The best description for the relationship between the products and the reactants in an exothermic reaction is C. The potential energy of the products is less than the potential energy of the reactants.
Atoms cannot be created or destroyed by chemical reactions. They just have a some bonds rearranged is all.
Beta radiation / decay would likely occur when the ratio of protons to neutrons is below the band of stability.
Density = mass/volume = 80 g/10cm³ = 8 g/cm³
