PERIODIC CLASSIFICATION OP THE ELEMENTS. 11
multiple of the charge of the proton. Speculations regarding the
structure of the nucleus have been made in abundance but have
shed little light as to the nature of the incredible forces that hold
the nucleus together, — for the nuclei are almost immutable, or
at least just as immutable as the elements. The nucleus possesses
practically the entire mass of the atom but its bulk is excessively
minute. It occupies a space in the center of the atom much as
the sun does in the center of the solar system and the relative bulk
is comparable.
Surrounding the nucleus are "planetary" electrons, equal in
number in the neutral atom to the number of unit charges of the
nucleus. These electrons are relatively as far removed from the
nucleus as the planets are from the sun.
The properties of the elements, both physical and chemical, with
the exception of mass, are believed to depend entirely on the
arrangement of the electrons around the nucleus. Mass depends
on the number of protons in the nucleus.
The arrangement of the planetary electrons is restricted to
conditions imposed by the so-called "quantum" theory which
deals with the relation between the emission and absorption of
radiant energy and the structure of matter. There is no known
law of physics which would determine the radius of the orbit of a
planet. A planet might fall into any orbit whatever, depending
solely on the energy it might happen to possess. Apparently the
atom differs from the solar system in this respect; the quantum
requirements limit the number of orbits into which the electrons
may fall so that the radii (or major axes of elliptic orbits) of the
possible orbits are to each other in proportion to the squares of the
simple integral numbers, that is, 1 : 4 : 9 : 16 : 25...
Although physicists today are rather strongly committed to the
orbital theory, it may still be better to refer to the possible positions
of the electrons as levels (i.e., measured out from the nucleus).
The number of electrons which can occupy each level is what
determines the configuration of the atom and this in turn deter-
mines all the physical and chemical properties of the atom.
The maximum number of electrons which can occupy the suc-
ceeding levels is twice the square of the successive whole numbers
or 2, 8, 18, 32, 50 .... The mutual repulsion of electrons, how-
ever, counteracts the tendency for the nucleus to pull the maximum