Alpha decay Definition in detaisl

Alpha decay is a spontaneous decay of a radioactive atom resulting in the loss of 2 in the atomic number and four in the mass number of an atom, it is a natural decay process that occur without the effect of heat or pressure, atoms undergo radioactive decay when they became unstable, therefore they decay continuously until they attain stability

²³⁵₉₂U →²³¹₉₀Th +⁴₂He

You can see from the above equation, the thorium atom as atomic number of 90 and mass number of 231 which 2 and 4 less than that of the uranium.