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the metallic elements ; but chemically considered they are both metallic and non-metallic. They differ considerably from all the other elements.
The properties of these elements are in keeping with the position assigned them by the periodic law : each section follows a series of elements the lower embers of which are metallic, and the higher members are physically metallic but chemically both negative and positive.
We have now learned something of the classification of 497 elements and compounds based on the periodic law.
The elements fall into series and groups : each series, with the exception of the first, is composed of seven elements; Series 1 contains a single element, hydrogen ; each group is divided into two families, those elements which occur in even series, and those which occur in odd series; a complete family consists of six elements. The nine elements, iron, nickel, cobalt, rhodium, ruthenium, palladium, iridium, osmium, and platinum, are placed to some extent apart from the others; the first section of this group (iron to cobalt) belongs both to Series 4 and Series 5, the second section (rhodium to palladium) belongs to Series 6 and 7, the third section (iridium to platinum) belongs to Series 10 and 11.
The properties of the elements in any series vary from that with the smallest atomic weight to that with the largest, so that the first and last members of the series are the most unlike one another. When the gradation of properties has been completed through a series, the properties, as it were, swing back to the starting point, and exhibit a similar gradation in the next series. The properties of the first members of Groups II. to VII., and of the first and second members of Group I., are to a certain extent typical of the properties of all the other members of these groups.
The properties of an element and its compounds are connected with the general properties of the group to which the element belongs and also with the gradation of properties in that group; they are likewise connected with the general properties of the series and with the gradation of properties in the series to which the element belongs. Inasmuch as the properties of each group and series are connected with the properties, and the gradation of properties, of the other groups and series, it follows that the relations of any element to other similar elements are to be elucidated only by studying the M. E. C.
position of the specified element in the complete scheme of classification.
The periodic law seeks to connect the changes in the properties of the elements, and in the compositions and properties of compounds, with the changes in the atomic weights of the elements; and it endeavours to make this connexion definite and to present it in accurate terms.
If the atomic weight of an element is known, the place of the element in the classificatory scheme is determined, and therefore the properties of the element and its compounds can be stated in a general way. If the properties of an element and its compounds are determined, the position of this element in the orderly sequence of elements can be found, and thereby an approximately correct value can be deduced for the atomic weight of the element. We have had examples. of the application of the periodic law both to the classification of elements and to the determination of the best values of the atomic weights of elements.
The valencies of the atoms of the elements are undoubtedly important factors determining the compositions of compounds. These valencies probably vary periodically with variations in the atomic weights of the elements; but the valencies of so few elements have been definitely determined that we are not at present able to state the connexions between changes of atomic valencies and of atomic weights.
The maximum valencies of all the elements in Series 2, except lithium, have been determined by considering the compositions of gaseous molecules of compounds of these elements with monovalent atoms. The valencies are as follows;
'I. II. III. IV. V. VI. VII. Series 2. Li Be B C N o F Valency. (? one) two three four three two
The atom of lithium is probably monovalent; assuming this atom to be monovalent, we see that the maximum valency of the atoms of the elements of Series 2 increases from the first to the middle member, and then decreases to the last member.
We cannot assert that maximum atomic valency varies in all the series exactly as it does in Series 2; nevertheless, the assumption that it does thus vary would certainly in some cases lead to results which are confirmed by observation.
Thus, the assumption requires the atoms of the elements in Group IV. to be tetravalent; the maximum valencies of the atoms of all the members of this group, except cerium, have been determined and the atoms have been found to be tetravalent. On the other hand, the assumption requires the atoms of the elements in Group VI. to be divalent; but the existence of the gaseous molecules MoCl;, TeCl,, WCI, UCI,, proves . that the maximum valency of some at least of these atoms is greater than two.
The conception of atomic valency--the conception, that is, that every atom in a molecule directly interacts with a limited number of other atoms—has been deduced from the study of yaseous molecules, and is strictly applicable to gaseous molecules only. But the greater number of the compounds of inorganic chemistry are non-gasifiable bodies; the conception of atomic valency cannot therefore, at present, be made use of, otherwise than in a broad and general way, in questions regarding the conditions which determine the compositions of compound molecules.
The periodic law asserts that the compositions of com- 499 pounds vary periodically with variations in the atomic weights of the elements. If we are content to use the expression composition of a non-gasifiable compound as meaning the ratio of the numbers of atoms forming the chemically reacting weight of the compound, then we can express the compositions of classes of similar compounds by general formulae, and
can trace connexions between these formulae and the atomic weights of the elements.
O S SO
2' 2' 2 CO2
&c., then the compositions of a great many important
LiCl, Na,O, K,SO,
PtPCI, PŁCISO,(OH). The compositions of the well marked compounds of the elements of a group become more varied as we pass from the lower to the higher groups.
It would however be out of place in an elementary book to attempt to generalise the connexion between the changes in the forms of compounds and the variations in the atomic weights of the elements. Let it suffice to note that the arrangement of the elements in accordance with the periodic law indicates the existence of such a connexion, and points the way by which the nature of this connexion may be elucidated.
If the properties of the elements and their compounds vary periodically with variations in the atomic weights of the elements, accurate determinations of the atomic weights of all the known elements must be demanded in chemistry. The
values given for these constants in the tables on pp. 57, 59, and 60, are given in round numbers only. The atomic weights of many elements have been determined with great accuracy, those of other elements only with a fair degree of accuracy, and those of a few elements with but little accuracy. The following table presents the most trustworthy results of the various determinations.
Atomic weights of the elements.
126.53 Selenion 136.8 Iridium 192.5 Silicon 9:08 Iron
55.9 Silver 208
Lanthanum 138.5 Sodium 10.9 Lead
206.4 Strontium 79.75 Lithium
7.01 Sulphur 112 Magnesium 24
Tantalum 132.7 Manganese 55
Tellurium 39.9 Mercury 199.8 Thallium 11.97 Molybdenum 95.8 Thorium 139.9 Nickel
15.96 Vanadium 144
Palladium 106.2 Yttrium
Phosphorus 30.96 Ytterbium
Potassium 39.04 Zirconium
At. Wt. 104:4 44 78.8 28:3 107.66 23 873 31.98 182 125 203.64 231.8 117.8
48 183.6 240 51.2 89.6 173 64.9 90