Which is stable more stable ?

[vkp]

The entropy of universe is continuously increasing and thermodynamics is an important part of chemistry which finally is the base for researches in universe.

So I wanted to have an answer for a question .....

Out of the two , which one is more stable and why ?

H₂⁺

or

H₂^-

Secondly, Can the bond order of any molecule be zero ?

[alainprice]

Molecular hydrogen that has lost an electron is not stable. You have a single electron bonding the 2 nuclei. One electron cannot be in 2 places at once. I'm just gonna stop here.

[snarkophilus]

The entropy of universe is continuously increasing and thermodynamics is an important part of chemistry which finally is the base for researches in universe.

So I wanted to have an answer for a question .....

Out of the two , which one is more stable and why ?

H₂⁺

or

H₂^-

Secondly, Can the bond order of any molecule be zero ?

I'm not certain what entropy has to do with it, but I'm going to venture that H₂+ is the more stable of the two, in a sense. The energy gap between the 1s and 2s orbitals is pretty large, so that extra electron is very likely to be picked off by just about anything else in the area. On the other hand, H₂+ can be observed as a metastable species.

Can the bond order of a molecule be zero? I assume you mean, "can two species be bound, but not share any electrons?" Well, the answer is yes and no. According to QM, there is always some electron sharing, but there are cases where it's so close to zero that one might as well call it zero.

The simplest case is van der Waals complexes. It's possible for two helium atoms, for instance, to join up in a complex of He₂, contrary to what you may have learned in early chemistry. It's more likely to occur in liquids than in gases (lower energy and more collisions), and indeed, it is possible to observe He₂ complexes in liquid helium. However, with crossed beam experiments, you can get these things in gas form, too (I'm not sure which species have been done, exactly).

If you want references to papers about He-H₂ van der Waals complexes, I can give you pages of them. The H₂ has a bond order of 1, but the He is not bound to it by significant electron sharing.

Apparently, when a gecko sticks to a wall, that's because of a number of (almost) zero order bonds (that is, it uses van der Waals bonding). So says Wikipedia, anyway.

[vkp]

As far the two species are concerned I am not very sure but according to Molecular Orbital Theory (what I am studying in class XII) is this

1) H₂⁺ - IT can be considered to form from hydrogen atom and hydrogen ion which is having a positive charge.Since antibonding orbitals are empty for this molecule if we go by electronic configuration(which cause instability), it can be considered stabler than H2-

2) In H2- the antibonding orbital is filled with 1 electron ... so this causes instability.

Can someone improve on this and give me a better explaination ?

Bond order zero : what does it mean ? Does it mean that since bond doesnot exist ..... species cannot form ?? Or there is more to it ?

[snarkophilus]

As far the two species are concerned I am not very sure but according to Molecular Orbital Theory (what I am studying in class XII) is this

1) H₂⁺ - IT can be considered to form from hydrogen atom and hydrogen ion which is having a positive charge.Since antibonding orbitals are empty for this molecule if we go by electronic configuration(which cause instability), it can be considered stabler than H2-

2) In H2- the antibonding orbital is filled with 1 electron ... so this causes instability.

Can someone improve on this and give me a better explaination ?

Bond order zero : what does it mean ? Does it mean that since bond doesnot exist ..... species cannot form ?? Or there is more to it ?

Your MO theory explanation is a good one, except that typically in MO theory, you are looking at the difference between bonding and anti-bonding orbitals (divide that by 2 to get the order). H₂+ has one electron in a bonding orbital, giving it a bond order of 1/2. H₂- has two electrons in a bonding orbital and one in an anti-bonding orbital, also giving it a bond order of 1/2.

So the question is: is the difference between the bonding orbital and the initial atomic orbitals bigger than the difference between the anti-bonding orbital and the atomic orbitals? The answer is no, the anti-bonding orbital differs by just a little more. I can't remember why this is exactly. (Maybe has something to do with entropy.) That means the more stable species is... H₂+, which has slightly lower energy than H₂-.

Ignore my post and the thing about the 2s orbital... I was thinking of H₄ complexes (and shouldn't have used the s notation even in that context). Stupid mistake.

As to bond order, think of H₂^2-, which has bond order 0. It has two electrons in a bonding orbital, lowering the energy by 2 x E_bonding. It also has two electrons in an anti-bonding orbital, raising the energy by 2 x E_anti. Your total energy is therefore 2 x E_anti - 2 x E_bonding. Since 2 x E_anti > 2 x E_bonding, it's energetically more likely that your atoms will stay apart.

So, one could conclude from MO that you can't have a stable species with bond order 0.

[PhantomWolf]

I suspect that snarkophilus is correct in that the H₂⁺ is slightly more stable as it has the lower energy state. H₂⁺ is likely to fall apart though, unless it can grab an electron form something, which is unlikely. I'd suspect that on contacting another molecule of attom the bond will break and it'll attach to whatever it found. The H₂^- will quickly lose the extra electron because it won't be able to hold onto it for long.

[Tobin Dax]

I suspect that snarkophilus is correct in that the H₂⁺ is slightly more stable as it has the lower energy state. H₂⁺ is likely to fall apart though, unless it can grab an electron form something, which is unlikely. I'd suspect that on contacting another molecule of attom the bond will break and it'll attach to whatever it found.

That's an interesting point you mention. A friend of mine is studying H₃⁺, and so I have to wonder how likely a collision of H₂⁺ and HI is to dissociate the molecule or to combine into the new one. I'll have to remember to ask if he knows.