An atom is considered stable when it has a full outer shell of electrons, also known as the valence shell. The number of valence electrons needed for stability depends on the specific atom and its position on the periodic table.

Here's a general rule:

* Atoms strive to have 8 valence electrons (except for hydrogen and helium, which only need 2). This is known as the octet rule.

For example:

* Oxygen (O) has 6 valence electrons. It needs 2 more electrons to complete its outer shell and achieve stability.

* Sodium (Na) has 1 valence electron. It will readily lose this electron to achieve a stable configuration with 8 valence electrons in its next lower shell.

* Helium (He) has 2 valence electrons, which is the maximum number for its outer shell. It's already stable.

Exceptions to the octet rule:

* Elements in the third row and beyond can have more than 8 valence electrons due to the availability of d orbitals.

* Some atoms can be stable with fewer than 8 valence electrons, such as in the case of boron (B) and beryllium (Be).

Understanding the octet rule helps to predict how atoms will bond with each other to achieve stability and form molecules.