386 solid element proportional to the atomic weight of the element. The most striking way of exhibiting the connexion between the variable, atomic weight, and each of the variants, melting point and atomic volume, is to represent the values of the former as lengths marked off on a horizontal line, and the values of the latter as lengths on a vertical line; then, from each pair of points so marked to produce lines until they meet, and to draw a curve cutting the points of intersection of these lines. The curves thus obtained are drawn in the plate on page 269. The melting points are calculated from the initial temperature -273°; the numbers so obtained are divided by 7 in order to bring the curve within manageable limits. The values of the atomic volumes are multiplied by 4 to make the scale of the curve comparable with that of the curve of melting points. Lack of data is indicated by a broken line, or by a gap in the curve. Thus if the elements are arranged in order of increasing atomic weights, nitrogen, oxygen, and fluorine come after carbon and before sodium; but the atomic volumes of these elements are unknown, hence the dotted line in the curve of atomic volumes. Similarly a number of elements come between didymium and tantalum; the gap in the curve indicates that the atomic volumes of these elements have not been determined. The melting points of only about two-thirds of the elements have been determined; hence many parts of the curve of melting points are shewn as dotted lines. The curves shew that the melting points, and the atomic volumes, of the elements vary periodically with variations in the atomic weights of the elements. The value of either variant does not exactly repeat itself at definite intervals; but the elements fall into periods, in each of which the values of the melting point and the atomic volume decrease from a maximum to a minimum, and then again increase to a maximum. The nature of this periodical variation is best shewn by the curve of atomic volumes, as the data are here more abundant. The first period comprises the elements from lithium to sodium; the second, the elements from sodium to potassium; the third, the elements from potassium to rubidium; the fourth, the elements from rubidium to caesium; after caesium there is a great want of data. Elements, the values of whose atomic weights place them about midway between the first and last element of a period, have atomic volumes THE PERIODIC LAW. Os.396 ATOMIC VOLUMES ARE MULTIPLIED BY 4. MELTING POINTS (Calculated from-273) ARE DIVO BYT 270 250 240) 230 220 210 200 Cu 190 170 160 150 140 140 150 160 170 180 190 200 210 220 230 240 Thick line curve shews atomic volumes. Thin melting points. Dotted lines indicate that data are wanting THIS POINT SHOULD BE PLACED 66 DIVISIONS HIGHER 387 approximately equal to those of elements which occupy a similar position in another period; compare, for instance, the positions and atomic volumes of chromium, manganese, iron, nickel, and cobalt, with the positions and atomic volumes of rhodium, ruthenium, palladium, and silver; or compare the positions and atomic volumes of sulphur, selenion, and tellurium. If the only properties of elements which it was necessary to study were their melting points and atomic volumes, it is evident that the connexion between the values of these and the values of the atomic weights of the elements is so marked and definite that a system of classification might well be based on this connexion. But the periodic law asserts that the properties of the elements and their compounds in general, and not only one or two properties in particular, vary periodically with variations in the atomic weights of the elements. Let the 14 elements from lithium (Li=7) to chlorine (Cl = 35.5) be arranged in two series or periods of seven in each; thus Li = 7 Be = 9 B=11 C 12 N = 14 0 = 16 F = 19. P= 31 S = 32 Cl = 35.5. The difference in the values of the atomic weights of two consecutive elements varies from 1 to 3.5, the mean difference is about 2. The difference between the values of the atomic weights of two elements placed one under the other varies from 15 to 17; the mean difference is about 16. The following statement gives a general indication of the chemical properties of lithium and sodium, beryllium and magnesium, boron and aluminium, &c. Lithium and sodium: very light, soft, easily melted, metals; rapidly decompose cold water, thus 2 M+H2O = MOH + H; oxides, MO, alkali-forming and strongly basic; form salts MSO, MCO, MCI, &c.; do not combine with H. 2 Beryllium and magnesium: fairly hard metals, of low spec. gravity but high melting points; oxides basic but not alkali-forming; form salts MSO,, MCO,, MC, &c.; do not combine with H. Boron and aluminium: Al metallic, B non-metallic; B forms BH,, Al does not combine with H; Al forms salts 3 A1,380, Al,C ̧, &c. B forms BC, and perhaps B.3SO.; oxides are M2O,, BO, is acidic, Al,O, basic but feebly acidic towards strong alkalis; both dissolve in KOHAq forming borate or aluminate of K and evolving H. 2 3 Carbon and silicon: non-metals; both exhibit allotropy; neither forms salts by replacing H of acids; compounds are MH, MC, MO̟,, &c.; oxides are acidic; acids H‚MO̟ are very weak. 3 Nitrogen and phosphorus: non-metals; P exhibits allotropy; neither forms salts by replacing H of acids; both form strong oxyacids HMO,; N also forms HNO,, and P forms H.PO, H.PO, &c. ; oxides M,O, and M,O, are acidic; both combine with H forming MH ̧. 3 3 Oxygen and sulphur strongly negative non-metals; both exhibit allotropy; compounds with H are MH,, one neutral, the other a feeble acid; form many analogous compounds, e.g. P.M., As,M,, CuM, &c. &c. ვი Fluorine and chlorine: non-metals; very negative; neither exhibits allotropy; compounds with H, MH, are strong acids; oxides CIO and CIO, are acidic, no oxide of F is known. This statement shews that there is a very similar gradation of properties in each of these series or periods of seven elements; the first member of each period is a strongly positive metal, the last is a markedly negative non-metal; there is a regular decrease in the metallic, and an increase in the non-metallic, character of the members as each period is ascended, i.e. as the atomic weights increase. The relations between the chemical properties of a pair of consecutive members of one series are on the whole very similar to those of the corresponding pair of consecutive members of the other series. Thus if the symbol of an element is used to represent the general chemical character of that element, then we may say that Li Be = Na: Mg; or C : N = Si: P ; or O: FS: Cl. If the elements are arranged in order of atomic weights 388 from hydrogen (H = 1) to uranium (U = 240), and if they are then marked off into series or periods each of which contains 7 elements, it is found that in some cases the properties of one period are to a great extent a repetition of the properties of the preceding period, but in other cases no such repetition of properties is to be noticed; in other words, it is found that series of 7 elements sometimes form periods in which the 389 390 properties vary periodically with the variation of the atomic weights of the elements, but that other series do not shew any clear connexion of a periodic kind between the variation of atomic weights and the variation of properties. But if it is assumed that there are several gaps in the list of elements, to be filled up by the discovery of elements at present unknown, and if a peculiar, and at first sight abnormal, position is assigned to about a dozen elements, (s. Chap. xxvi.), then it is possible to arrange the elements in order of increasing atomic weights in periods, so as to shew a distinct connexion of a periodic kind between variation of atomic weights and variation of properties. The following table (p. 273) shews the arrangement of the elements in accordance with the periodic law. The values of atomic weights are given in round numbers. The elements in a vertical column form a Group; the elements in a horizontal column form a Series; the connotation of these terms will be discussed later. Hydrogen is placed in a series by itself. The elements in Group VIII. must be considered to some extent apart from the other elements. All the elements in the same group resemble each other in their chemical properties; there is a gradation of properties from the first member (that with smallest atomic weight) to the last. The elements in a series differ from each other; the difference becomes more marked as the series is ascended, that is as atomic weight increases, so that the greatest difference is that between the first and last members of the series. 5 2 5 Thus, taking Group V. we find that all the elements in this group form oxides of the composition M2O, (where M=N, P, V, &c.); that these oxides as a class are acidic, but that when M is one of the higher members of the group (Di to Bi) M.O, are very feebly acidic, and at the same time are also basic; that the lower members of the group are non-metals, the intermediate members are both metallic and non-metallic, and the element with highest atomic weight (Bi) is a metal; and that these elements taken as a whole are more like each other than they are like any other element or class of elements. Then, taking say Series 2, we have already seen how very different fluorine, the last member of the series, is from lithium, the first member of the series; and we have learned that no two of these elements could be placed in the same |