Here's why:

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

* Octet Rule: This rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons. This configuration is similar to that of the noble gases, which are very unreactive.

So, when an atom has eight valence electrons, it is considered to be very stable and less likely to react with other atoms.