Niels Bohr of Denmark publishes theory of atomic structure combining nuclear theory with quantum theory. Using x-ray tubes, H. G. J. Moseley determines charges on the nuclei of most atoms. Accordingly, the periodic table was reorganizaed to be based upon atomic number instead of atomic mass. Albert Einstein publishes general theory of relativity. Ernest Rutherford studies visible scintillations when a zinc sulfide film is struck by alpha particles.
Occasional scintillations are brighter when the alpha particles pass through rarified nitrogen.
Rutherford conjectures that it happens when an atom of nitrogen is changed into oxygen by an alpha particle, the first observation of artificial transmutation of an element. Aston discovers existence of isotopes through the use of a mass spectrograph. 1921 Ernest Rutherford and James Chadwick of Great Britain achieve transmutation of all elements except carbon, oxygen, lithium and beryllium. 1922 P. M. S. Blackett of Great Britain begins experiments on transmutation of elements. 1931 Albert Einstein urges all scientists to refuse military work.