What type of decay is involved when Polonium-214 decays into lead?
This is an example of alpha decay.
One of the common forms of radioactive decay involves splitting off a particle that has two protons and two neutrons. This is called an alpha particle, and it is identical to a helium-4 nucleus. Eventually when an alpha particle loses its energy and neutralizes its charge by picking up a couple electrons, it becomes just plain helium.
Because an alpha particle contains two protons and two neutrons, a nucleus that emits it has its atomic number (number of just protons) drop by two and its mass number (protons plus neutrons) drop by four. For example, polonium-214 with atomic number 84 and mass number 214 decays to lead-210 with atomic number 82 and mass number 210.