Hydrogen as a halogen

[weiming1998]

This site (http://hydrogentwo.c...odic-table.html) claims that hydrogen should actually be put in group 17 instead of group 1, basically claiming that hydrogen is a halogen.

 

Whilst hydrogen do not exactly fit in group 1 as an "alkali metal" (it's a gas instead of a metal, except in special conditions, it forms negatively-charged ions, and it is far less electropositive than lithium), it absolutely cannot be a halogen. Firstly, electronegativity decreases as you go down the periodic table, and the element becomes more and more metallic. Hydrogen would have to be more electronegative than fluorine should it be a halogen. Second, it doesn't even behave like a halogen, being mainly in the +1 oxidation state (doesn't happen to halogens, the most stable compounds of halogens have either -1 or a very high oxidation state, like +7 or +5. An oxidation state of +1 is very unstable in all halogens. It is also reactive with non-metals like oxygen, but not with metals like iron (still reacts slowly though, to form hydrides), unlike all halogens.

 

Hydrogen fits in its current position on the periodic table (group 1) much better than in group 17 (not a perfect fit, though). For example, it forms stable/meta-stable peroxides/superoxides, like alkali metals, particularly potassium and beyond. It has the same most common charge (+1) as all alkali metals, is mainly reducing like them, and reacts with halogens and oxygen vigorously, along with a few other examples.

 

But a few of its properties doesn't fit with either the halogens or the alkali metals. Its boiling point, for example. It is also much less electropositive than lithium but only about as electronegative as iodine (in fact, I think iodine is more electronegative). I think hydrogen should just be in its own group (since when its electron shell is filled, there are only 2 electrons instead of 8), with it being the most similar to alkali metals. So, I'm wondering what other people's opinions on the matter is.

Edited November 2, 2012 by weiming1998

[mississippichem]

I say keep it in group I. In the ground state a hydrogen atom has one electron in a 1s orbital. All the group I elements and hydrogen have the valence configuration ns¹. All the halogens have the valence configuration ns²np⁵. Ultimately it doesn't really matter as nature doesn't seem to care how we arbitrarily classify things.

 

Another element whose position could be disputed is helium. In the ground state it has no p-orbitals occupied though it is often grouped on the far right with the p⁶ noble gases.

[Anders Hoveland]

Helium is the only noble gas that does not have 8 electrons in its outer shell. And hydrogen does form a metal-like state under extremely high pressure.

An argument certainly can be made that hydrogen has chemistry in common with both halogens and alkali metals. But really, hydrogen is its own unique group. This is one of the problems when you try to classify different unique sets of properties into defined groups. The periodic table is like racism against chemical elements.

[dirtyamerica]

interesting concept. I've noticed that on some color-coded periodic tables they don't assign H a color.

[Peanuts]

Hydrogen is considered to be in a group of its own.

 

-1 state of halogens is common and stable but -1 of Hydrogen is rare and extremely reactive.

 

Still, If I'd be Moseley I'd place it above the alkali metals. Metallic hydrogen was discovered in 1996. Its +1 oxidation state is much stable like the others. Other properties are much similar.

 

The only discrepancies here are electronegativity and covalency.

[elementcollector1]

Hydrides do exist, if I recall. According to Wiki, in fact, crystallization for all alkali hydrides follows the motif of NaCl (cubic). An interesting correlation...

Of course, it behaves more like an extremely strong base, as well as hydrolyzing, and is used as an effective drying agent, so I suppose it's not entirely like table salt.

There are also unstable versions of halogens in the +1 oxidation state, for example bromine in bromine chloride (BrCl). So, could hydrogen be an 'inverse halogen' of sorts?

[Anders Hoveland]

Something else to consider is hydrocarbons, which are relatively chemically inert, similar to fluorocarbons.