Subject: Valence electrons and energy levels??

We know that Shell 1 (K), in an atom's electron cloud, can only hold two electrons, Shell 2 (L) can hold 8, and the Shell 3(M) can hold a total of 18 electrons. Why is it that most of our materials state that the Shell 3 can only hold 8 valence electrons? When creating an electron dot diagram, you can only show a maximum of 8 valence electrons. It seems that Shell 3 should be able to hold 18 valence electrons. Thanks!

Re: Valence electrons and energy levels??