD v
T
Browh University,
Providence, Rhade Island
HE deplorable inconsistencies in the use of the term valence cannot be tolerated by the logically minded scientist, teacher, or student. The scientist may avoid the term but the teacher has the disagreeable task of teaching freshmen all the tangled definitions and misstatements of the introductory textbook. The teacher can hardly avoid discussing such a widely used term. Indeed "valence" has the prestige of tradition as well as a real use in certain fields. How can the teacher surmount the diEculty? Very simply. It is only necessary to define the term in the restricted sense in which it is actually used to advantage by the initiated, and thus avoidattempts to teach the beginner more than is actually known by the leaders of our science. If we examine most of our current difficulties in regard to valence we will find that they are attributable to one or more of three sources. The 6rst source of
confusion is the attempt to generalize the properties of isolated atoms. The formation of a given compound cannot be ascribed wholly to the properties of any one atom, chemical union depending as it does on the nature of all the atoms in the compound as well as on external conditions. The "combining power" is not a property of a single atom but is a function of a t least two atoms. Any attempt, then, to discuss chemical combination must take into account all the atoms involved. This may be accomplished by focusing our attention on the compound rather than on its constituent atoms. The compound, under some specified conditions, represents an actual case where all the observable "constitutional influences" of the atoms are accounted for. A simple illustration will demon. strate the point. A certain specified number of atoms may be combined to form a number of isomers. Clearly the properties of these distinctly different substances
cannot be wholly ascribable to the constituent atoms. like bonds associated with an atom in a given comThe arrangement, the constitution of the given isomer pound. His units are bonds which may differ in is an inseparable character of that compound and alone strength but are equivalent as structural or relational distinguishes the several isomers. units controlling arrangements of atoms in molecules. Turning our attention to the compound we now have These bonds he represents by means of lines in his the problem of describing its constitution and of bring- formulas or struts in his models. The limes and struts ing out the relation of this constitution to the proper- are all alike; two lines give a valence of two; a valence ties of the constituent atoms. For this purpose a of four implies four struts in his model. His difficulties number would he very convenient, if it could be used. begin when his structures cannot be determined or when his bonds do not behave normally as in the This leads us directly to the second source of the aculties attending "valence" today, the question of the central C - C bond in hexaphenyl ethane. He then valence number. In order to use a valence number it wisely points out the abnormality, counts the remainis necessary that it represent countable equivalent ing normal struts, and says the central carbon atom is entities or units. This is obviously where a great deal trivalent, leaving others to account for the properties of confusion exists. The freshman definition impels us of the material if considered tetravalent, or he may say to count hydrogen-like atoms. Thus, carbon, as ex- it is a mixture of two different substances, triphenylemplified by ethylene, (CH& is combined with two methyl and the dimer, the central carbon valence hehydrogen atoms and, counting hydrogen atoms, its ing three in the free group and four in the dimer. valence is two. The organic chemist is concerned with The inorganic chemist is somewhat less consistent bonds and in this compound four bonds are associated in his use of valence. When referring to ions, however, with carbon. Since the bond is his unit of valence he his method is clear and meaning definite. The valence finds the carbon to have a valence of four. The of an ion is clearly used to indicate the number of unit chemist who deals with ions counts unit charges. The charges carried by the ion. And since many of his reexpert on coordination compounds counts neutral actions are the reactions of ions the number of charges groups. Where so many valence units are in vogue on the ion will be a determining factor in the reaction. one can hardly expect universal accord in the valence The behavior of these ions is conveniently generalized number. by speaking of the valence of the ion. Electrolytes The third source of trouble lies in the generality of are commonly classed as uni-univalent, mi-divalent, the definition and in the vagueness of "combining . etc., the classification referring to the number of power." When there are so many clearly differentiated charges on the ions. Little or no ambiguity arises types of chemical union it is not to be expected that all when valence is used in this sense. There is practically can be included in any one term unless the term be as no other generally accepted use of a valence number and general as chemical union itself. Certainly any at- when the term is applied to substances whose constitempt to describe all modes of behavior of an element or tutions are doubtful, misunderstandirigs arise. The atom by a simple number will fail. The greatest un- coordination number is used with some success but certainty in the application of valence is met with in chemical compounds of the type classified by the cocases where the compound in question is neither a ordination number are very different from the typical typical organic compound nor a typical salt but a com- organic compounds. Moreover, the coordination numpound such as a complex hydrated silica. ber is controlled by such factors as size and charge, Having examined some of the undesirible aspects of and shows little or no periodicity, whereas the number "valence" let us discuss some of its betterdeatures. of bonds associated with an atom in a compound and The most outstanding and practical use of valence is the number of charges carried by an ion are markedly to be found in organic chemistry. The amazingly periodic phenomena. Weshouldnot use thesame termin self-consistent theory of the structural formula is based connection with such unrelated types of chemical union. almost wholly on the idea that a constant number of When the term valence is confined to the periodic bonds is associated with a particular atom in a com- type of union it will be found more generally useful. pound. The organic chemist distinguishes between In other types of compound such as intermetallic comsingle, double, and triple bonds. The properties of pounds, hydrates, some of the compouuds formed by the his bonds are well known to him. They are structural smaller electronegative elements, polyiodides, polyrelationships somewhat resembling mechanical struts. sulfides, intermolecnlar compounds, and the like, the His double bond is well established, accounting for re- valence number concept is quite useless. Can we define valence so as to include only the deactivities, geometric isomerism, abnormal specific heats, abnormal parachor values, Raman lines, heats of sirable aspects of the older definition and also so that dissociation, and many other properties. The organic the new definition will define the term as most generally chemist must know the structural formula before he used? Yes, very simply. While many better de& can be sure of the valence of some particular atom in a nitions will probably occur to the reader the following compound and he must know whether or not the bonds definition of valence has been given to students in elementary classes for several years and has been found associated with the atom in question are typical C-H or C-C like bonds. To the organic chemist valence is satisfactory. The mlme of an atom in a given com@und is a number a number representing the number of C-C, or C-H
representing the number of typical C-C, or C-H like bonds associated m'th that atom i n the cumpound. The valence of a positive or negative ion i s a number representing the number of positinre or negative unit charges associated with the ion. This definition is neither new nor startling in any way. In fact, the reader will probably say "old stuff," thereby indicating that it is high time to teach our students this "old stuff and give up "deSinitionsU of valence that do not define valence. The above deiinition necessarily excludes the valence number from application to atoms in compounds of unknown constitution as well as excluding its application to coijrdinated groups where the bonding is not of the usual type. Where uncertainty as to the nature of the bond occurs, the valence will be uncertain. The valence bonds are of course the typical covalent or electron-pair bonds. With the above deiinition of valence it would be permissible to speak loosely of the valence of carbon as being four, to indicate that in most of its compounds carbon is quadrivalent; similarly, the statement that sodium is univalent means that generally the sodium ion carries one charge. Obviously the two types of union, ionic and covalent or bonded, give periodic valence numbers, and the valence number of the atom is usually the same as that of the elementary ion. Thus the valence of C1 in methyl chloride is one and in NaCl i t is also one. In our definition of valence we have two distinct valence units. While in many ways this is undoubtedly undesirable, no serious difficulties will arise if we distinguish sharply between the two valence units and avoid attempts to combine them when dealing with complex ions. The addition of the number of charges carried by an ion to the number of bonds associated with a particular atom in the ion to obtain the valence number of that atom assumes that the charges are associated only with the atom in question. In most cases this assumption is definifely wrong. In cases where an ionic linkage takes the place of an ordinary bond the charge may perhaps be localided and in any case probably little or no misunderstandmg will occur if the charge is counted as the bond of the parent compound in arriving a t the valence of the atom in the ionic derivative. However, this is not generally advisable since in one case we are dealing with an atom in a compound and in the other with an atom in an ion, the ion itself having a valence number. For example, a number of bonded compounds of the trimethyltin radical are known, such as (CH3)~SnH,(CHa)aSnCl, (CH3)3SnOH, (CH&SnCaH6, where the valence of the tin atom is plainly four. In the compound (CH3)3SnNawe might consider the valence of the tin to be four in order to indicate its relation to other compounds of the trimethyltin group. Smce this is an ionic compound strict adherence to our definition requires a valence number for the ion as well as valence numbers for the atoms in the ion and the valence of the (CHa)3Snis one, while the valence of ti in the ion must be considered three. If we knew with certainty that the ion
charge is located on the tin atom we could say that the valence of tin in the compound is four, not otherwise. In the case of Sn(CH& the sum of the valences of the ion and of the central atom in the ion, give the usual valence number of that atom but this is by no means a general rule. For example, if the percblorate
ion is assigned the usual structnre &!-0,
1
the
0 valence of the chlorine atom must be considered four in the ion, the ion valence is of course one, and the sum five. The ammonium ion is, of course, univalent; the nitrogen in the ion is qnadrivalent if we consider all the bonds equivalent, or trivalent if we have any reason to suppose that the fourth bond is neither the typical single nor multiple bond. Similarly the valence of nitrogen in an amine oxide is four if we know that the 0-N bond is a single electron-pair bond similar to other covalent bonds, or i t is three if we know the bond to be very different from an ordinary bond. The valence of the sulfate ion is two but the valence of sulfur in the ion cannot be determined without a knowledge of the structure of the ion. Since the most generally ac0 cepted structnre for this ion is O:S:O, .. the valence of 0 sulfur must be considered four. This is not quite the usually accepted valence number and may not be quite so convenient as the process of adding and snbtracting valence numbers to arrive a t the ion charge. The ion charge can be ascertained in practically any way that actually involves counting the electrons. It is certainly ridiculous to say that chlorine in the perchlorate ion has a positive valence of seven, irrespective of the question as to the constitution of the oxide C120i. It cannot be overemphasized that the question of the valence relations in these complex ions is by no means settled by the outstanding chemists of the day. How then can we expect to give the student rules to cover these cases? Indeed, this is one of the more serious errors common to the trained pedagogue; the teaching of more than is known by the teacher or any one else. The advantages gained by this definition of valence are many and it may be used in introductory courses without difficulty and without any very great change in the general language of chemistry. In treating of the complex ions it is only necessary to state the facts or classify the i ~ types. n Where electronic configurations of the atoms are discussed, the difficulties conceming the complex ions disappear. It may be said that some of the older rules of valence facilitate teaching, but teaching which is facilitated by inconsistencies, self-contradictions, and directly misleading statements is little more than a social pastime.
