Most elements need 8 valence electrons to become stable, a concept known as the octet rule.

However, there are exceptions to this rule, especially for elements in the first and second rows of the periodic table:

* Hydrogen and Helium: They only need 2 valence electrons to fill their outermost shell and become stable.

* Elements with less than 4 valence electrons: They tend to lose these electrons to achieve a stable configuration.

* Elements with 5 or more valence electrons: They tend to gain electrons to complete their octet.

The octet rule is a helpful guideline, but it's important to remember that it's not always strictly followed.