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I'm currently reading in-depth about the layout of the Periodic Table, and I wondered why the table has 3 columns in its Group VIII:

enter image description here

As I understand, this is an old notation of the table, now deprecated. But why was Group VIII a triple-sized group?

The answer might be trivial, but it's interesting.

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    $\begingroup$ Note that this is an old naming system; the current IUPAC names for the groups are simply 1 through 18. $\endgroup$ – orthocresol Apr 30 '16 at 15:38
  • $\begingroup$ The reason for having them all in one group was, that they are very similar in each of the three periods, much more than to their neighbours on the left and right. Co is even heavier than Ni, so the order was somewhat unclear in the beginning. $\endgroup$ – Karl Apr 30 '16 at 20:24
  • $\begingroup$ Does anybody know what the bottom number means? $\endgroup$ – gsurfer04 Jul 1 '18 at 22:56
  • $\begingroup$ @gsurfer04 The number written below each element is the mass number of that element. $\endgroup$ – Gaurang Tandon Jul 2 '18 at 16:09
  • $\begingroup$ The bottom number, below the electron structure. $\endgroup$ – gsurfer04 Jul 2 '18 at 22:51
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Dimitri Mendeleev designed his periodic table so that elements with similar properties were grouped together and group VIII demonstrated the most similar characteristics within each element, more so than other transitional metals.

Things such as the oxidation state of +2 and +3 being common among those specific metals-this is the best reason I could find and they are more consistent with the 18 electron rule than other transitional metals, particularly Fe, Co, and Ni.

The old system was removed because we thought there was sound reasoning to Mendeleev's decision (maybe he saw things we didn't), but there wasn't, and neither did we find enough cause to keep it that way. It has only led to people being misled or distracted because their immediate thought is: "why are they together?". When really, there is no solid justification.

From what I could find, the rationalisation for it being removed wasn't discussed at length or maybe even at all, so the above paragraph is just my assumption.

References:

https://answers.yahoo.com/question/index?qid=20060609045851AAOi9a7

p.s. I'll add to this as/if I discover more.

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