Balancing Decay Equations

Let us look at that decay in more detail:

Before decay, the Uranium-235 nucleus had 92 protons, and (235-92) 143 neutrons.

After decay, the daughter nucleus, Thorium-231, has 90 protons and (231-90) 141 neutrons.

The two protons and two neutrons missing are, of course, now part of the alpha particle.