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to points of connexion between the properties and the atomic weights of the elements.

In 1864 Newlands arranged a number of elements in order of their atomic weights, and shewed that these elements might be divided into groups of seven, each of which groups to some extent repeated the properties of the next group. "The eighth element” said Newlands “starting from a given element is a kind of repetition of the first, like the eighth note of an octave in music.” In subsequent papers Newlands insisted on the general applicability of what he called the law of octaves.'

It is however chiefly to Mendelejeff that we owe the systematic correlation of the atomic weights with the chemical and physical properties of the elements, and the properties of their compounds.

The properties of the elements and their compounds vary 384 periodically

with variations in the atomic weights of the elements. This statement is the outcome of the work of Mendelejeff, Lothar Meyer, Newlands, and many other chemists.

This statement, or a statement equivalent to this, is known as the periodic law. The rest of this book will be devoted to an attempt to amplify and explain the periodic law.

The properties of the chemical elements and their com- 385 pounds is the phenomenon to be examined; the variable is the atomic weight of the elements: the law asserts that with a continuous change of the variable the phenomenon repeats itself at definite intervals.

A quantitative value cannot be given to the phenomenon 'properties of the elements and their compounds.' To illustrate the statement of the periodic law, it is necessary to choose a definite measurable property of the elements, or of a series of similar compounds, and to trace the relation between the variation in the values of this property and the variation in the values of the atomic weights of the elements.

We shall choose two properties, one physical, the other partly chemical and partly physical. The properties are (1) the melting points, (2) the atomic volumes, of the elements. The atomic volume of an element is defined to be spec. gravity of solid element

-; atomic weight this quotient represents the volume occupied by a mass of the

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

386

THE PERIODIC LAW.

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ATOMIC VOLUMES ARE MULTIPLIED BY MELTING POINTS (Calculated from:273) ARE DIVENT.

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120 130 150 160 170 190 200 210 220 230 240
Thick line curve shews atomic volumes.
Thin
Dotted lines indicate that data are wanting

melting points. THIS POINT SHOULD BE PLACED 66 DIVISIONS HIGHER

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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.
Na = 23 Mg=24 Al= 27 Si = 28 P=31 S = 32Cl = 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

M +H0 = MOH + H; oxides, MO, alkali-forming and strongly basic; form salts M.SO, M,CO,, MCI, &c.; do not combine with H.

Beryllium and magnesium : fairly hard metals, of low spec. gravity but high melting points; oxides basic but not alkali-forming; form salts MSO, MCO, MOI, &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

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A1,380., Al,C., &c. B forms BCl, and perhaps B,350,; oxides are M,Og, B, O, is acidic, A1, 0, basic but feebly acidic towards strong alkalis; both dissolve in KOHAq forming borate or aluminate of K and evolving H.

Carbon and silicon : non-metals; both exhibit allotropy ; neither forms salts by replacing H of acids; compounds are MH,, MCI,, MO,, &c.; oxides are acidic; acids H,MO, are

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 MHz.

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.

Fluorine and chlorine: non-metals; very negative; neither exhibits allotropy ; compounds with H, MH, are strong acids; oxides cl,o 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 0 :F=S: 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

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