Each #alpha# particle has 4 unit mass and 2 unit positive charge.So after emission of 3 #alpha# particles the #"_92^238U# turns into new element having mass no.#238-3*4=226# and atomic no.#92-3*2=86#.The new atom formed is Radon(Rn).
Each #beta# particle has negligible mass but one unit negative charge.Rn atom gains two unit nuclear charge and gets transformed into Ra of mass no.226 and atomic no.88.when 2 #beta# particles are emitted from its nucleus.