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Most periodic tables only feature one Hydrogen atom, on the top of the first group. But some, like the one I was given, also show Hydrogen in the 7th group, to left of Helium.

Why are there two Hydrogen atoms? What's the difference between the two, and why do we work with the left one more often than the right?

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The two hydrogens are the same, but some periodic tables show hydrogen in both places to emphasize that hydrogen isn't really a member of the first group or the seventh group.

Hydrogen is a diatomic gas in it's elemental state, which is different from the other group one metals (and similar to the group seven elements). At the same time, hydrogen usually loses its electron, similar to sodium and the other group one metals.

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    $\begingroup$ I agree with this statement. I just want to add that hydrogen can (occasionally) form the hydride ion H-, much similar to the group seven elements. $\endgroup$ – Cardtrick Apr 29 '12 at 23:29
  • $\begingroup$ Also hydrogen has metallic properties if you put it under enough pressure. $\endgroup$ – Canageek Apr 30 '12 at 4:57
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    $\begingroup$ ""Also hydrogen has metallic properties if you put it under enough pressure."" This is not a chemical reason. One should avoid the terms metals or nonmetals when discussing chemical properties. Metal or not is a property of the elementary state, after some reaction it is no langer inmportant. $\endgroup$ – Georg May 1 '12 at 11:39
  • $\begingroup$ @Cardtrick Alkali metals form singly charged anions, too: alkalides $\endgroup$ – hBy2Py Aug 27 '15 at 15:18
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    $\begingroup$ I have seen a few tables where the hydrogen is slightly lifted off the row of alkali metals. $\endgroup$ – Pritt Balagopal Jun 3 '17 at 4:23

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