Alpha decay happens when a radioactive particle decays into an alpha particle (helium-4 nucleus) and a new particle. The new particle has an atomic number that is two less than the original radioactive particle and an atomic mass equal to four less than the original particle.
A radioactive material of atomic number A and atomic mass B will decay into an alpha particle and a new particle with an atomic number two less than A, and an atomic mass four less than B.
Alpha particles and helium-4 nuclei are the same; they have an atomic number of two and an atomic mass of four. When a radioactive material decays, it loses two protons and two neutrons, so it becomes a new particle.
One isotope of thorium is thorium-232, which has an atomic mass of 232 and an atomic number of 90. This isotope will decay into an alpha particle and radium-228, with atomic number 88 and atomic mass 228.