how do you know if an atom is unstable or stable
An atom is stable if the forces between its individual particles are balanced. What this means is that the atom is not going to lose any of its components (electrons, protons, or neutrons). There’s a concept known as the “Island of Stability”, a link to this is below. Basically, if an atom/isotope is on the blue line of the island of stability, it will not decay- but on either side of this blue line and it will decay!