Why are there two Hydrogen atoms on some periodic tables? 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?
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.