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Why do we find big spaces in the periods of the Periodic Table between $\ce{H}$ and $\ce{He}$, $\ce{Be}$ and $\ce{B}$, and $\ce{Mg}$ and $\ce{Al}$?

What is the logic of such organization of the periods?

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  • $\begingroup$ I believe that you’re meaning groups rather than periods - unless my chemistry is rusty, the groups are the vertical arrangements of elements whereas the periods are horizontal. But I could be mistaken. $\endgroup$ – kbh Apr 15 '15 at 14:35
  • $\begingroup$ related: chemistry.stackexchange.com/questions/901/… $\endgroup$ – Mithoron Apr 15 '15 at 16:18
  • $\begingroup$ I don't think that this is really a duplicate of that previous question - at least people react nervously over there that mine is not an answer. Anyway, it is an answer to this question, see: chemistry.stackexchange.com/questions/901/… $\endgroup$ – Luboš Motl Apr 16 '15 at 9:22
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From a historical perspective, this was done to account for the commonalities between how the various elements behaved. In other words, the Alkali metals all exhibited a strong reaction with $H_2O$, the noble gasses all exhibited inertness in reactions, and so on.

This is explained in modern day science via the quantum description of electronic structure which indicates that elements within the same group tend to have the same number of valence electrons in the valence shell. Simply put, the reason why there are such gaps between $H$ and $He$; $Be$ and $B$, and otherwise is because there are simply different quantum arrangements of the electrons which are readily accessible to the element. That is, the ground state of $He$ has both accessible $l=0$ states occupied by electrons, which constitutes the valence shell and so is in an energetically favorable state to remain inert - becoming of the noble gasses. Hence, it’s moved far to the right from $H$ to the 8th group; consistent with the historical structure of the table.

That’s a crude description, but about what’s happening here. I agree however that the chemists may feel more at home discussing the periodic table. I’ll stick to the chart of nuclides.

Also, the Wikipedia article is fairly informative on this topic: http://en.wikipedia.org/wiki/Periodic_table

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