# Why is the 8-electron rule more important than the 2- or 18-electron rule

Why is the fulfilled electronic configuration of only $p$ orbital is stable. I mean why $II-B$ group with fulfilled $d$ orbital,$II-A$ group with fulfilled $s$ orbital...are not stable. Why makes $p$ orbital so special?

• Both the rules are differently used. Octet rule is for Lewis and only Lewis structures for the main group elements, specially the lighter ones like Carbon, nitrogen etc. The Eighteen electron rule is used in inorganic chemistry and organometallic chemistry of transition metals only. They rarely crash into each other... – user3459110 Apr 19 '14 at 10:26

By the time the $p$ orbital is filled it is still the valence shell, you can see this by filling order e.g. $3s^2$ $3p^6$ $4s^2$ $3d^{10}$ ; You can see there isn't really an '18 electron rule' because there can never be 18 electrons in the valence shell and since chemistry is under most circumstances about the interaction of valence electrons then you can see that the $d$ shell electrons rarely have much to do with reactions because they are always 'one level down' from the valence shell.
Your question is a little unclear so I can't really say much more but I think you seem to be trying to ask why Neon or an analogous noble gas is so stable whilst Magnesium with a full $s$ orbital is not. Think about the difference in energy levels between going from one shell to another and you'll realise that an $\ce{Mg^2+}$ ion and Neon are very similar in terms of their stability because they are both in a low energy state.