Atomic nuclei are made of protons and neutrons, and their numbers determine the element and its mass. Soddy systematically studied “transmutation,” in which radioactive atoms emit alpha or beta radiation and turn into new elements. He proposed that atoms with the same atomic number but different mass numbers exist, introducing the concept of isotopes and redefining what an element is. This idea later enabled technologies such as medical radio-isotope production and carbon-14 dating. Through the Soddy–Fajans displacement law he showed that alpha decay lowers atomic number by two while beta decay raises it by one, deepening our grasp of decay series. His work helped establish radiochemistry, the borderland between chemistry and physics. The knowledge continues to influence nuclear energy, environmental analysis and many other fields, linking science directly to society.

Together with Rutherford, Soddy formulated the disintegration theory, showing that radioactive elements decay exponentially and form elaborate decay series. Using advanced chemical separation he identified short-lived daughter products in the radium, thorium and uranium chains and mapped the entire sequences. Observing chemically identical yet mass-different nuclides, he coined the term “isotope,” clarifying that chemical identity is tied to atomic number, not mass. The idea meshed perfectly with Moseley’s X-ray determination of atomic numbers and triggered a reorganization of the periodic table. The Soddy–Fajans displacement law succinctly relates changes in Z and A during decay and underpins later nuclear-fission theory and mass-table construction. Geochronological methods such as U–Pb dating rely on decay constants first established by Soddy. His discussion of mass defect and released energy helped to verify E = mc² and provided theoretical support for nuclear power. Spanning chemistry, physics, earth science and medicine, his legacy continues to grow after more than a century.

Soddy repeatedly fractionated short-lived nuclides such as 210Po, determining decay constants λ with high precision and empirically validating N(t)=N0e^{-λt}. His 1913 Nature article demonstrated chemically indistinguishable yet mass-different species, introducing the isotope concept. In the same year, with Fajans, he formulated the displacement law (α: Z−2, A−4; β⁻: Z+1, A constant), an essential scaffold for later mass-table compilations. Density measurements of radium emanation (222Rn) secured its assignment to the noble-gas group and fixed its atomic weight at 222, founding inert-gas isotope chemistry. Carrier addition and selective precipitation at the ppt level in his protocols prefigure modern tracer methodology. By correlating mass defect Δm with decay energy Q he pioneered quantitative tests of E=Δmc², consistent with Aston’s mass-spectrograph data. Decay constants derived for the U–Pb chain remain reference values in geochronology today. Collectively, Soddy imposed systematic and quantitative rigor on nascent nuclear chemistry, catalyzing advances in nuclear medicine, isotope geoscience and nuclear physics.