Fluorine vs hydrogen

[Moontanman]

I'm gonna ask a stupid question, fluorine lacks one valence electron hydrogen lacks one electron yet hydrogen is not considered a halogen...  yet helium has  two electrons in it's outer shell and is a Nobel gas...why? 

[StringJunky]

17 minutes ago, Moontanman said:

I'm gonna ask a stupid question, fluorine lacks one valence electron hydrogen lacks one electron yet hydrogen is not considered a halogen...  yet helium has  two electrons in it's outer shell and is a Nobel gas...why? 

On the helium question, the outer shell is full and so it doesn't need to react with anything.

[OldChemE]

The arrangement of elements in the periodic table was done before we had extensive knowledge of electron orbitals and all that.  The original(s) of the periodic table were based on atomic weights and the reactive properties of the elements, not their electron structures. The Halogens all have nearly complete outer electron shells (missing only one electron), and are rather aggressive about stealing an electron to fill that outer shell.  Thus, they tend to form ions with a -1 charge.  That led to the classification called halogens.  Technically, Hydrogen is also just 1 electron away from filling its outer (and only) electron shell, but Hydrogen usually (but not always) gives up one electron forming a +1 ion instead of stealing a second electron-- a behavior more like lithium, sodium and the others in its group.

[hypervalent_iodine]

Technically, hydrogen can behave as either a halogen or an alkali metal, though it fits neither category completely and tends to be more of the latter. I would say one of main differences, and probably the most important one, between it and the halogens is in the fact that hydrogen only has an incomplete s orbital. This means that in its ground electronic configuration it only has 1 valence electron. Halogens have incomplete p orbitals, and have 7 valence electrons. A halogen is much more able to grab one single electron to fill its outter valence orbitals than it is to lose 7. Hydrogen, having only one valence electron, can go either way, but prefers to lose one. Couldn’t tell you why losing one is more energetically favoured than gaining one, though I guess it could be due to an extra electron resulting in additional repulsive forces (or maybe not, I’m no physicist). 

[StringJunky]

21 minutes ago, hypervalent_iodine said:

Technically, hydrogen can behave as either a halogen or an alkali metal, though it fits neither category completely and tends to be more of the latter. I would say one of main differences, and probably the most important one, between it and the halogens is in the fact that hydrogen only has an incomplete s orbital. This means that in its ground electronic configuration it only has 1 valence electron. Halogens have incomplete p orbitals, and have 7 valence electrons. A halogen is much more able to grab one single electron to fill its outter valence orbitals than it is to lose 7. Hydrogen, having only one valence electron, can go either way, but prefers to lose one. Couldn’t tell you why losing one is more energetically favoured than gaining one, though I guess it could be due to an extra electron resulting in additional repulsive forces (or maybe not, I’m no physicist). 

I had a look on Quora and it seems  it has middling electronegativity (2.2), so the atom it combines with determines which way it goes.

Edited March 8, 2019 by StringJunky

[hypervalent_iodine]

1 hour ago, StringJunky said:

I had a look on Quora and it seems  it has middling electronegativity (2.2), so the atom it combines with determines which way it goes.

Yes, although it is worth noting that a hydride is, as far as I understand, more reactive (and thus less stable) than H⁺. You could not, for example, have something like NaH just sitting out in the atmosphere without it being protected in some way by paraffin or some such. As I said, I believe this due to electrostatic repulsion of the two electrons, and possibly something to do with nuclear charge.

Anyway, point being that as you and I have noted, hydrogen has the capacity to act as either a Group 1 or 7 element to some extent, and is kind of in its own special category within the periodic table. 

 

[Moontanman]

Thanks, for the answers, just something I have long wondered about...