Explain why noble gases are chemically stable and what the term "stable arrangement" refers to in this context.

Noble gases are stable because their outermost electron shell is completely filled. This is often referred to as an octet (eight valence electrons) for most noble gases, or a duet (two valence electrons) for helium.

The stable arrangement refers to the state of their valence electrons.

Noble gases have a stable arrangement of a full outer electron shell.

The final answer is $\boxed{\text{a full outer electron shell}}$