Simple question about the P table

I have forgotten why the element K the last shell holds 1 electron which seems logical, but two elements later in Sc the3rd shell begins adding electrons, and even drops an outer shell electron in Cr...I know that I know this...but am drawing a blank(Duh :?)

periodically confused in Fort Wayne IN

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cause thats what the smart people said todo :D actually im not sure so wait for WebElements to show up he seems to be on top of things

Re: Simple question about the P table

[quote="Rockenrollen"]I have forgotten why the element K the last shell holds 1 electron which seems logical, but two elements later in Sc the3rd shell begins adding electrons, and even drops an outer shell electron in Cr...I know that I know this...but am drawing a blank(Duh :?)

periodically confused in Fort Wayne IN[/quote]

For neutral gaseous atomic K the 4s level lies below the 3d level and so is filled first. After the 4s level is filled, the next lowest available level is 3d (rather than 4p) - and so it fills next.

The 3d level fills more or less in an orderly fashion across the d-block but Cr and Cu adopt 3d54s1 and 3d104s1 structures. The reaons for this are just not simple, despite what is said in many text books, and I don't think I know how to explain this in a coherent fashion.

The transfer of a 4s electron to a 3d orbital between the third and fourth rows (as well as the 8th and 9th rows) of the transition metals is due to the fact that the 3d orbitals, due to their lower principal quantum number, drop in energy faster than the 4s orbital with increasing charge on the nucleus (see Slater's Rules). In the case of the third and fourth columns, the 3d orbital just happens to drop below the energy of the 4s orbital plus the paring energy for the "second" electron in the 4s in-between the third and fourth columns of transition elements (in the case of the "transfer" from the 8th to 9th columns, the pairing energy for a 3d electron plus the energy of the 3d orbital drops below the energy of the pairing energy of a 4s electron plus the energy of the 4s orbital). This small effect is ONLY valid for neutral species of such transition metals, usually in the gas phase, which never occur in 99% of chemistry. It is a general rule that the 3d orbital is lower in energy than the 4s orbital for all cationic transition metals, which is how these metals are usually found in solid state and coordination chemistry.

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