What particle is emitted when Ra221 decays to Rn217?

Answer 1

Radon has two protons less than radium; and there is a total difference of 4 nucleons, so it is obvious that it would emit an alpha particle (which takes away 4 nucleons, 2 of them protons). Note: I didn't check whether this reaction actually occurs, or even whether these isotopes actually exist - I just did the subtraction.

Answer 2

An alpha particle