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63

A comparison of the chemical properties with the position. on the curve of atomic volumes shows that the difficultly-reducible elements occur on the descending, and the easily-reducible elements on the ascending, portions of the curve, but the change from positive to negative is seen on both portions of the

curve.

§ 38. Theoretical Prediction of Properties.-The close connection between atomic weights and properties renders it possible to predict the unknown properties of an element as soon as its atomic weight is ascertained, and, on the other hand, the atomic weight can be deduced approximately from the chief properties of an element.

When the Periodic Law' was first propounded, scandium, gallium, and germanium were unknown, and the position these elements now occupy was represented by blank spaces. Mendeléeff ventured to predict the properties of these undiscovered elements, and his predictions were afterwards verified when the elements were discovered and investigated. The speedy recognition of the value of this systematic arrangement of the elements was, to a great extent, the result of this happy verification of Mendeléeff's predictions. On the other hand, some atomic weights had been incorrectly determined, and attention was called to this fact by the circumstance that these elements did not fit into the system.

The atomic weight of caesium was ten units too low; indium was only two-thirds of the value now in use. Earlier determinations placed platinum before iridium and iridium before osmium, but the properties of these metals indicated that the order should be reversed, and this has been confirmed by the new atomic weight determinations of K. Seubert. The question whether the atom of beryllium corresponds to two or three equivalents—that is, whether the atomic weight is 2 × 4·54=9·08 or 3 x 4.54 13.62-has been decided by the followers of the periodic system in favour of the first assumption, because there is no space for an element with the atomic weight, 13.6, between carbon (C=11.97) and nitrogen (N=14.01), and an element possessing the properties of beryllium would be out of place in such a position. The question was definitely settled by the determination of the vapour density of beryllium chloride

by Nilson and Pettersson. This result has also materially influenced the recognition of the Periodic System.

§ 39. Periodicity of Valency.-The elements differ widely in their combining power. The atoms of some elements can only combine with a single atom, but the atoms of other elements can each unite with two, three, four, or more other atoms. They have double, treble, &c., the power of the other atom, and are said to be di-, tri-, tetra-, penta- or hexa-valent, or they are said to have two, three, or more affinities. Hydrogen again forms the standard of comparison, as it does in the case of the equivalent and atomic weights. The combining powers of the chemical elements vary regularly with the atomic weights.

All those elements are called 'monovalent' that have an atomic weight equivalent to one atom of hydrogen; if their atoms unite with or displace two atoms of hydrogen, they are said to be divalent.

The determination of this property of chemical valency is simple enough in principle. For, if the atomic weight of an element is equal to its equivalent weight (§ 11), the element is monovalent; if the atomic weight is double the equivalent weight, it contains two equivalents and the element is divalent.

Generally speaking, the chemical valency is determined by the number of equivalent weights contained in the atomic weight. The chemical valency determined by this method is a periodic function of the atomic weight. Before studying this relationship, it is necessary to consider the methods of determining valency.

§ 40. Determination of Chemical Valency.-The valency of an element is most easily determined from the composition of the molecule of its hydrogen compounds. These hydrides are not numerous; they may be divided into four types :—

As the hydrogen atom can only unite with a single atom of its own class to form the molecule H2, we must assume that the hydrogen atoms contained in the compounds under II., III.,

and IV. are united to the other constituents. These compounds may be represented graphically by the formula

H

H-C-H

H

The dashes indicate the manner in which the atoms are supposed to be united together. As the elements in the four types are incapable of combining with a larger number of hydrogen atoms, we regard fluorine, chlorine, bromine, and iodine as monovalent; oxygen, sulphur, selenium, and tellurium as divalent; nitrogen, phosphorus, arsenic, antimony as trivalent; carbon, silicon as tetravalent; in their compounds with hydrogen.

As F, Cl, Br, and I are monovalent in the compounds in the first group and have the same valency as hydrogen, they may be used like hydrogen, for the purpose of determining the valency of other elements. This is very desirable, as their compounds are much more numerous than those of hydrogen. A comparison of the fluorides, chlorides, bromides and iodides of the first four families shows that their compounds correspond to the four types which have just been mentioned, e.g. :

The chemical valency is constant for each family, and, with an increasing atomic weight, rises from one family to the next, by one unit.

The following families behave in a similar way, but at first sight the relationship appears somewhat more complicated, the composition of the typical hydrides indicating that the valency decreases, thus :

F

But these and similar compounds do not contain the maximum amount of chlorine or other monovalent atoms, with which the elements are capable of uniting. Compounds of the following composition are known:

Some of these compounds are very unstable: ammonium chloride, phosphorus pentachloride, and iodine trichloride decompose on volatilisation, e.g.: NHCl=NH ̧+HC1; PC1=PC1 ̧ +Cl2; ICl2=ICl+Cl2; others can be converted into vapour without decomposition, e.g.: PF, NbCl, and TaCl, TeCl, MoCl ̧, WC. This difference in the behaviour of the compounds is explained by the fact that the affinity of the negative elements for chlorine and its analogues is feeble, and consequently the chlorine atoms are easily separated from the compound. The behaviour of sulphur is very remarkable: the tetrachloride SCI, can only exist at 20° C.; even at 0° C. it begins to decompose into SCI,, and on distillation half the residual chlorine is lost. Sulphur does not form any definite compounds with bromine and iodine. Phosphorus combines with five atoms of fluorine to form a stable compound. PC1, and PBr, easily part with two atoms of chlorine or bromine respectively: but phos

OXIDES AND HYDROXIDES

67

phorus can only unite with three atoms of iodine; as a rule it only combines with two, forming PI.

We may assume that the difference in the behaviour of the elements belonging to families V. and VI. towards the non-metals is not due to a difference in their valency, but is probably caused by a difference in the force with which they attract the monovalent elements.

This assumption is confirmed by an examination of the oxygen compounds. As an atom of oxygen is equivalent to two atoms of hydrogen and is divalent, any other atom which has the power of uniting with one atom of oxygen is also divalent. One trivalent atom could combine with oxygen in the proportion of one to one and a half, or, more correctly, two trivalent atoms can unite with three atoms of oxygen. The radical hydroxyl -OH is formed by the union of one atom of hydrogen with one atom of oxygen; as the oxygen still has the power of uniting with a second atom, the radical is monovalent. The valency of an element is represented by the number of hydroxyls with which an atom combines, or by twice the number of oxygen atoms which in its oxide would be united with one atom. In order to permit of a uniform comparison, the formula of the oxides in the following table are given as though they contained two atoms of the other element, even where the molecule may only contain one atom.

This table shows the remarkable relation which exists between valency and atomic weight. The number of each family indicates its valency. In families I. to VII. only oxygen and