* Electron Configuration: Atoms are most stable when their outermost electron shell (also called the valence shell) is completely filled. This arrangement minimizes their energy and makes them less likely to gain, lose, or share electrons.

* Octet Rule: For most elements, a full outer shell means having eight electrons. This is known as the octet rule. However, some elements, like helium, are stable with only two electrons in their outer shell.

* Noble Gases: The noble gases (helium, neon, argon, krypton, xenon, and radon) are excellent examples of chemically stable atoms. They naturally have full outer shells, which is why they are very unreactive.

Examples:

* Sodium (Na): Sodium has one electron in its outermost shell. It readily loses this electron to form a positively charged ion (Na+) with a full outer shell, making it stable.

* Chlorine (Cl): Chlorine has seven electrons in its outermost shell. It readily gains one electron to form a negatively charged ion (Cl-) with a full outer shell, making it stable.

Exceptions:

* Transition Metals: Many transition metals have partially filled outer shells and are less stable than noble gases. They can participate in a variety of chemical reactions.

* Large Atoms: Large atoms, especially in the later periods of the periodic table, can have more than eight electrons in their outer shells. These atoms can still be relatively stable depending on the specific electron configuration.

In summary, an atom is chemically stable and unreactive when its outer electron shell is filled, typically with eight electrons (octet rule). This configuration makes it energetically favorable for the atom to remain in its current state and not participate in chemical reactions.