An atom of radon gas emits an alpha particle as shown below. radon-222 has 86 protons and 136 neutrons. what daughter product will form from this decay?

The answer is Polonium-218 (84 protons).

Alpha particles consist of two protons and two neutrons. So, mass is 4 (2+2) amu .
When an atom of emits an alpha particle, its atomic number will be reduced by 2 and a mass number will be reduced by 4:
 →  + .

After emission of the alpha particle from Radon-222 with 86 protons, the daughter product will have the atomic number reduced by 2: 86-2 = 84 and the mass number reduced by 4: 222-4=218:
  →  + .