An atom that has 84 protons and 86 neutrons undergoes a reaction. At the end of the reaction, it has 82 protons and 84 neutrons. What happened to the atom?

The atom changed from 84 protons and 86 neutrons to 82 protons and 84 neutrons, so it lost 2 protons and 2 neutrons. An alpha particle contains 2 protons and 2 neutrons, so emitting one accounts for the change. This is a nuclear reaction, not a chemical reaction.

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