* It has a full outer electron shell: This is the most common and important factor. Atoms strive to achieve a stable configuration with a full outer shell of electrons, like the noble gases. This means it has the maximum number of electrons its outermost energy level can hold.

* Its nucleus is stable: This refers to the balance between protons and neutrons within the nucleus. Too many or too few neutrons can make the nucleus unstable, leading to radioactive decay.

* It has a low energy state: Atoms naturally seek the lowest possible energy state. When an atom has a full outer shell and a stable nucleus, it is in a relatively low energy state.

Here's a breakdown:

* Full outer shell: The outermost energy level of an atom contains electrons that participate in chemical bonding. Atoms with a full outer shell (like Helium, Neon, Argon) are very unreactive because they are already stable.

* Stable nucleus: The nucleus of an atom is composed of protons (positively charged) and neutrons (no charge). The strong nuclear force holds these particles together. If the ratio of protons to neutrons is not balanced, the nucleus can become unstable and decay, releasing energy.

* Low energy state: Stable atoms have a low energy state. Atoms with an incomplete outer shell or an unstable nucleus tend to have higher energy states. They will react or undergo radioactive decay to achieve a lower energy state.

In summary: Stable atoms are those that have a full outer shell of electrons, a stable nucleus, and are in a low energy state. This means they are not likely to undergo chemical reactions or radioactive decay.