The four lower types RX, RX2, RX3, and RX4 are met with in compounds of the elements R with chlorine and oxygen, and also in their compounds with hydrogen, whilst the four higher types only appear for such acid compounds as are formed by chlorine, oxygen, and similar elements.
Among the oxygen compounds the saline oxides which are capable of forming salts either through the function of a base or through the function of an acid anhydride attract the greatest interest in every respect. Certain elements, like calcium and magnesium, only give one saline oxide—for example, MgO, corresponding with the type MgX2. But the majority of the elements appear in several such forms. Thus copper gives CuX and CuX2, or Cu2O and CuO. If an element R gives a higher type RXn, then there often also exist, as if by symmetry, lower types, RXn-2, RXn-4, and in general such types as differ from RXn by an even number of X. Thus in the case of sulphur the types SX2, SX4, and SX6 are known—for example SH2, SO2, and SO3. The last type is the highest, SX6. The types SX5 and SX3 do not exist. But even and uneven types sometimes appear for one and the same element. Thus the types RX and RX2 are known for copper and mercury.
Among the saline oxides only the eight types enumerated below are known to exist. They determine the possible formulæ of the compounds of the elements, if it be taken into consideration that an element which gives a certain type of combination may also give lower types. For this reason the rare type of the suboxides or quaternary oxides R4O (for instance, Ag4O, Ag2Cl) is not characteristic; it is always accompanied by one of the higher grades of oxidation, and the compounds of this type are distinguished by their great chemical instability, and split up into an element and the higher compound (for instance, Ag4O = 2Ag + Ag2O). Many elements, moreover, form transition oxides whose composition is intermediate, which are able, like N2O4, to split up into the lower and higher oxides. Thus iron gives magnetic oxide, Fe3O4, which is in all respects (by its reactions) a compound of the suboxide FeO with the oxide Fe2O3. The independent and more or less stable saline compounds correspond with the following eight types :—
R2O; salts RX, hydroxides ROH. Generally basic like K2O, Na2O, Hg2O, Ag2O, Cu2O; if there are acid oxides of this composition they are very rare, are only formed by distinctly acid elements, and even then have only feeble acid properties; for example, Cl2O and N2O.
R2O2 or RO; salts RX2, hydroxides R(OH)2. The most simple basic salts R2OX2 or R(OH)X; for instance, the chloride Zn2OCl2; also an almost exclusively basic type; but the basic properties are more feebly developed than in the preceding type. For example, CaO, MgO, BaO, PbO, FeO, MnO, &c.
R2O3; salts RX3, hydroxides R(OH)3, RO(OH), the most simple basic salts ROX, R(OH)X3. The bases are feeble, like Al2O3, Fe2O3, Tl2O3, Sb2O3. The acid properties are also feebly developed; for instance, in B2O3; but with the non-metals the properties of acids are already clear; for instance, P2O3, P(OH)3.
R2O4 or RO2; salts RX4 or ROX2, hydroxides R(OH)4, RO(OH)2. Rarely bases (feeble), like ZrO2, PtO2; more often acid oxides; but the acid properties are in general feeble, as in CO2, SO2, SnO2. Many intermediate oxides appear in this and the preceding and following types.
R2O5; salts principally of the types ROX3, RO2X, RO(OH)3, RO2(OH), rarely RX5. The basic character (X, a halogen, simple or complex; for instance, NO3, Cl, &c.) is feeble; the acid character predominates, as is seen in N2O5, P2O5, Cl2O5; then X = OH, OK, &c., for example NO2(OK).
R2O6 or RO3; salts and hydroxides generally of the type RO2X2, RO2(OH)2. Oxides of an acid character, as SO3, CrO3, MnO3. Basic properties rare and feebly developed as in UO3.
R2O7; salts of the form RO3X, RO3(OH), acid oxides; for instance, Cl2O7, Mn2O7. Basic properties as feebly developed as the acid properties in the oxides R2O.