- Dec 1, 2005
Atomic Weights and Valency
The most important fact concerning the formation of various chemical compounds from elements is contained in the so-called law of constant proportions, which states that the relative amounts of different chemical elements needed to form a definite chemical compound always stand in a certain given ratio. Thus, when we place a mixture of hydrogen and oxygen gases in a thick-walled container and ignite the gases with an electric spark, we produce a rather violent chemical reaction (or explosion) which results in the formation of water. If the original proportions of hydrogen and oxygen are 1:8 by weight, the reaction will be complete and there will be nothing left over of either of the two gases. If, however, there is originally more hydrogen or more oxygen than is specified in the 1:8 proportion, then a corresponding excess of either gas will be left over. (There exists, however, another compound of hydrogen and oxygen known as hydrogen peroxide in which the ratio of the two elements is 1:16.)
The law of constant proportions was interpreted by the British chemist, John Dalton (1766-1844), as being due to atom-to-atom union in the formation of chemical compounds. To explain the above-described facts concerning water and hydrogen peroxide, one can assume that the weight ratio of the atoms of hydrogen and oxygen is 1:16 and that there is one atom of hydrogen per each atom of oxygen in hydrogen peroxide, while there are two hydrogen atoms per each oxygen atom in the case of water. Therefore, writing H for a hydrogen atom and O for an oxygen atom and using a subindex to denote the number of atoms of each, we can express the chemical composition of these two substances as:
water molecule = H2O
hydrogen peroxide molecule = HO (or H2O2, as it can be shown to be by other methods)
The second way of writing the expression for hydrogen peroxide indicates that this molecule has one oxygen atom too many in comparison with the much more common compound, water. And, indeed, hydrogen peroxide is an unstable substance that decomposes spontaneously according to the equation:
H2O2 → H2O + O
The free oxygen atoms that are liberated in this reaction possess strong oxidative properties, which make H2O2 useful in various bleaching processes, not the least of which is the turning of a dark-haired girl into a platinum blonde.
Similarly, the union of carbon and oxygen may result either in carbon dioxide, CO2, or, in the case of burning with an unsufficient supply of oxygen, in carbon monoxide, CO. In contrast to hydrogen peroxide, CO molecules lack one oxygen atom and are anxious to rob that extra oxygen atom from any other molecule which does not hold it strongly enough. The ratio by weight of carbon to oxygen is carbon monoxide is 3:4, which can also be written as 12:16. Since the atomic weight of oxygen was established as 16 (i.e., it weighs 16 times as much as a hydrogen atom, which for the present we can consider to be of unit weight), the atomic weight of carbon must be 12. Carbon also unites with hydrogen, giving rise to a gas known as methane or "marsh gas." The ratio of hydrogen to carbon in methane is 1:3 or 4:12, and, since 12 is the weight of one carbon atom, the formula of methane must be CH4. Let us now consider a slightly more complicated example presented by an analysis of ethyl alcohol, which is 52.2 percent carbon, 34.8 percent oxygen, and 13.0 percent hydrogen. By noticing that the ratio 52.2/34.8 is 1.50, whereas the ratio of the atomic weights of carbon and oxygen is only 0.75, we can conclude that there must be two carbon atoms for each oxygen atom. If there were only one hydrogen atom for each oxygen atom, the ratio of corresponding percentages would have to be 1/16 = 0.0625, but the ratio is actually 13.0/34.8 = 0.375, i.e., six times larger. Therefore there must be six hydrogen atoms per oxygen atom, and the formula for ethyl alcohol is C2OH6.
The ability of atoms to unite with one or more other atoms is known as chemical valency and can be represented in an elementary way by drawing on each atom a number of hooks that can be coupled with the hooks of other atoms. In the examples so far considered, we have ascribed to hydrogen atoms a valency of 1, to oxygen 2, and to carbon 4. The way atoms are then bound into molecules (the so-called structural formula of the molecule) is shown in Table 1-1.
Valence "hooks" can also act between identical atoms and bind them into "diatomic" or "triatomic" molecules of a simple chemical substance, as indicated in the last three items of Table 1-1. Similar relations can be found for other chemical elements and for more complicated chemical compounds.
Table 1-1 Molecular Structure of Various Compounds
In speaking about chemical valency, we must mention six very peculiar elements: argon, helium, krypton, neon, radon, and xenon. These do not possess any chemical valency whatsoever. The atoms of these elements despise any chemical intimacy and prefer to remain alone; they do not even form pairs between themselves as other atoms often do, so their molecules are always "monatomic." Closely connected with this chemical inertness is the fact that all these six substances are gases and liquefy only at very low temperatures. Using the self-apparent analogy, we call these elements noble gases or, sometimes, rare gases, since, indeed, they all are rather rare on the earth. As everybody knows, helium is used for filling balloons and dirigibles to avoid fires, and neon, which emits a brilliant red light when subjected to an electric discharge, is used for making luminous signs for advertising.