ATOM
ATOM
numbers of atoms in the molecule. Water is an
example of a compound substance, or chemical com-
pound. Its molecule contains three atoms; two
atoms of hydrogen, and one atom of oxygen. If a
quantity of these two elements were mixed, the re-
sult would be a mechanical mixture of the molecules
of the two. But if heat, or some other adequate
cause were made to act, chemical action would fol-
low, and the molecules, splitting up, would combine
atom with atom. Part of a molecule ol ox-ygen —
one atom — would combine with part of two mole-
cules of hydrogen — two atoms. Ihe result would be
the production of a quantity of molecules of water.
Each water molecule contains one atom of oxygen
and two atoms of hydrogen. The spUtting-up of
the elemental molecules into atoms is sjaichronous
with their combining into molecules, so that an atom
never exists alone. The molecules of the elements,
oxygen and hydrogen, have disappeared, and in
their places are molecules of water. There are
about eighty kinds of atoms known, one kind for
each element, and out of these the material world is
made.
IxvARiABiLiTT OP COMPOSITION. — The invariability of composition by weight of chemical compoimds is a fundamental law of chemistry. Thus water under all circumstances consists of 8S.8S% of oxj-gen and 11.11% of hydrogen. This establishes a relation between the weights of the atoms of hydrogen and oxygen in the water molecule, which is 1: 8. Oxj-- gen and hydrogen are gaseous under ordinarj' con- ditions. If water is decomposed, and the gases are collected and measured, there will always be two volumes of hydrogen to one of oxygen. This illus- trates another fundamental law — the invariabiUty of composition by gaseous volume of chemical com- pounds. From the composition by volume of water its molecule is taken as composed of two atoms of hydrogen and one of oxygen, on the assumption that in a given volume of any gas there is the same number of molecules. As there are two atoms in the mole- cules of both of these elements, the above may be put in a more popular way thus: the atoms of hy- drogen and oxj'gen occupy the same space. The ratio spoken of above, of 1: 8, is therefore the ratio of two atoms of hydrogen to one of oxygen. It follows that the ratio of one atom of hydrogen to one atom of oxj'gen is 1: 16. The numbers 1 and 16 thus determined, are the atomic weights of hydrogen and oxj^gen respectively. Strictly speaking they are not weights at all, only numbers expressing the relation of weights. Atomic weights are determined for all the elements, based on several considerations, such as those outlined for the atoms of oxygen and hydrogen. Thus the term atom indicates not only the constituents of molecules, but has a quantitative meaning, the proportional part of the element which enters into compounds. The sum of the weights of the atoms in a molecule is the molecular weight of the substance. Thus the molecular weight of water is the sura of the weights of two hydrogen atoms, which is two, and of one oxygen atom, which is six- teen, a total of eighteen. If we divide the molecular weight of a compound into the atomic weight of the atoms of any element in its molecule, it will gi\e the proportion of the element in the compound. Taking water again, if we divide its molecular weight, 18, into the weight of the atoms of hydrogen in its molecule, 2. we obtain the fraction ^'s. w-liich expresses the proportion of hydrogen in water. The same process gives the proportion of oxj-gen in water as jf ,
Even,- element has its own atomic weight, and the invariability of chemical composition by weight is explained by the invariability of the atomic weights of the elements. Tables of the atomic weights of the elements are given in all chemical
text-books. The relations of the atomic weights
to each other are several. The atom of lowest
weight is the hydrogen atom. It is usually taken
as one, which is very nearly its exact value if oxj-gen
is taken as sixteen. On this basis one quarter of
the other elements will have atomic weights that are
whole numbers. This indicates a remarkable sim-
plicity of relationsliip of weights, which is carried out
by the close approach of the rest of the elements to
the same condition, as regards their atomic weights.
The range of the atomic weights is a narrow one.
That of hydrogen is 1.008 — that of uranium 238.5.
The latter is the liea\-iest of all. Between these all
the other atomic weights lie. Many of the elements
resemble each other in their chemical relations. It
might appear that those nearest to each other in
atomic weight should be of similar properties. This
is not the case. If the elements are written down
in the order of their atomic weights, beginning with
the lightest and ending with the heaviest, it will be
found that the position of an element in the series
■will indicate pretty clearly its properties. The
elements will be found to be so arranged in the list
that any element will be related as regards its chemi-
cal properties to the element eight places removed
from it. Tills relationsliip may be thus expressed:
the properties of an element are a periodic fimction
of its atomic weight.
Mendeleeff's Table. — This relation is called Mendel^eff's Law, from one of two chemists who independently developed it. The elements may, as before said, be written down in the order of their atomic weights, but in eight vertical columns. Along the top line the eight elements of lightest atomic weights are written in the order of their weights, followed on the second line by the next eight, also in the order of their atomic weights. Tins arrangement. ob\'iously, when carried out brings the elements eight atomic weights apart, into vertical columns. It will be found that all the elements in any vertical column are of similar chemi- cal properties. When Mendel&ff made out liis table it was supposed that several elements were as yet undiscovered. The table also brought out clearly certain numerical relations of the atomic weights. These together with other factors caused him to leave blank spaces in liis table, which none of the known elements could fill. For these places h)-potheticaI elements were assumed, whose general properties and atomic weights were stated by him. One by one these elements have been discovered, so that Mendeleeff's Law predicted the existence of elements later to be discovered. These discoveries of predicted elements constitute one of the greatest triumphs of chemical science. Up to witliin a very- recent period the atom was treated as the smallest division of matter, although the possibility of the transmu- tation of the elements in some way, or in some degree, has long been considered a possibility. It was con- jectured that all the elements might be composed of some one substance, for which a name, protyle, meaning first material, was coined. This seemed to conflict with the accepted definition of the atom, as protyle indicated something anterior to or preceding it. The idea rested in abeyance, as there was little ground for buikling up a tlieorj- to include it. Re- cent discoveries have resuscitated this never quite abandoned theory; protyle seems to have been discovered, and the atom has ceased to hold its place as the ultimate division of matter.
CoRPUSCULES. — The most recent theory holds that the atom is composite, and is built up of still minuter particles, called corpuscules. As far as the ordinarj' processes of chemistni- are concerned the atom remains as it was. But investigations in the field of radio-activity, largely physical and partly chemical, go to prove that the atom, built up of
