The equation is balanced because the mass and charge are equal on both sides of the reaction equation.
In a nuclear reaction, one nucleus is transformed into another. In this case of the equation shown, the radon nucleus is being transformed into a polonium nucleus with the emission of an alpha particle.
The equation is balanced because a total mass of 219 is found on both sides of the reaction equation and a total charge of 86. Recall that an alpha particle is the same as a helium nucleus with a mass of four units and a charge of two units.
