Analysis of information sources in references of the Wikipedia article "Isotope" in English language version.
p. 208: Up to 1913 we used the phrase 'radio elements chemically non-separable' and at that time the word isotope was suggested in a drawing-room discussion with Dr. Margaret Todd in the home of Soddy's father-in-law, Sir George Beilby.
Thus the chemically identical elements - or isotopes, as I called them for the first time in this letter to Nature, because they occupy the same place in the Periodic Table ...