Synthetic Inorganic Chemistry

354 APPENDIX

PERIODIC CLASSIFICATION OF THE ELEMENTS AC-

CORDING TO THEIR ATOMIC NUMBERS AND

THE ARRANGEMENT OF THEIR ELECTRONS

The periodic recurrence of similar properties in passing through
the series of chemical elements suggests a recurrence of similar
structural units within the atom, the properties of the element
being dependent on the extent to which each of the units in turn
has been developed. Modern knowledge of the structure of the
atom, although still very inadequate, indicates very clearly
certain principles which render the periodicity of the elements
more comprehensible.
Atoms of Electricity. Atoms of matter are believed to be built
entirely of electrical atoms, of which there are two kinds: the
electron, or atom of negative electricity, with a charge of 4.774 X
10~^10 electrostatic unit and a mass T^¥u tna* of the hydrogen atom;
and the proton, or atom of positive electricity, with a charge equal
in magnitude to that of the electron but opposite in sign, and a
mass equal that of the hydrogen atom. In fact, the proton is
identical with the hydrogen ion — the neutral hydrogen atom
consisting of one proton and one electron.
Atoms of Matter. Electrically neutral atoms consist of two
parts: the nucleus, and the planetary electrons. There is a certain
analogy between the atom and the solar system, the nucleus cor-
responding to the sun and the planetary electrons to the planets,
and an atom is mostly space just as our solar system is.
The Nucleus. The nucleus of hydrogen consists of a single
proton with unit positive charge; the nucleus of helium consists
of an aggregate of four protons and two electrons with mass four
and two units of net positive charge. The nuclei of all the elements
consist of closely packed aggregates of protons and electrons, with
protons always in excess, so that the net charge is always positive.
The atomic number of an element is equal to the number of net
positive charges of the nucleus; the atomic weight is approximately
equal to the number of protons of the nucleus. Elements are now
known representing practically all the atomic numbers from
hydrogen 1 to uranium 92.
Variation of Mass in Close Packing of Nucleus. The atomic
weight of hydrogen is 1.008, that is to say, 6.06 X 10^23 (Avogadro's
number) hydrogen atoms weigh 1.008 grams. The atomic weight