Atom.
The ultimate particle or division of an elementary substance; the
smallest part that can exist in combination, and one which cannot exist
alone. An elementary substance is composed of molecules just as truly as
a compound one, but the atoms in the molecule of an elementary substance
are all precisely alike. Hence atoms are the units of chemistry, they
have to do with combinations, but the physical unit, the smallest
particle of matter that can have an independent existence, is the
molecule. The two are often confounded, especially by writers of a few
years ago, so that by "atom" the molecule is often meant. There is
nothing to be said of their size or mass. All such calculations refer to
the molecule, q. v., often spoken of and called the atom.
[Transcriber's note: Yet to be discovered: electron--1897 (5 years),
proton--1920 (28 years), neutron--1932 (30 years), quark--1961 (69 years).]
Atomic Attraction.
The attraction of atoms for each other, in virtue of which they combine
into molecules; chemical affinity, q. v., treats principally of this,
although molecular attraction also plays a part in it.
Atomic Heat.
The product of the atomic weight of a substance by its specific heat.
This product is approximately the same, 6.4; this approximation is so
close that it is of use in determining the valency and atomic weights of
substances. The atomic weight of a substance therefore represents the
approximate number of gram-calories required to raise one gram-atom, q.
v., of such substance through 1° C. (1.8° F.)
Atomicity.
The quantivalence or valency of the atoms; the number of combination
bonds, or bonds of affinity, possessed by the atoms of any substance.
Thus two atoms of hydrogen combine with one atom of oxygen, and three of
oxygen with one of sulphur, forming saturated compounds. Therefore,
taking hydrogen as of single atomicity or a monad, oxygen is of double
atomicity or a dyad, and sulphur is of six-fold atomicity, or a hexad.
The elements are thus classified into seven orders of atomicities, thus:
1, Monads or Univalent elements, Hydrogen, etc.
2, Dyads or Bivalent " Oxygen, etc.
3, Triads or Trivalent " Nitrogen, etc.
4, Tetrads or Quadrivalent " Lead, etc.
5, Pentads or Quinquivalent " Phosphorous, etc.
6, Hexads or Sexivalent " Chromium, etc.
7, Heptads or Septivalent " Chromium, etc.
The same element often possesses several atomicities. Barium is
generally a dyad, sometimes a tetrad; nitrogen acts as a monad, dyad,
triad, tetrad and pentad. The familiar electrolysis of water, giving two
volumes of hydrogen to one of oxygen, is one of the illustrations of the
theory indicating that two atoms of hydrogen are combined with one of
oxygen.