Editing Atomic number (section)

=== The proton and the idea of nuclear electrons ===
In 1915, the reason for nuclear charge being quantized in units of ''Z'', which were now recognized to be the same as the element number, was not understood. An old idea called [[Prout's hypothesis]] had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one. However, as early as 1907, Rutherford and [[Thomas Royds]] had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times. If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms.

In 1917, Rutherford succeeded in generating hydrogen nuclei from a [[nuclear reaction]] between alpha particles and [[nitrogen]] gas,<ref>[http://www.nzhistory.net.nz/people/ernest-rutherford Ernest Rutherford | NZHistory.net.nz, New Zealand history online] {{Webarchive|url=https://web.archive.org/web/20121201203746/http://www.nzhistory.net.nz/people/ernest-rutherford |date=1 December 2012 }}. Nzhistory.net.nz (19 October 1937). Retrieved on 2011-01-26.</ref> and believed he had proven Prout's law. He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles). It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of [[hydrogen]] nuclei, and thus there was required a hypothesis for the neutralization of the extra [[protons]] presumed present in all heavy nuclei. A helium nucleus was presumed to have four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two charges. At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of +79, consistent with its atomic number.