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The chemical elements and their compounds exhibit certain contrasts in their nature. These are indicated by the terms 'positive' and 'negative.' The use of these terms arises from the close relation between chemical and electrical properties. As a rule, when two or more bodies of different composition are brought in contact with each other, both are electrified -one becoming positive, the other negative. The greater the difference between the composition of the two substances the greater the electrical excitement will be. The difference which exists between the two bodies is called the electro-chemical difference. The direct measurement of the electric charge produced by the contact of heterogeneous bodies is diffi cult.

But the electro-chemical nature of the substances can be determined by another method. Many liquids and some solids are decomposed by the electric current. In this act of electrolysis, those constituents which are positive on contact are liberated with the positive electricity and those which become negative with the negative electricity, so that the electro-chemical nature of the constituents is easily recognised. The difference in the electro-chemical nature of a substance is not absolute, but merely relative, so that one element can be positive with regard to a second element and negative to a third. It has also been observed that a positive element in a compound can generally be replaced by a more positive and a negative by a more negative. This replacement of one element by another affords another means of ascertaining the electro-chemical nature of an element. The oxides and hydroxides of the positive elements have basic properties (i.e. they neutralise acids); the oxides, hydroxides, and some of the hydrides of the negative elements are acids.

If the elements are divided into the two classes-electronegative and electro-positive-these properties are regularly divided in the periods. In the atomic volume table, the positive elements are denoted by + and the negative by. positive nature changes in the same way as the metallic nature and malleability, i.e. twice in the large periods of atomic volumes.

The

The first family in the table on page 56 consists of positive elements, the alkali metals, Li, Na, K, Rb, Cs; the positive

character increases with the atomic weight, and caesium is not only the most electro-positive metal of this group but of all the elements. The metals Be, Mg, Ca, Sr, Ba, in the second family, closely resemble these metals in their electro-positive nature; again, we find the metal with the highest atomic weight, barium, is the most electro-positive. In the third family, containing Bo, Al, Sc, Y, La, Yb, the electro-positive nature is much feebler. The hydroxide of boron is a feeble acid, and aluminium hydroxide exhibits the properties of a weak acid as well as of a strong base: here again the negative character grows feebler and the positive stronger, as the atomic weight increases. In the fourth family C, Si, and Ti yield acids, but the higher members Zr, Ce, and Th have a more positive character. The elements in all these four families have one property in common: they form very stable compounds with oxygen, and consequently their oxides are difficult to reduce.

The four sub-groups, which are entirely composed of heavy metals, are of an opposite character:

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These elements are easily obtained from their oxides and analogous compounds by reduction. In the first two groups, the facility with which the compounds are reduced increases with the atomic weight. In the first and second family the positive character decreases as the atomic weight increases. This is exactly opposite to the behaviour of the refractory light metals of this family. Nothing is known of the third group in this respect, but in the fourth group lead (Pb) is more positive than tin (Sn); the positive character, as usual, increasing with the atomic weight.

The same contrasts occur in the following three families, only with this difference, that the first members do not belong to the refractory, but to the easily reducible, elements, with the exception of phosphorus, which is not so easily reducible as arsenic, antimony, and bismuth, but more easily than

vanadium, niobium, and tantalum. The chief group is here formed of the easily reducible elements :

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The sub-group is composed of the refractory elements:

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The three groups generally embraced in family VIII. are easily reducible :

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A comparison of the chemical properties with the position on the curve of atomic volumes shows that the refractory 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.

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When the Periodic Law' was first propounded, scandium, gallium, and germanium were unknown, and the positions these elements now occupy were 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 de

termined, 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 × 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 hexavalent, or they are said to have two, three, or more affinities. Hydrogen also 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

F

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:

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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 formulæ :

H

H-F H-O-H H-N-H

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, so far as their compounds with hydrogen are concerned, fluorine, chlorine, bromine, and iodine as monovalent; oxygen, sulphur, selenium, and tellurium as divalent; nitrogen, phosphorus, arsenic, antimony as trivalent; carbon, silicon as tetravalent.

As F, Cl, Br, and I are monovalent in the compounds in the first group and have the same valency as hydrogen, they

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