Here's why:

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

* Octet Rule: The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their outermost shell.

* Exceptions: There are some exceptions to the octet rule, particularly for elements in the third and higher periods (rows) of the periodic table. These elements can sometimes have more than eight valence electrons due to the involvement of d orbitals in bonding.

However, the general principle holds true: most atoms strive for a stable configuration of eight valence electrons, and this is the maximum number they can have.