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One of the most interesting controversies I've seen about periodic table schemes is the placement of helium. It usually sits above neon as a noble gas. However,
by apo 8y ago
One of the most interesting controversies I've seen about periodic table schemes is the placement of helium. It usually sits above neon as a noble gas. However, not everyone agrees with that:
"Two things are surprising about Mendeleev’s classification of the chemical elements according to the Periodic Law,” Bent writes. “One is how much of the classification has not needed revision. The other is how long the scheme’s second element has been misplaced, above neon, despite Mendeleev’s Rule of Light-Element Distinctiveness; despite his statement that the Periodic System is about atoms, not simple substances; despite classification by atomic physics of helium atoms as s2 systems (not p6 systems); despite appearance of the Left-Step Periodic Table nearly eight [now over nine] decades ago; and despite numerous implications of the LSPT that require placement of helium above beryllium.”
https://cen.acs.org/unassigned/Reactions/97/i3 https://cen.acs.org/unassigned/Reactions/97/i3
How you view the placement of helium (and other more difficult elements) comes down to what you believe the table represents:
- empirically-derived reactive trends of elemental substances (in which case helium above neon makes much more sense); or
- a summary of theoretically-derived electronic configuration (in which case helium above beryllium makes perfect sense).
You can see aspects of both views in the modern canonical table. It's weird exceptional cases like this that can, among other things, make chemistry such a difficult (and fun) subject to learn.
- trophycase 8y agoWhy does electronic configuration put it above Beryllium? Helium has a full first shell of electrons, and other noble gases have their subsequent shells filled with electrons, this is why they all have their property of being more or less inert. The reactive trends of noble gases aren't "emperically derived" as you say but as a result of being extremely energetically stable due to having full electron orbital shells. EDIT: Perhaps I misunderstood and you meant that its initial placement was due to empirical data. But for good theoretical reason it has remained where it is IMO.
- gus_massa 8y agoBeryllium has 2 electrons and 6 holes in the last shell. Neon has 8 electrons and 0 holes in the last shell. Helium has 2 electrons and 0 holes in the last shell. How do you align Helium, in the 2 electrons column over Beryllium or in the 0 holes column over Neon? [I vote for aligning it over Neon.]
- trophycase 8y agoIf the number of holes tells you more about the reactivity and properties of the element than the number of electrons in the outermost shell, then they should be arranged that way. AFAIK there is no theoretical mandate that the periodic table should be the way that it is, so just organize it in the way that makes sense while also providing maximum utility.
- jcranmer 8y agoThe noble gases had yet to be discovered when Mendeleev started the periodic table. Actually, the original periodic table is quite inaccurate; that it works at all is largely a coincidence that the d block has 10 elements and that the lanthanides were generally unknown (so the screwing that the f block does wouldn't crop up). Electronic configuration is already screwed up in the periodic table more than just helium; there are several elements that don't match the expected electronic configuration, particularly in the f block. The f block itself is really annoying to actually place in the table, since there's arguments to be made as to whether La/Ac belong in the d block or if Lu/Lr move there, or if you should just cram the entire block as being a group 3 element, or if you should do something just for the lanthanides but not the actinides, etc.
- mannykannot 8y agoIf it were not for the reactive trends, would there be much benefit in ordering the elements periodically?
- yoz-y 8y agoI remember having a periodic table as a kid on which helium was present twice. Seems like a good compromise.
- Pulcinella 8y agoI’ve also seen ones with hydrogen placed twice, above lithium and above fluorine. In reactions hydrogen likes to gain an electron like the halogens rather than lose one like the alkali metals.