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Having never given it a though before, I recently discovered (in a different context) that the prefix halo- actually means 'salt' or 'sea' and the suffix -gen means 'to form' or 'to generate'. So the Halogens are the elements that 'form salts'. But there are salts that do not have halogens in them, like $\ce{Na2S}$ or $\ce{(NH4)2SO4}$. Was it known at the time that other salts existed, and what other names for this group might have been considered?

On a side note, why aren't Group I metals called halogens? Doesn't it take two to tango (I mean, to form salts?)

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This seems like a bit of a rhetorical question, so this isn't a terribly formal or authoritative answer, but anyhow - a lot of chemical nomenclature is like lava flow. It solidified and people just worked around it.

The halogens are so named because they have a rich chemistry of ionic compounds (fluorine through iodine, anyhow). However, both the halogens and the group I metals can form a wide range of things that aren't salts. The noble gases can form compounds with elements of low birth. The rare earth elements aren't particularly rare. Oxygen ('acid-former') is not a necessary component of acids. Technetium ('artificial' + ium) is produced in nature in significant quantities.

In addition, a lot of chemistry defies our human efforts to succintly categorise things, so at some point chemists have tried their best to find pragmatic general descriptors that unite groups of elements or molecules on the basis of the properties or constitution. The divide between organic and inorganic chemistry and the resulting exceptions and edge cases in classification (such as mellitic anhydride) is a good example of this. The map is not the territory.

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    $\begingroup$ "nomenclature is like lava flow. It solidified and people just worked around it." - oh, that is a delightful turn of phrase. I'll quote that (crediting you) in my next lecture. $\endgroup$ – EnergyNumbers Oct 22 '13 at 11:14
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    $\begingroup$ @EnergyNumbers - That was an oblique reference to the lava flow antipattern in software engineering, where it's too costly/complex to overhaul crappy or obsolete code/systems, so people just work around it. So, the phrase is not my invention. $\endgroup$ – Richard Terrett Oct 23 '13 at 2:23
  • $\begingroup$ @RichardTerrett Still, you transplanted it over to chemical nomenclature - that's not nothing! $\endgroup$ – hBy2Py Dec 13 '16 at 14:37
  • $\begingroup$ +1 for lava flow. As @hBy2Py points out, you applied it elegantly. $\endgroup$ – MarsJarsGuitars-n-Chars Feb 17 '18 at 17:49

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