So far we have considered the transitions possible for individual atoms.

Quite simply, the core energy levels for atoms of a particular element depend on its environment.

Techniques using this are most effective for light atoms such as hydrogen.

This value is less for larger atoms at lower temperatures, a system considered more ideal.

It's like saying the half life of an isotope might be ten years, but that's only for individual atoms.

The temperature is still too high for atoms to form.

Almost every research team has devised its own combination of systems for cooling atoms.

I'm not sure how large you are asking about, but stability is a very difficult thing to predict for huge atoms.

Such a mechanism is not available for helium-3 atoms, which are fermions.

Let me just list a few of the other bits of evidence for atoms.