Page:Outlines of Physical Chemistry - 1899.djvu/52

 32 OUTLINES OF PHYSICAL CHEMISTRY

��Lothab Meyer's Classification

It has already been mentioned that not only the chemical character but also the physical properties of the elements vary periodically with the atomic weights. L. Meyer has shown this interdependence most clearly by making use of a conception which we have not yet noticed, namely, the atomic volume.

The space occupied by an atom is obviously directly proportional to the atomic weight and inversely proportional to the density of the element considered. The atomic volume is, therefore, obtained by dividing the atomic

weight (a) by the density (d\ i.e. it is equal to -^ Putting

a

it more concretely, we may say that the ratio -, represents

the space (in cubic centimetres) occupied by one gram- atom of the element whose atomic weight is a and whose density is d.

In order to obtain comparable numbers for the atomic volumes, we must take the density of the elements in the solid state.

We obtain a graphic representation of the changes which the atomic volumes undergo as the atomic weight rises, by marking off abscissae proportional in length to the atomic weights and raising corresponding ordinates proportional in length to the atomic volumes. By joining the co-ordinate points thus obtained we get a curve like that shown in the figure l (page 33).

The density in the solid state of certain elements such as H, 0, N, F, cannot be experimentally determined ; but we can foresee by theoretical considerations that this

1 Note by Translator. — BelUtoff and Scherbatscheff (Butt. Acad. St. Pitersbourg, 1894 [5], 1) have found that the specific gravity of caesium is 2*366 (and not 1*88 as given by Settler) : its atomic volume is consequently 56, and not 75.

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