size

[ydoaPs]

what is the division between quantum rules and classical rules? atoms, molecules, ect.? is there a blurry area where it is kinda both or is it a division at x size?

[gib65]

Isn't it just the plank length?

[Meir Achuz]

For bound states, the classical region is when the level of excitation is so large that a change in N of one unit seems continuous. Just where this occurs depends on what accuracy you want. For instance, for hydrogen E is proportional to me^4/n^2.

When n gets large, this looks classical, since changes in n look continuous.

[swansont]

QM always applies. Classical rules are the result of some parameter getting large enough that the system looks continuous.