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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?

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  • $\begingroup$ Well, are the shells full seems only option IMO $\endgroup$
    – Mithoron
    Sep 21 '15 at 19:01
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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.

enter image description here

The electron configurations of the group 15 elements are:

\begin{array}{|c|c|} \hline Element & Electron~configuration \\ \hline \ce{N} & 1s^2~2s^2~2p^3 \\ \hline \ce{P} & 1s^2~2s^2~2p^6~3s^2~3p^3 \\ \hline \ce{As} & 1s^2~2s^2~2p^6~3s^2~3p^6~3d^{10}~4s^2~4p^3 \\ \hline \ce{Sb} & 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} & 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.

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