During a nuclear reaction, an atom must emit an alpha particle, which consists of 2 protons (positively charged) and 2 neutrons (neutrally charged). Thus, the alpha particle has a charge of +2 (there are no electrons, which are negatively charged, to balance out the two protons). When an atom emits an alpha particle, it will lose this particle, thus losing 2 protons and 2 neutrons as well. Since the atom in the equation goes from 84 protons to 82, and 86 neutrons to 84, it lost 2 protons and 2 neutrons, indicating that it emitted an alpha particle.
