Since the atom consisted of a positively charged nucleus at the center,
and a number of negatively charged electrons outside, the next step was
to find the exact size of the nuclear charge and the exact number of
electrons for the different varieties of atoms.
The answer came through a line of research that began with the English
physicist Charles Glover Barkla (1877-1944). In 1911 he noted that when
X rays passed through atoms, some were absorbed and some bounced back.
Those that bounced back had a certain ability to penetrate other matter.
When the X rays struck atoms of high atomic weight, the X rays that
bounced back were particularly penetrating. In fact, each different type
of atom seemed associated with reflected X rays of a particular
penetrating power, so Barkla called these “characteristic X rays”.
In 1913 another English physicist, Henry Gwyn-Jeffreys Moseley
(1887-1915), went into the matter more thoroughly. He measured the exact
wavelength of the characteristic X rays by reflecting them from certain
crystals. In crystals, atoms are arranged in regular order and at known
distances from each other. X rays reflecting from (or more accurately,
diffracting from) crystals are bent out of their path by the rows of
atoms. The longer their waves, the more they are bent. From the degree
of bending the wavelength of the waves can be determined.
[Illustration: _Charles Glover Barkla_]
[Illustration: _Henry Gwyn-Jeffreys Moseley_]
Moseley found that the greater the atomic weight of an atom, the shorter
the waves of the characteristic X rays associated with it and the more
penetrating those X rays were. There was such a close connection, in
fact, that Moseley could arrange the elements in order according to the
wavelength of the characteristic X rays.
For some 40 years prior to this, the elements had been listed in order
of atomic weight. This was useful especially since the Russian chemist
Dmitri I. Mendeléev (1834-1907) had arranged them in a “periodic table”
based on the atomic weight order in such a way that elements of similar
properties were grouped together. The elements in this table were
sometimes numbered consecutively (“atomic number”) but this was
inconvenient since, when new elements were discovered, the list of
atomic numbers might have to be reorganized.
[Illustration: _Dmitri Mendeléev and Bohuslav Brauner in Prague in 1900.
Brauner was a professor of chemistry at the Bohemian University in
Prague._]
The Danish physicist Niels Bohr (1885-1962) had just advanced a theory
of atomic structure that made it reasonable to suppose that the
wavelength of the characteristic X rays depended on the size of the
nuclear charge of the atoms making up a particular element. Moseley
therefore suggested that these X rays be used to determine the size of
the positive charge on its nucleus. The atomic number could then be set
equal to that charge and be made independent of new discoveries of
elements.
Hydrogen, for instance, has an atomic number of 1. Its nucleus carries a
unit positive charge, +1, and the hydrogen atom possesses 1 electron to
balance this. Helium, with an atomic number of 2, has a nuclear charge
of +2 and 2 electrons, with a total charge of -2, to balance it. (The
alpha particle released by radioactive atoms is identical with a helium
nucleus.)
The atomic number increases as one goes up the line of atoms. Oxygen
atoms, for instance, have an atomic number of 8 and iron atoms have one
of 26. At the upper end, thorium is 90 and uranium is 92. Each uranium
atom has a nucleus bearing a charge of +92 and contains 92 electrons to
balance this.
Once the notion of the atomic number was worked out, it became possible
to tell for certain whether any elements remained as yet undiscovered
and, if so, where in the list they might be.
Thus, when Moseley first presented scientists with the atomic number it
turned out that there were still 7 elements that were not discovered. At
least elements with atomic numbers of 43, 61, 72, 75, 85, 87, and 91
were still not known. By 1945, all seven had been discovered.
It quickly turned out that the atomic number was more fundamental and
more characteristic of a particular element than was the atomic weight.
[Illustration: _Niels Bohr_]
[Illustration: _Bohr’s study._]
Since Dalton’s time it had been assumed that all the atoms of a
particular element were of equal atomic weight and that atoms of two
different elements were always of different atomic weight. The first
inkling and the first proof that this might not be so came through the
study of radioactivity.
[Illustration: showing Helium atom, Hydrogen atom; Nucleus, Proton,
Neutron, Electron labelled]