Saturated and unsaturated elements? I was reading about Group 15 and came upon the following: 


  The penultimate shell in N contains 2 electrons (saturated), in P contains 8 (saturated), in Arsenic 18 (saturated) while antimony and bismuth contain 18 electrons (unsaturated) each. 


What does the author mean by saturated and unsaturated?
Although I've never heard this terminology before, I'm pretty sure I can see what it means.

As you can see from this diagram, each increase in the principal quantum number (n), adds a new type of orbital to that shell.



The electron configurations of the group 15 elements are:

\begin{array}{|c|c|} \hline
Element &amp; Electron~configuration \\ \hline
\ce{N} &amp; 1s^2~2s^2~2p^3 \\ \hline
\ce{P} &amp; 1s^2~2s^2~2p^6~3s^2~3p^3 \\ \hline
\ce{As} &amp; 1s^2~2s^2~2p^6~3s^2~3p^6~3d^{10}~4s^2~4p^3 \\ \hline
\ce{Sb} &amp; 1s^2~2s^2~2p^6~3s^2~3p^6~3d^{10}~4s^2~4p^6~4d^{10}~5s^2~5p^3 \\ \hline
\ce{Bi} &amp; 1s^2~2s^2~2p^6~3s^2~3p^6~3d^{10}~4s^2~4p^6~4d^{10}~4f^{14}~5s^2~5p^6~5d^{10}~6s^2~6p^3 \\ \hline
\end{array}

$\ce{N, P}$ and $\ce{As}$ all have full penultimate shells, whereas $\ce{Sb}$ and $\ce{Bi}$ do not -  the $4 f$ and $5 f$ orbitals are unnocupied respectively. Therefore 'saturated' seems to refer to elements where the penultimate shell is fully occupied, and 'unsaturated' to elements where it is not.