THE STRUCTURE OF MATTER: A BRlEF REVIEW OF PRESENTDAY CONCEPTIONS. 111. CHEMICAL COMBINATION MAURICEL. Huccms, Srmmno UNNERSITY, C~IPORNU
As was indicated in Table I1 of the first paper' of this series, atoms may be divided into nine diierent classes, according to the charges on their kernels. Expressing the same idea somewhat diierently, we can imagine all compounds to have been built up from hypothetical neutral atoms such as those diagrammatically represented in Fig. 2.
RIG. 2.-Illustrating the nine classes of neutral atoms, from which all compounds might theoretically be derived. The small circles represent valence electrons, outside of kernels having the net charges indicated.
Two Important Tendencies2 Of prime importance in considering combinations between atoms are two tendencies: (1) the tendency of the valence electrons to pair off, and (2) the tendency to form octets, i. e., to add electrons to the valence shell until it contains eight, this tendency being, in general, greater the greater the kernel charge. It should be emphasized that these are merely tendencies, for there are exceptions to both, as we shall see. These tendencies are sometimes satisfied, in whole or in part, by the transfer of electrons from one atom to another, producing charged atoms or ions. Thus a sodium atom or a hydrogen atom may give up an electron to a chlorine atom.3 Na + . & : - + ~ a + f :&:..
..
A pair of electrons can be shared between two atoms belonging to the valence shells of both at the same time. In sodium chloride or hydrogen 'THISJOURNAL. 3,1110-6 (Od.,1926).
' Our present views of combination between atoms, as sketched in the following paragraphs, are due almost entirely to G . N. Lewis. The p r e s s is ordinarily not so simple as represented here, but the principle is the same.
TAB STRU~WRE OP MATTER
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chloride vapor the molecules probably have structures which can be represented as follows: Na
:c!:
H
:G:
Other examnles are:
Sometimes two or three pairs of electrons are shared between two atoms, where a t least one of the atoms (usually both) so joined is carbon, nitrogen, or ~ x y g e n . ~
H:.c::o::
H.
:
:o::c::o.:
~:ciic:~
The atoms, other than hydrogen, in these compounds, as in the others mentioned above, have tetrahedral valence shells, being bonded together through tetrahedron comers, edges, or faces, according to whether the bonds are single, double, or triple. A pair of electrons shared between two atoms is often held more tightly by one than by the other. We then say the bond is semi-polar. All degrees of polarity can exist, from complete polarity, where there has been a complete transfer of an electron from one atom to another, to complete non-polarity, where an electron pair (or two or three pairs, in double or triple bonds) is shared equally by the two atoms. The atom having the larger kernel charge usually holds onto a shared electron pair more tightly than the other atom. This polarity is of great importance in determining the chemical and physical properties of substances. The formation of polyatomic ions from neutral elements involves both the transfer and the sharing of electrons. Thus the following groups of atoms all possess a negative charge due to the removal of one or more electrons from an electropositive atom or atoms, but the atoms within each monp are held together by semi-polar bonds:
As exceptions to the "rule of eight" or the "octet rule" may be mentioned the followine:
4 Some chemists prefer to assumesia-electronvalence shells in these compounds with only single bonds, but in the writer's opinion the evidence is strongly against such an-idea.
Other examples will be given among the "secondary valence compounds." A few molecules contain an odd number of valence electrons. These must be exceptions to the "rule of twou-the tendency of the valence electrons to form pairs. NO, NOz, ClOz, and (CsHa)aC are examples of such "odd molecules." The arrangements of valence electrons in these substances are still in the field of speculation. Secondary Valence6 Most of the compounds containing non-polar or semi-polar honds whose structures have been illustrated above can be synthesized on fiaper from the hypothetical neutral atoms (Fig. 2) by reactions in which one electron of each honding pair comes from each of the two atoms joined by that pair: A+.B+A:B
To obtain any of the poly-atomic ions illustrated, however, one must postulate reactions in which hoth electrons of one or more honding pairs coke from the Sam; &om: .
A + :B---tA:B
Although such reactions are quite as reasonable as those of the other kind, old-fashioned valence theory does not take them into account. In fact;'whenmer either reactions of the second type or a complete transfer of one or more electrons frmn atom to atom (ionization) must be assumed, the structural formulas as ordinarily written do not correctly represent tXe actual structures. Where there is ionization, a t least one of the "bonds" in the structural formula must he considered to represent not an actual bond, but a positive charge on one atom or group of atoms and a negative charge on another. The whole problem of the nature of "secondary valence" is now solved. "Secondary h e n c e c6mpounds" are those which cannot be thought of as having been formed from neutral atoms entirely by reactions of the lirst kind. Once formed, the honds formed by the two types of reaction are identical. These statements may be illustrated by a few examples of "secondary valence reactions:"
6
Huggins, J. Phys. Chem., 26, 601 (1922).
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It will be noted that in a number of the foregoing compounds an atom has more than four electron pairs in its valence shell. This is fairly common among the more electro-positive atoms but is relatively rare among the more electro-negative atoms.
