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The molecular and atomic theory asserts that a quantity of any gaseous element or compound is constituted of a very great number of minute particles, all having the same masses and the same properties, and all in constant motion. These particles, or molecules, are constituted of smaller particles which have a certain freedom of motion among themselves ; these smaller particles, or atoms, are of one kind and of equal masses when the molecule formed by their union is the molecule of an element; but the atoms are of different kinds and different masses when the molecule formed by their union is the molecule of a compound.

Chemical change, according to the molecular and atomic theory, is an interaction between molecules, and it results in the formation of new molecules. In very many cases of chemical change the interacting molecules are separated into their constituent atoms, and these atoms rearrange themselves to form new molecules; but in some cases the interaction of the original molecules probably consists in the direct formation of more complex molecules. Thus the interactions of hydrogen and chlorine to produce hydrogen chloride, and of hydrogen and oxygen to produce water-gas, are represented thus by the theory of atoms and molecules :

(1) H. + Cl, = 2HCl; (2) 2H, +0, = 27,0. The symbols H, Cl,, 0, HCI, H,O, each represents the atomic composition of a molecule of an element or compound. But the interactions of water and cobalt chloride, or water and copper sulphate, are probably best represented by equations which assume the change to consist in the combination of molecules of the interacting substances to produce more complex molecules : thus,

(1) COCI, + 27,0 = CoCl. 24,0;

(2) CuSO, +5HO=Cuso,.57O. The theory regards most physical changes as changes in the 313 rates of motion, without changes in the atomic compositions, of molecules. But changes usually called physical may result in the coalescence of molecules into more or less complex aggregations which are stable under definite conditions of temperature, pressure, &c.

The theory of molecules and atoms does not therefore give us a means of sharply distinguishing between physical and chemical change. The typical chemical change results in a redistribution of the atoms of the interacting molecules so as to form new molecules; the typical physical change results in changes in the rates of motion of molecules without any redistribution of the parts of these molecules. But there are many changes which cannot be placed wholly in one or other of these classes. Every chemical change is accompanied by physical change : the portion of the change we call chemical is only one part of the complete occurrence. Even if the theory gave a sharp and clear definition of each kind of change, it could not give a means whereby we might classify all actually occurring changes into chemical on the one hand, and physical on the other.

The laws of chemical combination find a simple explana- 3 tion in terms of the molecular and atomic theory. The atom is the ultimate particle of matter of which we take cognisance in chemistry. The properties of a molecule depend, among other conditions, on the nature and number of the atoms which form it; this is the law of fixity of composition. If two or more different kinds of atoms combine to form several different molecules, each molecule must be composed of atoms of one kind + ac' atoms of another kind + x" atoms of another kind

+ ac''' atoms of another kind &c., and x, ac', ac'', cm' must be whole numbers, because the atom is, by definition, indivisible; this is the law of multiple, and the law of reciprocal, proportions.

The molecular and atomic theory throws light on the 315 conceptions of combining and reacting weights. The reacting weight of a gas is the molecular weight of that gas. The combining weight of an element, as the term was defined in Chap. vi. par. 79, is the atomic weight of that element.

Thus we found (s. Chap. VI. par. 86) that the reacting weight of water is 18; and that one reacting weight of this compound is composed of two combining weights of hydrogen united with 1 c. w. of oxygen. Translated into the language of the molecular and atomic theory this statement reads as follows :—the molecular weight of water-gas is 18, and a molecule of water-gas is formed by the union of 2 atoms of hydrogen with 1 atom of oxygen.

We know that 36.5 parts by weight of hydrogen chloride are formed by the combination of 1 part by weight of hydrogen with 35:5 parts by weight of chlorine. In order to express this fact in terms of combining and reacting weights, we say that one c. w. of hydrogen combines with one c. w. of chlorine to produce one reacting weight of hydrogen chloride. The molecular and atomic theory expresses the same fact by saying that one molecule of hydrogen interacts (not combines) with one molecule of chlorine to produce 2 molecules of hydrogen chloride.

We formerly applied the term reacting weight to compounds only. We now apply the term molecule to elements as well as to compounds. But when we are dealing with solid bodies which have not been gasified, we cannot in strict accuracy speak of the interactions of molecules of these bodies. Thus when boron and aluminium are strongly heated together under proper conditions two compounds, AlB, and AlB,, are produced. As neither boron nor aluminium has been gasified, and as neither of the borides of aluminium has been gasified, we do not know the molecular weights of any of the bodies taking part, or formed, in this reaction : we cannot therefore say how many molecules of each element have taken part in the change nor how many molecules of each compound have been formed. Again, when solutions of barium chloride and sodium sulphate, in aqueous solutions, are mixed in the ratio BaCl, : Na, SOX, the change of composition which occurs may be represented thus:

BaCl, Aq + Na SO, Aq = BaSO, + 2NaClAq. We may read the equation as meaning :—one reacting weight of barium chloride interacts with one reacting weight of sodium sulphate to produce one r. w. of barium sulphate and 2 r. ws. of sodium chloride; but the equation cannot be read as certainly meaning, one molecule of barium chloride interacts with one mol. of sodium sulphate to produce one mol. of barium sulphate and 2 mols. of sodium chloride. As none of the bodies taking part in this change have been gasified we do not know the molecular weight of any of them. We might read the equation thus; atomic aggregates of barium chloride and sodium sulphate interact to produce atomic aggregates of barium sulphate and sodium chloride.

The definition of molecule is a physical definition; it is stated in terms which have an accurate meaning only when used of gaseous elements and compounds. If we choose to use the term in speaking of the phenomena of liquid and solid bodies we must not forget that the term cannot then be accurately defined. The definition of reacting weight is a chemical definition; but the term reacting weight is much vaguer than the term molecule. The reacting weight of a solid or liquid compound is doubtless an aggregation of atoms which interacts, as a whole, with other aggregations of atoms; but whether the number of atoms in this aggregation is the same in all chemical changes we do not know.

We have seen in Chap. xv. that the atomic weights of 316 most of the elements have been determined, either by the method based on the law of Avogadro, or by that founded on the generalisation 'atomic weight into spec. heat = a constant.' But the molecular weights of only a few elements have been determined. About 70 elements are known; 14 of these have been gasified; therefore the molecular weights of only 14 are known. The specific gravities in the gaseous state, and hence the molecular weights, of some of these 14 elements are constant through a wide range of temperature; the specific gravities, and hence the molecular weights, of others have very different values at different temperature-intervals. The most probable explanation of the changes in the values of the molecular weights of certain elements is that the atomic compositions of the molecules of these elements are different at different temperatures. Thus sulphur-gas from about 450° to about 600°, is 96 times heavier than an equal volume of hydrogen at the same temperature; therefore the molecular weight of sulphur-gas between 450° and about 600° is approximately 96 x 2 = 192. But from about 800° and upwards sulphur-gas is only 32 times heavier than hydrogen; therefore the molecular weight of sulphur-gas at temperatures above 800° is approximately 32 x 2 = 64. The atomic weight of sulphur is 31.98; this number is determined by applying Avogadro's law to many gaseous compounds of sulphur, and it is verified by determinations of the spec. heat of sulphur.

Now 31.98 x 6 = 191•88, and 31.98 x 2 = 63.96 ; therefore we conclude ; (1) that sulphur-gas at 450° to 600° has the molecular weight 191.88, and at 800° and upwards the molecular weight 63.96 ; and (2) that the molecule of gaseous sulphur at 450° to 600° is composed of 6 atoms, or is hexatomic, and that the molecule at 800° and upwards is composed of 2 atoms, or is diatomic.

The expression atomicity of a molecule is used to denote the number of atoms which form the gaseous molecule of an element or compound. The data for classifying the molecules of elements in accordance with their atomicities are presented in the following table.



Atomicity of elementary gaseous molecules.
Monatomic Diatomic Triatomic Tetratomic Hexatomic

Hydrogen Oxygen Phosphorus) Sulphur
Cadmium Chlorine (as ozone) Arsenic (about 4000 to 6000)

Selenion (at temps. below
Iodine Iodine

(7000 to about 8000). white heat)
(at about 15000 (2000 to about
and upwards) 10009)
(? Bromime Oxygen
at 18000 and Sulphur

(8000 and upwards)
(12000 and
Arsenic 3

(at white heat)
The molecular weights of some gaseous compounds also
vary with variations of temperature. Thus nitrogen tetroxide
at very low temperatures is about 46 times heavier than hydro-
gen, but at higher temperatures it is only 23 times heavier than
hydrogen ; therefore this gas has two molecular weights which
are approximately equal to 92 and 46 respectively. Determina-
tions of the atomic weights of nitrogen and oxygen, and
accurate analyses of the compound nitrogen tetroxide, shew
that the composition of the molecule of this gas at very low
temperatures is represented by the formula N O. (N, = 28.02,
0. = 63.84) = 91.86, and at higher temperatures by the formula
NO,=45.93. We shall examine the relations between changes
of molecular weight and changes of temperature in more detail
later (s. pars. 334 to 337).

The conception expressed in the term atomic weight is now



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