In the Fajans-Soddy law, alpha emission lowers an atom’s atomic number by two places.
An alpha particle is the nucleus of a helium-4 atom: two protons and two neutrons. When a radioactive nucleus emits one, it loses two protons, so its atomic number falls by 2 and its mass number falls by 4. Uranium-238, for example, becomes thorium-234: 92 protons become 90, while the mass number changes from 238 to 234.
The rule was formulated independently in 1913 by Frederick Soddy and Kazimierz Fajans. It helped explain how radioactive decay could transform one chemical element into another and clarified the positions of radioelements in the periodic table.
A common mix-up is to confuse atomic-number change with mass-number change. Alpha decay changes them by two and four, respectively. In ordinary beta-minus decay, the mass number stays the same while atomic number rises by one.