atom of phosphorus and each atom of fluorine, between the atom of tungsten and each atom of chlorine, and between the atom of cadmium and each atom of bromine, in the respective molecules.

Now, as no gaseous molecule has been obtained composed of one atom of carbon and more than 4 atoms of hydrogen, fluorine, chlorine, bromine, or iodine; as no gaseous molecule has been obtained composed of one atom of phosphorus and more than 5 atoms of hydrogen, fluorine, &c.; as no gaseous molecule has been obtained composed of one atom of tungsten and more than 6 atoms of chlorine, &c.; and as no gaseous molecule has been obtained composed of one atom of cadmium and more than 2 atoms of chlorine, &c.; we seem justified in asserting that an atom of carbon can directly interact with not more than 4 monovalent atoms, an atom of phosphorus with not more than 5 monovalent atoms, an atom of tungsten with not more than 6 monovalent atoms, and that an atom of cadmium can directly interact with not more than 2 monovalent atoms, in gaseous molecules.

We thus arrive at the notion of a limit to the number of 359 atoms between which direct interaction can occur in a gaseous molecule.

If we now apply this notion not only to gaseous molecules composed of one polyvalent atom united with monovalent atoms, but to all gaseous molecules, we find ourselves in possession of a good working hypothesis regarding the arrangement of atoms in molecules.

The hypothesis may be stated thus;-each atom in a gaseous molecule can directly interact with a limited number of other atoms.

Let us at once widen our conception of atomic valency, 360 and say that the valency of an atom is a number expressing the maximum number of other atoms between which and the given atom there is direct interaction in any gaseous molecule. But while thus widening the meaning of the term valency, let us agree to determine the valency of any atom by finding the maximum number of monovalent atoms with which it directly interacts, i.e. in this case with which it combines, in a gaseous molecule.

We have thus a perfectly definite method for finding the valencies of atoms, and we have also a wide range of application for these values.

It is customary to place small roman numerals above the 361

362

symbol of an element to represent the valency of an atom of that element; thus OI, CIV, Biш, WV, mean a divalent atom of oxygen, a tetravalent atom of carbon, a trivalent atom of bismuth, a hexavalent atom of tungsten, respectively. When a numeral is not placed over the symbol of an atom that atom is taken to be monovalent.

The table in par. 354 gives the results of the determinations of the valencies of atoms; the applications of these values will now be shewn by one or two examples.

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Two compounds exist each having the composition expressed by the formula C2HO; as both compounds have been gasified, this formula represents the atomic composition of the molecule of either compound. From the data given in the table in par. 354 it is evident that the atom of carbon is tetravalent, and the atom of oxygen is divalent; the atom of hydrogen is one of the standard monovalent atoms. In other words an atom of carbon can directly interact with not more than 4 other atoms, an atom of oxygen can directly interact with not more than 2 other atoms, and an atom of hydrogen can directly interact with not more than one other atom, in a gaseous molecule. We have agreed to represent direct interaction between two atoms in a gaseous molecule by the use of lines proceeding from the symbols of these atoms. How then can we represent the arrangement of one divalent 2 tetra- and 6 monovalent atoms? Let us assume (1) that each carbon atom directly interacts with the other carbon atom; (2) that neither carbon atom directly interacts with the other carbon atom. On assumption (1) the only possible representation of the atomic arrangement of the molecule CIVHO is

Now two compounds, and only two, exist having each the molecular composition C2HO. So far the facts are in keeping with the deduction from the hypothesis of valency.

But how can we tell which of the two compounds is I. and which is II. If the chemical properties of a molecule depend, partly, on the arrangement of the atoms which constitute the molecule, the chemical properties of compound I. must differ from those of compound II. To determine which compound is represented by formula I. and which by formula II., we must study the chemical properties of each isomeride CHO. The isomerides in question are called ethylic alcohol, and methylic ether. Each interacts with phosphorus pentachloride, but the products of the interactions are very different: ethylic alcohol interacts thus,

C2H2O + PCI = POC1 ̧ + HCl + C2H ̧C1 ;

5

methylic ether interacts thus,

3

The first interaction consists in the withdrawal of an atom of oxygen and an atom of hydrogen from the molecule CHO, and the putting in the place of these atoms of an atom of chlorine; the second interaction consists in the withdrawal of an atom of oxygen from the molecule CHO, the putting in the place of this atom of 2 atoms of chlorine, and the simultaneous separation of the group of atoms CHCl into two parts, each of which is composed of one atom of carbon united with 3 atoms of hydrogen and one atom of chlorine. Now symbol I. represents an atom of oxygen as directly interacting with an atom of carbon and also with an atom of hydrogen; as the atom of chlorine can directly interact with a single other atom only, the withdrawal of the group of atoms OH, and the substitution for it of the atom Cl, seems a very probable change. If this change proceeds the resulting molecule will be represented H H

by the symbol H-C-C

H H

Cl. On the other hand symbol

II. represents an atom of oxygen as directly interacting with 2 atoms of carbon, and with these atoms only; if this atom of oxygen were withdrawn and 2 atoms of chlorine put in its

but this molecule cannot exist (by hypothesis) because the chlorine atom is monovalent; hence the substitution of two atoms of chlorine for a single atom of oxygen in the molecule represented by symbol II. must result in the production of two

This conclusion is borne out by a further study of the properties of the two isomerides. Thus, sodium rapidly interacts with ethylic alcohol to produce C2H NaO+H; but sodium and methylic ether do not interact. The formula given for ethylic alcohol represents one, and only one, of the 6 hydrogen atoms as directly interacting with an atom of oxygen; we should therefore expect that sodium would replace either 5 atoms, or one atom, of hydrogen from the molecule of ethylic alcohol. As sodium replaces only a single atom of hydrogen we conclude that the atom replaced is that which is represented in the formula as directly interacting with an atom of oxygen. But if this is so, we should conclude that none of the hydrogen atoms in the molecule of methylic oxide would be replaceable by sodium. Another reaction which favours this conclusion regarding the interaction of sodium

2

with ethylic alcohol is that which occurs between sodium and water. This change is formulated thus H2O + Na = NaOH + H. Now the only possible way of representing the molecule Н ̧1 is H-O-H. The interaction between this molecule and an atom of sodium must be represented thus,

H-O-H+Na Na -O- H+H;

=

the atom of hydrogen which is replaced by an atom of sodium must be represented as directly interacting with an atom of oxygen. Therefore, as only one hydrogen atom in the molecule of ethylic alcohol is represented as interacting directly with an oxygen atom, it is fairly probable that this is the atom of hydrogen which is replaced by sodium.

Let us take another example of the application of the 363 conception of atomic valency, that is that each atom in a gaseous molecule can directly interact with a limited number of other atoms. A certain hydrocarbon has the molecular composition C.H. Can more than one compound exist having this composition? In other words, can we represent the arrangement of the atoms C,IVH in more than one way? As the atom of carbon is tetravalent, and that of hydrogen is monovalent, we must represent the two atoms of carbon in the gaseous molecule CH, as directly interacting; the only possible way of doing this is to write the formula thus,

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Therefore the hypothesis of valency, when applied to the compound C,H,, asserts that one and only one compound having this molecular composition can exist. As a matter of fact only one compound C2H, does exist.

There is another hydrocarbon C,H,; what are the ways 364 in which 2 tetra- and 4 mono-valent atoms can be arranged? Each carbon atom in the molecule C2H, must interact with another carbon atom; we may write the formula as