An important thing we must keep note of is that an α-particle (alpha particle) is a helium nucleus. This means that it contains 2 protons and 2 neutrons; giving it a mass number of 4.
During alpha decay, an atomic nucleus emits an alpha particle. Essentially, it decays into an atom with an atomic number of 2 less than its original amount and a mass number of 4 less than its original amount.
In our case, Curium-243 undergoing alpha radiation decay must abide the following equation:
²⁴³₉₆Cm → ⁴₂He + x
～ The (number at the top/mass number) of x must be 243 - 4 = 239
～ The (number at the bottom/atomic number) of x must be 96 - 2 = 94
～ Element 94 is Pu (Plutonium). Therefore, the equation is: ²⁴³₉₆ Cm → ⁴₂ He + ²³⁹₉₄ Pu
