The that describes the of Polonium-210 can be written like this:
##””_84^210Po -> “”_82^206Pb + _2^4He##
Po-210 has 84 protons and 126 neutrons in its nucleus. During alpha decay, an ##alpha”-particle”##, which is essentially a Helium-4 nucleus, is emitted by the nucleus.
Since the Helium-4 nucleus has 2 protons and 2 neutrons, the alpha decay process will cause the to decrease by 2 and the to decrease by 4.
takes place after an aplha decay, which explains the formation of Pb-206.