Stable atoms typically have 8 valence electrons, except for hydrogen and helium. This is known as the octet rule.

Here's why:

* Valence electrons are the electrons in the outermost shell of an atom, and they are the ones involved in chemical bonding.

* Atoms are most stable when their outermost shell is full. For most elements, a full outer shell contains 8 electrons. This is because having 8 valence electrons creates a very stable electron configuration, similar to the noble gases.

* Hydrogen and helium are exceptions because they only have one electron shell, which can only hold a maximum of 2 electrons. Therefore, they are stable with just 2 valence electrons.

For example:

* Oxygen has 6 valence electrons. To become stable, it needs to gain 2 more electrons.

* Sodium has 1 valence electron. To become stable, it needs to lose 1 electron.

The octet rule helps us understand why atoms bond with each other and how they form molecules.