Here's why:

* Chemically inert atoms are indeed those with a full outermost electron shell. This is because having a full shell makes them very stable. They don't need to gain or lose electrons to achieve a stable configuration.

* The exception: Helium (He) only has 2 electrons in its outermost shell. This is because it has only one shell, which can only hold 2 electrons. Even though it's not "full" in the same way as other inert gases, it's still stable and considered chemically inert.

In summary:

* Chemically inert atoms usually have a full outermost electron shell (8 electrons, except for Helium, which has 2).

* This full shell makes them very stable and less likely to participate in chemical reactions.

Let me know if you have any other questions!