well, im not sure exactly when it becomes non reactive and stable, but i know that it is always stable and non reactive when it has the same number of protons, neutrons, and maybe, maybe electrons, not sure

An atom consists of energy levels (the amount of energy levels it posesses depends on the atom you are talking about). However, when we are talking about reactivity and stability, we are only looking at the OUTER energy levels. Energy levels can hold a limited number of electrons. When an atom has a FULL outer energy level (ie the outer most energy level consists of the maximum number of electrons it can hold), then the molecule is stable. Eg1, elements in group 8 of the periodic table have 8 electrons in their outer energy level. This means it has a FULL outer energy level which is why they are very stable/unreactive. Eg2, atoms in group 1 of the table have only 1 electron in their outer energy level. theres are very reactive since they would like to get rid of this electron such that they have an empty outer energy level (or full outer level since the outer most level is now going to be the penultimate energy level). There are a few other more complicated rules though. For example, a HALF FILLED outer energy level also makes the atom semi-stable (this is complicated to explain but it's beacuse each SUB-energy level has one electron). To summarise, the reactivity / stability of an atom is LARGELY determined by how many electrons are in it's outer energy level. Atoms with completely full or completely empty outer energy levels are VERY stable.

When it gets married