At equilibrium, individual molecules still contain the ability to change constantly. Equilibrium is a dynamic state that refers to the overall average of no net change.
In order for a reaction to reach equilibrium, it must be a reversible reaction that exists in a closed system. A closed system moves spontaneously towards equilibrium due to its desire to reach its lowest energy state. Equilibrium is achieved in a system not when the reaction stops, but rather when the rate at which products form and decay is equal.
The ratio of products to reactants, known as the equilibrium constant, is specific to a system and fluctuates slightly due to uneven forward and backward reaction rates. It is dependent upon the nature of the reagents and products, temperature, and pressure. In a standard state, the equilibrium constant is always the same for a given reaction. Every chemical species involved in a reaction has the ability to shift to a different chemical species within the reaction at any moment; however, when the total number of products and total number of reactants are counted at any moment, they always yield the equilibrium constant. The large number of molecules present in the closed system allow for slight changes to occur on the molecular level without disturbing the equilibrium.