Here's why:

* Alpha decay is a type of radioactive decay where an atomic nucleus emits an alpha particle, which is essentially a helium nucleus (2 protons and 2 neutrons).

* The atomic mass of Pu-240 is 240, and the atomic mass of U-236 is 236. The difference in mass (4) corresponds to the mass of an alpha particle.

* The atomic number of Pu is 94, and the atomic number of U is 92. The loss of 2 protons in the alpha particle explains the change in atomic number.

Therefore, the decay of Pu-240 to U-236 can be represented by the following equation:

²⁴⁰Pu → ²³⁶U + ⁴He