When is an atom considered to be in its most stable state?

An atom is considered to be in its most stable state when its valence shell, or outermost electron shell, is completely filled with electrons. This arrangement makes the atom less likely to react chemically with other atoms, as it fulfills the octet rule (for most elements), where atoms tend to prefer having eight electrons in their valence shell. When the valence shell is full, the atom achieves a lower energy state and increased stability, leading to a preference for remaining as it is rather than participating in chemical reactions.

In contrast, an atom with a net charge is typically unstable because the imbalance of protons and electrons leads to a tendency to seek out other atoms or molecules to balance the charge. An atom that contains no neutrons can still exist, but neutrons contribute to the stability of an atomic nucleus; certain isotopes without neutrons may be unstable and radioactive. Lastly, while isolating an atom from other atoms can impact its surrounding environment, it doesn’t guarantee stability. The stability is fundamentally tied to the arrangement of electrons in the valence shell, making the state of a completely filled valence shell the definitive characteristic of a stable atom.