As far as atoms are concerned, happiness is a full outer orbit of electrons. To achieve this state, they either have electrons removed from them or attract electrons from another atom. The ease of doing this determines the reactivity.
Example: rubidium has just one electron in its outer orbit. This is shielded from the attractive force of the positive nucleus by inner orbits of electrons. So it doesn't take much to remove this outer electron.. so rubidium is chemically very reactive.
Example: rubidium has just one electron in its outer orbit. This is shielded from the attractive force of the positive nucleus by inner orbits of electrons. So it doesn't take much to remove this outer electron.. so rubidium is chemically very reactive.