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Physical Chemistry

Lanthanum's Appropriate Spot

May 7, 2007 | A version of this story appeared in Volume 85, Issue 19

I was browsing C&EN's website when I came upon the periodic table ( and was somewhat dismayed to see that it continues to perpetuate the dubious placement of lanthanum and actinium in the d-block, below yttrium. If you check their respective electron configurations, you will note that both lanthanum and lutetium have their highest energy valence electron in a 5d-orbital, but the configuration of lanthanum would be considered as "anomalous," while that of lutetium is in the "normal" progression of filling the entire 5d subset. Therefore, lanthanum more properly belongs in the f-block of elements preceding cerium, and lutetium more properly belongs in the d-block below yttrium. An analogous argument can be made for actinium and lawrencium.

A number of recent commercially produced texts and periodic tables have addressed this long-standing inconsistency and have begun to place lutetium and lawrencium in the d-block, below yttrium; and concordantly place lanthanum and actinium as the first entries in the f-block, preceding cerium and thorium.

I would hope that, rather than lagging behind commercial publishers, ACS would lead the way in reversing the inconsistency of older periodic tables toward a more proper arrangement based on the general rule of following the diagonals in the mnemonic device common to all standard undergrad texts:

7s 7p 7d 7f 7g 7h 7i
6s 6p 6d 6f 6g 6h
5s 5p 5d 5f 5g
4s 4p 4d 4f
3s 3p 3d
2s 2p

This arrangement unambiguously places lanthanum and actinium in the f-block, while placing lutetium and lawrencium in the d-block.

John Michael Tamine
New Freeport, Pa.



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