sub-group. Both the hydrogen valence and the oxygen valence are separately recognized in the headings of the vertical columns. The many breaks in the list indicate hypothetical elements that may yet be discovered. Both physical and chemical properties may be determined to a very marked degree by the periodic law. This is very clearly shown in the cases of the measurable physical properties, which manifest either a maximum or a minimum in the middle of both large and small periods, as may be seen in considering the specific gravities of some of the elements: Si Р S CI Na Mg Α1 Cr Mn Fe Co Ni Cu Zn Ga As Se Br 5.5 6.8 7.2 7.9 8.5 8.8 8.8 7.1 5.9 5.6 4.6 2.9 K Ca Sc Ti V .86 1.6 3.8 Also in the case of the chemical properties, maxima and minima may be shown in the case of valence, the hydrogen valence being at the maximum in the middle of the period, thus: NaR MgR AIRS SiH, PH, SH2 CIH While in the case of the salt-forming oxides the maximum valence increases with the small periods, thus: Na2O MgO AlgО SiO2 P2O5 SO, Cl207 And in the large periods increases with a double periodicity: CuO ZnO GagOg ASO SеOg Br2O7 On account of the gradations in properties of the elements the existence, position, and properties of undiscovered elements may be predicted. In general the value of any property of an element is the mean between the values of the same property of the elements on all four sides of it in the table. When this table was first prepared there were gaps between boron and yttrium, between aluminum and indium, and between silicon and tin. these hypothetical elements Mendelejeff assigned the names of eka-boron, eka-aluminum, and eka-silicon, and minutely described their properties. In 1875, 1879, and 1886 gallium, scandium, and germanium were discovered respectively with the properties assigned by Mendelejeff. Το The only explanation of the agreement in the properties of the elements with the atomic weights is that all the elements are aggregations of the same primordial substance. Hydrogen was once believed to be this substance, chiefly because of the fact that it looked at first as if the atomic weights of all the elements were simple multiples of the hydrogen atom. This has been shown to be incorrect by recent accurate atomic weight determinations. (See reference to Prout's law, p. 109.) PART III. CHEMISTRY OF THE METALS.. CHAPTER I. THE ALKALI METALS. General Characters.-The alkali group of metals consists of Potassium, Sodium, Lithium, Rubidium, Cæsium, and the atomic group Ammonium. In this group, the valence of the individual members is the immediate reason for so classifying them. They have numerous other properties in common, for instance, their soft, waxlike consistence, their low specific gravity, all but rubidium and cæsium being lighter than water, their strong metallic lustre, and their low melting points. Of course, where the metals are to be compared, ammonium is excluded; but the compounds of all the members bear a close resemblance to one another. The metals all energetically decompose water, evolving hydrogen, and forming hydrates in solution which have a strongly alkaline reaction. The salts are nearly all soluble in water, the chief exceptions being lithium phosphate and carbonate. Potassium, ammonium, rubidium, and cæsium form insoluble tartrates and chloroplatinates, while sodium and lithium do not. Some other distinguishing characters of their salts will be noted when they come to be treated analytically. Symbol, K. POTASSIUM. Atomic Weight, 39.1. Valence, I. History. The salts of potassium have been known from the earliest times. They were probably first extracted from wood ashes under the name alkali. Previous to 1736 there was no distinction between the salts of potassium and sodium. The former then became known as potashes. |