Atoms become stable when they have 8 valence electrons, with the exception of hydrogen and helium which only need 2 valence electrons to be stable. This is known as the octet rule.

Here's why:

* Valence electrons: These are the electrons in the outermost shell of an atom. They are the ones involved in chemical bonding.

* Stability: Atoms are most stable when their outermost shell is filled with electrons. This is because a filled shell is energetically favorable.

* Octet rule: For most elements, a full outer shell contains 8 electrons. This is why atoms tend to gain, lose, or share electrons to achieve this configuration.

Examples:

* Sodium (Na): Has 1 valence electron. It wants to lose this electron to become stable, forming a +1 ion with a full outer shell.

* Chlorine (Cl): Has 7 valence electrons. It wants to gain 1 electron to become stable, forming a -1 ion with a full outer shell.

* Oxygen (O): Has 6 valence electrons. It wants to gain 2 electrons to become stable, forming a -2 ion with a full outer shell.

Keep in mind that the octet rule is a generalization, and there are exceptions, especially with transition metals and larger atoms.