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.
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.