In this example question it says determine if the following nuclide will undergo alpha decay, beta decay or electron capture and provide the nuclear reaction. Here we're dealing with radon 222. Now upon inspection we see that it's mass number is greater than 209. So remember this typically happens with isotopes that have very high mass numbers and the answer is alpha decay.

Here, nuclear fission is also possible, but that's not one of the options. All right, So we're going to have Radon 222. Radon has a atomic number of 86. It's undergoing alpha decay, so an alpha particle is emitted as a product. Here we need to balance the mass numbers and total number of protons on both sides. We have 222 for our mass number on the reactant side, so we need 222 total on the product side. Four of them come from the alpha particle, so we need another two. 18218 + 4 gives us 222.

We have 86 protons on the reactant side, so we need 86 total on the product side. Two of them already counted for in terms of the alpha particle, so we need another 84. If we look on the periodic table, we see that the element with an atomic number of 84 is polonium O. So this would represent our nuclear reaction where radon 222 undergoes alpha decay. And remember it undergoes alpha decay because its mass number or atomic mass in this case is greater than 209 AMU. It's incredibly heavy element and those typically do alpha decay or nuclear fission.

Rn 222 → Po 218 + α 4