356 APPENDIX
of helium is exactly 4, that is to say, 6.06 X 10^23 helium atoms weigh
4.00 grams. If it is true, as we believe, that the helium nucleus
contains four protons, which apart have a mass of 4.032 units, they
must, in the process of packing together with two electrons into the
minute space of the nucleus, have lost 0.032 unit of mass. When
we consider the incredibly great forces that must exist within the
nucleus, such a loss of mass cannot be considered as surprising.
In the building up of all the other atoms the protons always
suffer about the same diminution of mass as in the formation of
helium nuclei. Thus the masses of all atoms except hydrogen are
very nearly even multiples of the same unit, that is, one-fourth
the mass of the helium atom, or one-sixteenth the mass of the
oxygen atom.
Isotopes. Elements which like chlorine have uneven atomic
weight really consist of a mixture of atoms of different masses but
of the same atomic number. Ordinary chlorine, for example, of
atomic weight 35.45 consists of a mixture of atoms of masses 35
and 37 of which there are about three times as many of the former
as of the latter. These two atomic species are called isotopes.
It is the atomic number, or the charge of the nucleus, which
determines the chemical properties of the element; so the different
isotopes are identical in chemical properties and cannot be sepa-
rated by any chemical processes. Thus the uniformity of the
atomic weight of chlorine wherever found. The isotopes must
have been uniformly mixed at the creation of our earth and no
process since has ever sorted them out into separate fractions.
Permanency of Nucleus. The force which must be overcome in
forcing protons together against their enormous electrostatic re-
pulsion into the narrow confines of the nucleus is beyond our
comprehension. No adequate theory has been devised to explain
the stability of the nucleus; yet the fact of the permanency of the
nucleus exists. The identity of the element depends on the perma-
nency of the nucleus, and our well-known law of the conservation
of the elements expresses the almost absolute permanency of the
nuclei of the common elements. Only with the radioactive
elements is the nucleus subject to change, and this change is in
most ways entirely unaffected by any physical or chemical forces
which scientists are able to bring to bear.
Radioactive Disintegration. In radioactive disintegration a
nucleus suddenly explodes ejecting either an electron (beta par-