Period 2 elements like carbon, oxygen and nitrogen cannot expand their octet because they cannot make use of d orbitals in that energy level (they don't exist). However, period 3 elements like sulfur and chlorine can because they can utilize the d orbitals with the same principal quantum number. However, the the 4s orbitals are lower in energy than the 3d orbitals (from the order of filling of electrons) so why can't they be used/ why aren't they? Moreover, why can't period 2 elements use the 3s orbital?

  • 1
    $\begingroup$ Interesting, I upvote it then vote to close it! It can be answered by that Q, Rob. $\endgroup$
    – M.A.R.
    Jan 14, 2015 at 20:31
  • 1
    $\begingroup$ In reality there's no such thing as expanded octet see en.wikipedia.org/wiki/Orbital_hybridisation $\endgroup$
    – Mithoron
    Jan 15, 2015 at 0:39