Polonium is a rare element with 33 radioisotopes. the most common one, 210po, has 82 protons and 128 neutrons. when 210po decays, it emits an alpha particle, which is a helium nucleus (2 protons and 2 neutrons). 210po decay is tricky to detect because alpha particles do not carry very much energy compared to other forms of radiation. for example, they can be stopped by a single sheet of paper or a few inches of air. that is one reason why authorities failed to discover toxic amounts of 210po in the body of former kgb agent alexander litvinenko until after he died suddenly and mysteriously in 2006. what element does an atom of 210po change into after it emits an alpha particle?