According to Rutherford’s notion, the atom consisted of a single tiny positively charged lead shot at the center of a foam of electrons. It was Thomson’s notion in reverse. Still, the nucleus carried a positive charge of a particular size and was balanced by negatively charged electrons. Rutherford’s model of the atom explained the existence of ions just as easily as Thomson’s did and it explained more besides.
For instance, if all the electrons are removed so that only the nucleus remains, this nucleus is as massive as an atom but is so tiny in size that it can penetrate matter. The alpha particle would be a bare atomic nucleus from this point of view.
Rutherford’s model of the “nuclear atom” is still accepted today.
Atomic Numbers
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.
Charles Glover Barkla
